A Review of the Systematics

and Zoogeography of the Freshwater

Species of Palaemonetes Heller

of North America

(Crustacea: Decapoda)

NED E. STRENTH

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY • NUMBER 228
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SMITHSONIA N CONTRIBUTION S TO ZOOLOG Y • NUMBE R 22 8

A Review of the Systematics

and Zoogeography of the Freshwater

Species of Palaemonetes Heller

of North America

(Crustacea: Decapoda)

Ned E. Strenth

SMITHSONIAN INSTITUTION PRESS

City of Washington

1976
ABSTRAC T

Strenth, Ned E. A Review of the Systematics and Zoogeography of the Freshwater Species of Palaemonetes Heller of North America (Crustacea: Decapoda).

Smithsonian Contributions to Zoology, number 228, 27 pages, 6 figures, 1 table,

1976.—Three new species of freshwater Palaemonetes from Texas and Mexico

are described. A key to the known North American species is given along with

major taxonomic references on each species since the 1952 revision by Holthuis.

The subgenus Alaocaris Holthuis, 1949, is synonymized with the subgenus

Palaemonetes Holthuis, 1949. As a result of larval studies, it is concluded that

the freshwater species of Palaemonetes in at least North America are of monophyletic origin separate from the marine species. These larval studies also support a close relationship to at least one European and one Asian species. Laboratory studies on the freshwater Palaemonetes kadiakensis Rathbun indicate an

inability of this species to disperse across wide oceanic stretches. Origin and dispersal of the freshwater Palaemonetes is considered. Zoogeographical distribution

of the genus Macrobrachium Bate and its ecological relationship to Palaemonetes

as correlated with geological data support a much earlier origin of the genus

Palaemonetes than previously recorded in the literature. Several hypotheses are

reviewed in an effort to explain the worldwide distribution of freshwater

Palaemonetes. The distributional patterns of Palaemonetes and Macrobrachium

appear consistent with the theory of continental drift.

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded

in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea

cavernosa (Linnaeus).

Library of Congress Cataloging in Publication Data

Strenth, Ned E

A review of the systematics and zoogeography of the freshwater species of Palaemonetes Heller

(Crustacea, Decapoda) of North America.

(Smithsonian contributions to zoology ; no. 228)

Bibliography: p.

Supt. of Docs, no.: SI 1.27:228

1. Palaemonetes. 2. Crustacea—Classification. 3. Crustacea—Geographical distribution. 4. Crustacea—North America. I. Title. A review of the systematics and zoogeography of the freshwater

species of Palaemonetes Heller. II. Series: Smithsonian Institution. Smithsonian contributions

to zoology ; no. 228.

QL1.S54 no. 228 [QL444.M33] 591'.08s [595'.3843]
Contents

Page

Introduction 1

Palaemonetes Heller, 1869 2

Palaemonetes antrorum Benedict, 1896 2

Palaemonetes paludosus (Gibbes, 1850) 2

Palaemonetes kadiakensis Rathbun, 1902 2

Palaemonetes cummingi Chace, 1954 2

Palaemonetes suttkusi Smalley, 1964 3

Palaemonetes lindsayi Villalobos and Hobbs, 1974 3

Palaemonetes holthuisi, new species 3

Palaemonetes texanns, new species 5

Palaemonetes mexicanus, new species 7

Key to the Freshwater Species of Palaemonetes in the United States and

Mexico 9

Systematic Relationships of Marine and Freshwater Palaemonetes 9

Reexamination of the Subgenus Alaocaris Holthius and Affinities within

the North American Freshwater Species 12

Salinity Tolerance and Related Dispersal Abilities of Palaemonetes

kadiakensis Rathbun 15

Zoogeographical and Geological Considerations 18

Literature Cited 24

A Review of the Systematics

and Zoogeography of the Freshwater

Species of Palaemonetes Heller

of North America

(Crustacea: Decapoda)

Ned E. Strenth

Introduction

The North American species of Palaemonetes

represent what are probably the most poorly

known of our freshwater decapods. This is in part

due to their somewhat limited commercial value,

overall morphological similarity, lack of extensive

field work, and a poor understanding of currently

used taxonomic characters. At the time of Holthuis' (1952) revision of the subfamily Palaemoninae there were only three described freshwater

species known from the United States and Mexico.

Two of these, P. paludosus (Gibbes) and P. kadiakensis Rathbun, were placed in the typical subgenus Palaemonetes along with the North American marine species and South American saltwater

and freshwater species. The subgenus Alaocaris

was erected for the single aberrant subterranean

species P. antrorum Benedict. Three species of

freshwater Palaemonetes have been described

since 1952. These are P. cummingi Chace 1954,

P. suttkusi Smalley 1964, and P. lindsayi Villalobos

and Hobbs 1974. Identification of species by use

of appendix masculina spination was established

by Fleming (1969) and later used by Villalobos

and Hobbs (1974).

Ned E. Strenth, Marine Biomedical Institute, University of

Texas Medical Branch, Galveston, Texas 77550.

The genus Palaemonetes Heller represents what

is possibly one of the most systematically confusing

taxon of the natant decapods. Sollaud (1923a),

for example, divided the then-accepted members

of the subfamily Palaemoninae into two groups:

those with small eggs and those with large eggs.

Even though this served to split the species of

Palaemonetes, he proposed no taxonomic separation at the generic level. Sollaud (1923b:4) also

noted the difficulty and inability to satisfactorily

explain the geographical distribution of the freshwater species of Palaemonetes. Kemp (1925:315)

suggested the possibility that the genus may not

have been derived from a monophyletic origin.

Chace (1972) reviewed the difficulty involved in

establishing the systematic relationship of the

genus Palaemonetes with that of Palaemon Weber,

and Holthuis (pers. comm.) has expressed the

need for a thorough revision of both genera. A

polyphyletic origin of the genus Palaemon has

also been proposed by Tiwari (1955b: 238).

After considerable field work and laboratory

studies, it is the conclusion of this author that

several undescribed species are present in Texas

and Mexico; three new species are described herein. A reexamination of the systematic relationship

between the freshwater and marine species is

presented. As a result of larval studies, it is con-
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

eluded that the freshwater species of Palaemonetes

in at least North America are of monophyletic

origin separate from the marine species. These

larval studies also support a close relationship to

at least one European and one Asian species. The

subgenus Alaocaris Holthuis, 1949, is synonymized

with the subgenus Palaemonetes Holthuis, 1949.

A key to the known North American species is

given along with major taxonomic references on

each species since the 1952 revision by Holthuis.

Distributions of species as well as selected characters are examined in an effort to establish affinities among the North American species of

Palaemonetes.

Laboratory studies on the freshwater Palaemonetes kadiakensis Rathbun indicate an inability

of this species to disperse across wide oceanic

stretches. Origin and dispersal of the freshwater

Palaemonetes is considered. Zoogeographical distribution of the genus Macrobrachium Bate and

its ecological relationship to Palaemonetes as correlated with geological data support a much

earlier origin of the genus Palaemonetes than

previously recorded in the literature. Several hypotheses are reviewed in an effort to explain the

worldwide distribution of freshwater Palaemonetes. The distributional patterns of Palaemonetes

and Macrobrachium appear consistent with the

theory of continental drift.

Under "Disposition of Types," the USNM numbers refer to the specimens deposited in the National Museum of Natural History, Smithsonian

Institution, under the catalog numbers of the

former United States National Museum.

ACKNOWLEDGMENTS.—I am most grateful to

Merrill H. Sweet, Sammy M. Ray, Tai Soo Park,

and Jack W. Anderson of Texas A & M University

for their criticisms of the manuscript. Appreciation is extended to Willard Young and Glenn

Longley for making available larval specimens of

Palaemonetes antrorum. I wish to thank the Government of Mexico for extending permission to

make collections during the course of this study.

My most sincere thanks go to Horton H. Hobbs,

Jr., for his review of the manuscript.

Palaemonetes Heller, 1869

Palaemonetes Heller, 1869:157. [Type-species, by monotypy:

Palaemon varians Leach, 1814:432; gender: masculine.]

Palaemonopsis Stimpson, 1871:128.

Allocaris Sollaud, 1911:50.

Coutierella Sollaud, 1914:318.

Subgenus Alaocaris Holthuis, 1949:89.

Palaemonetes antrorum Benedict, 1896

Palaemonetes antrorum Benedict, 1896:615.

Palaemonetes (Alaocaris) antrorum.—Holthuis, 1949:89, 1952:

203, 1955:49.—Chace, 1954:323—Smalley, 1964:231.—Fleming, 1969:444.—Villalobos and Hobbs, 1974:15.

This species is known from the subterranean

waters within the city of San Marcos; also from

Ezell's Cave within the city limits of San Marcos,

Hays County, Texas (Holthuis, 1952).

Palaemonetes paludosus (Gibbes, 1850)

Hippolyte paludosa Gibbes, 1850:197.

Palaemonetes exilipes Stimpson, 1871:130.

Palaemonopsis exilipes.—Stimpson, 1871:130.

Hippolysmata paludosa—Howard, 1883:294.

Palaemon (Palaemonetes) exilipes.—Thallwitz, 1892:8.

Palaemon (Palaemonetes) paludosus.—Thallwitz, 1892:12.

Palaemonetes (Palaemonetes) paludosus.—Holthuis, 1949:91,

1952:207.—Smalley, 1964:231.—Fleming, 1969:444—Villalobos and Hobbs, 1974:15.

This species is widespread in the eastern United

States and is found as far west as eastern Texas

(Holthuis, 1952). It has been introduced into

southern California and northern Mexico (Hayden and Ringo, 1963, and St. Amant and Hulquist,

1969).

Palaemonetes kadiakensis Rathbun, 1902

Palaemonetes kadiakensis Rathbun, 1902:93.

Palaemonetes (Palaemonetes) kadiakensis.—Holthuis, 1949:92,

1952:212.—Smalley, 1964:231.—Rodriguez de la Cruz, 1965:

96.—Fleming, 1969:444.—Villalobos and Hobbs, 1974:15.

This species is widespread in the central United

States (Holthuis, 1952); its range extends southwest into northern Mexico (Creaser, 1932).

Palaemonetes cummingi Chace, 1954

Palaemonetes (Palaemonetes) cummingi Chace, 1954:319.—

Smalley, 1964:232.—Villalobos and Hobbs, 1974:15.

This species is known only from "Squirrel Chimney," a circular solution cavity in Alachua County,

Florida (Chace, 1954).
NUMBER 228

Palaemonetes suttkusi Smalley, 1964

Palaemonetes suttkusi Smalley, 1964:229.—Rodriguez de la

Cruz, 1965:92.

Palaemonetes (Palaemonetes) suttkusi.—Fleming, 1969:444.—

Villalobos and Hobbs, 1974:15.

This species was originally described from the

Rfo Nadadores north of Monclova, Coahuila, Mexico (Smalley, 1964). Minckley (1969:25) extended

the range to include the entire Cuatro Ctenegas

basin.

Palaemonetes lindsayi

Villalobos and Hobbs, 1974

Palaemonetes (Palaemonetes) lindsayi Villalobos and Hobbs,

1974:9.

This species is known only from the springs and

irrigation canals associated with the La Media

Luna watershed near the city of Rfoverde, San

Luis Potosf. Mexico (Villalobos and Hobbs, 1974).

Palaemonetes holthuisi, new species

FIGURE 1

DISPOSITION OF TYPES.—Male holotype, USNM

152391, and paratype USNM 152392; one male

paratype, Rijksmuseum van Natuurlijke Historic

Leiden, The Netherlands.

TYPE-LOCALITY.—Subterranean waters of Ezell's

Cave in the city of San Marcos, Hays County,

Texas.

DESCRIPTION.—Rostrum (Figure la) high, compressed and short, reaching only to distal margin

of basal segment of antennular peduncle; upper

margin bearing four to eight teeth, one or two

placed behind orbit; lower rostral margin devoid

of teeth and distally with only few setae. Antennal

spine slender and distinct, its base removed short

distance from anterior margin of carapace. Branchiostegal spine situated somewhat ventral to

branchiostegal groove and removed short distance

from anterior margin of carapace.

Pleura of fourth and fifth abdominal somites

forming acute to right angle; sixth somite twice

as long as fifth. Telson (Figure 16) atypical; posterior margin widened with six to ten small spines

and one enlarged lateral pair placed just mesial

to lateralmost smaller pair; two pair of dorsal

spines placed along lateral margins.

Eyes (Figure la) reduced, without pigment, and

bullet shaped in appearance; cornea completely

degenerated.

Stylocerite (Figure \e) of antennular peduncle

sharp at tip; upper antennular flagellum with

rami united for only five to six articles; free portion of shorter ramus consisting of 19 to 20 articles.

Scaphocerite (Figure Id) almost twice as long

as wide, lateral margin straight; blade extending

beyond lateral tooth; antennal flagellum exceeding 250 mm in length.

Mandible (Figure 1/) with molar process typical, incisor process reduced to a short triangular

structure. Maxillula (Figure lg) with mesial

lacinia greatly enlarged; median lacinia rather

slender. Maxilla (Figure \h) typical in size and

shape. First maxilliped (Figure It) somewhat

elongated with enlarged cup-shaped endites hooding greatly enlarged mesial lacinia of maxillula.

Second maxilliped (Figure 1;) greatly enlarged

due to elongation of virtually all segments, most

notable in the distal ones. Third maxilliped

(Figure \k) typical, extending anteriorly just past

end of antennal peduncle.

First pereiopod (Figure 1/) extending to distal

margin of scaphocerite; palm and finger bearing

numerous setae distally; carpus 2.5 times as long

as chela and subequal in length to merus. Second

pereiopod (Figure lm) similar in length of first

pereiopod; carpus 2.5 times as long as chela, merus

twice as long as chela. Third pereiopod (Figure

In) overreaching scaphocerite by half length of

propodus; propodus and merus of equal length

with carpus only five-sixths length of propodus.

Fourth and fifth pereiopods similar to third.

Appendix masculina (Figure \c) of male with

nine apical spines. Lateral ramus of uropod (Figure 16) without movable spine between fixed distolateral tooth and margin of blade.

SIZE.—Males with carapace length to 6 mm

(including rostrum, to 8 mm).

VARIATION.—This species like Palaemonetes tonkinensis (Sollaud, 1914) bears a telson, which is

atypical for the genus. The telson does resemble

that of larval palaemonids as noted by Holthuis

(1952:206) and is possibly of neotenic origin.

RANGE.—This species is known only from Ezell's

Cave in the city of San Marcos, Hays County,

Texas.

REMARKS.—At Southwest Texas State University,
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

FIGURE 1.—Palaemonetes holthuisi, new species, holotype, male: a, anterior region; b, telson and

uropods; c, appendix masculina; d, right antenna; e, right antennule; /, right mandible; g, right

maxillula; h, right maxilla; i, right first maxilliped; ;', right second maxilliped; k, right third

mzxilliped; /, first pereiopod; m, second pereiopod; n, third pereiopod. (Scale 1.0 mm.)
NUMBER 228

I came into possession of several collections of

shrimp which had been taken from Ezell's Cave

prior to its acquisition by the Nature Conservacy

in 1967. These specimens were either in very poor

condition, lacking complete labels, or both. An

examination of this material revealed the presence

of two morphological types.

Holthuis (1952:205) suggested the possibility

that a second species of subterranean shrimp was

present in Ezell's Cave in addition to Palaemonetes

antrorum Benedict. The type-specimens of P.

antrotum were collected from the artesian well

at the old Fish Hatchery (Benedict, 1896). Holthuis (1952) was able to examine only three specimens which had been collected from Ezell's Cave.

One of these proved to be identical to the typematerial of P. antrorum. The two remaining

specimens, however, were quite different. Because

of the similarity of the telson to that of larval

palaemonids and the inability at that time to

secure additional material, Holthuis with considerable doubt considered these two specimens

to be questionable juveniles of P. antrorum.

Recently I contacted Dr. Holthuis by letter to

advise him of the current situation concerning

this matter and to offer the additional material

to him. He has most generously suggested that I

describe the new species. It is with great pleasure

that this unusual and distinctive species is named

in his honor.

Palaemonetes texanus, new species

FIGURE 2

DISPOSITION OF TYPES.—Male holotype, USNM

152395, and one male and one female paratype,

USNM 152396; one male and one female paratype, Rijksmuseum van Natuurlijke Historic

Leiden, The Netherlands.

TYPE-LOCALITY.—San Marcos River within the

city limits of San Marcos, Hays County, Texas.

DESCRIPTION.—Rostrum (Figure 2a) straight;

slightly overreaching anterior margin of scaphocerite; dorsal margin bearing five to eight teeth,

one of which is placed behind orbit; lower margin

with two to four teeth. Antennal spine sharp,

distinct, and extending beyond anterior margin

of carapace. Branchiostegal spine just below branchiostegal groove on anterior margin of carapace

and extending anteriorly.

Pleura of fourth and fifth somites forming acute

to right angle; sixth somite 1.5 times as long as

fifth. Telson (Figure 2b) subequal in length to

sixth somite; anterior pair of dorsal spines posterior to midlength of telson; posterior pair usually

near posterior margin. Posterior margin of telson

with sharp median point flanked by one pair of

plumose setae and two pair of spines; mesial pair

of spines elongate and extending beyond median

point.

Eyes (Figure 2a) well developed; cornea globular and well pigmented.

Stylocerite (Figure 2e) sharp and extending

one-third length of basal segment of antennular

peduncle; distolateral spine strong. Lateral antennular flagellum with proximal 19 to 29 articles of

rami fused; free portion of shorter ramus consisting of three to six articles.

Scaphocerite (Figure 2d) about three times as

long as wide, lateral margin straight; blade well

overreaching lateral tooth.

Mouthparts (Figures 2f—k) typical.

First pereiopod (Figure 2/) extending to distal

margin of scaphocerite; palm and finger bearing

numerous setae distally; carpus twice as long as

chela and slightly longer than merus. Second

pereiopod (Figure 2m) longer than first, overreaching scaphocerite by most of chela; carpus

1.4 times as long as chela and 1.5 times as long

as merus. Third pereiopod (Figure 2n) extending

to base of third segment of antennular peduncle;

propodus and merus of equal length with carpus

0.6 length of propodus. Fourth pereiopod reaching anterior margin of scaphocerite; fifth pereiopod overreaching anterior margin of scaphocerite

by length of dactyl.

Appendix masculina (Figure 2c) of male with

six apical spines. Eggs of gravid females few and

large; 1.2 mm to 1.5 mm in length. Lateral branch

of uropod with or without movable spine between

fixed distolateral tooth and margin of blade.

SIZE.—Male with carapace length to 8 mm (including rostrum, to 14 mm); females, to 11 mm

(including rostrum, to 18 mm).

VARIATION.—The movable spine of the lateral

ramus of the uropod is quite variable; both movable spines may be present, both may be absent,

or only a left or right one may be present.
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

FIGURE 2.—Palaemonetes texanus, new species, holotype, male: a, anterior region; b, telson and

uropods; c, appendix masculina; d, right antenna; e, right antennule; /, right mandible; g, right

maxillula; h, right maxilla; i, right first maxilliped; ;', right second maxilliped; k, right third

maxilliped; /, first pereiopod; m, second pereiopod; n, third pereiopod. (Scale 1.0 mm.)
NUMBER 228

RANGE.—This species is known from the San

Marcos River in the city of San Marcos, Hays

County and Comal River in the city of New

Braunfels, Comal County, Texas. Both of these

rivers are fed by large springs associated with the

Balcones Escarpment.

Palaemonetes mexicanus, new species

FIGURE 3

DISPOSITION OF TYPES.—Male holotype, USNM

152393, and one male and one female paratype,

USNM 152394; one male and one female paratype, Instituto de Biologia de la Universidad Nacional Aut6noma de Mexico; one male and one

female paratype, Rijksmuseum van Natuurlijke

Historie, Leiden, The Netherlands.

TYPE-LOCALITY.—Small river 15 road miles (24

km) west of Ciudad Valles, San Luis Potosi, Mexico.

DESCRIPTION.—Rostrum (Figure 3a) high; upturned at end; extending to anterior margin of

scaphocerite; dorsal margin with six to seven

teeth, usually six, one of which is placed behind

orbit; ventral margin with one or two teeth, usually two. Antennal spine sharp and distinct. Branchiostegal spine placed on anterior margin of carapace a short distance below branchiostegal groove.

Abdomen normal; pleura of fifth somite forming right angle; pleura of fourth somite rounded;

sixth somite 1.5 times as long as fifth. Telson

(Figure 36) equal in length to sixth somite; anterior pair of dorsal spines located one-fourth of

telson length from posterior margin; posterior

pair on or near posterior margin. Posterior margin of telson with sharp median point flanked by

two pair of spines and one pair of plumose setae;

mesial pair of spines elongate and extending

much beyond median point; lateral pair of spines

extending to or slightly overreaching median point

of telson.

Eyes (Figure 3a) well developed; cornea globular and well pigmented.

Stylocertie (Figure Se) sharp and extending

one-third length of basal segment of antennular

peduncle; anterolateral spine strong, slightly overreaching anterior margin of segment; lateral

antennular flagellum with rami fused for 14 to 24

articles; free portion of shorter ramus consisting

of four to six articles.

Scaphocerite (Figure 3d) about three times as

long as wide, lateral margin slightly concave,

blade well overreaching lateral tooth.

Mouthparts (Figure Sf-k) typical.

First pereiopod (Figure 3/) extending to distal

margin of third segment of antennular peduncle;

palm and finger bearing numerous setae distally;

carpus twice as long as chela and 1.3 times as long

as merus. Second periopod (Figure 3m) overreaching scaphocerite by length of dactyl; carpus

1.6 times as long as chela; chela and merus equal

in length. Third pereiopod (Figure 3n) extending to midregion of third segment of antennular

peduncle; propodus 1.7 times as long as carpus;

merus twice as long as carpus. Fourth pereiopod

overreaching anterior margin of third segment of

antennular peduncle by length of dactyl; fifth

pereiopod overreaching anterior margin of scaphocerite by length of dactyl.

Appendix masculina (Figure Sc) of male with

five apical spines. Eggs of gravid females few and

large: 1.3 mm to 1.6 mm in length. Lateral ramus

of uropod with or without movable spine between

fixed distolateral tooth and margin of blade.

SIZE.—Male with carapace length to 6 mm (including rostrum, to 11 mm); females, to 11 mm

(including rostrum, to 18 mm).

VARIATION.—The movable exopod spine on lateral ramus of the uropod is quite variable; both

movable spines may be present, both may be absent,

or only a left or right one may be present.

RANGE.—This species is known only from a

small spring-fed river in the Sierra Madre Orientals, 15 road miles (24 km) west of Ciudad Valles,

San Luis Potosf, Mexico.

REMARKS.—This species in many respects is very

similar to P. texanus: the most notable of these is

in the variation of movable exopod spination of

the uropod. The two species, however, are quite

disjunct in their distributions and exhibit different

spination of the appendices masculinae and rostral ventral dentition, as well as different morphologies of epipodites of the first maxillipeds. The

type-locality is the same river as that given for

the type-locality of the gastropod Pachychilus

apheles Thompson, 1967. The native inhabitants

of the area call the river Nacimiento del Rio due

to the large spring from which it originates.
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

FIGURE 3.—Palaemonctes mexicanus, new species, holotype, male: a, anterior region; b, telson

and uropods; c, appendix masculina; d, right antenna; e, right antennule; /, right mandible;

g, right maxillula; h, right maxilla; i, right first maxilliped; /, right second maxilliped; k, right

third maxilliped; /, first pereiopod; m, second pereiopod; n, third pereiopod. (Scale 1.0 mm.)
NUMBER 228 ?

Key to the Freshwater Species of Palaemonetes in the United States and Mexico

The following key is provided to facilitate identification of the freshwater

species of Palaemonetes in the United States and Mexico. The key is an adaptation of previous keys by Holthuis (1952), Chace (1954), and Villalobos and

Hobbs (1974). Several characters may be given per couplet; initial characters,

although often difficult to observe, are the most reliable with subsequent characters decreasing in reliability. Collection locality appears quite reliable in most

species because of their rather restricted distributions; P. kadiakensis and P.

paludosus appear to be the only major exceptions.

1. Antennal scale of Form I larvae segmented at distal end; gravid females with large numbers

of small eggs; upper or lateral antennular flagellum with distal free portion of shorter

ramus equal to or longer than fused basal portion (see IB for exceptions)

Brackish and saltwater species

Antennal scale of Form I larvae lacking segmentation at distal end; gravid females with

small number of large eggs; upper or lateral antennular flagellum with distal free portion

of the shorter ramus shorter than fused basal portion in all except two aberrant subterranean species, P. antrorum and P. holthuisi, from Hays County, Texas

Freshwater species 2

2. Eyes degenerated; body depigmented; cave forms 3

Eyes well developed; body pigment normal; epigean forms 5

3. Distal free portion of shorter ramus of lateral antennular flagellum shorter than fused basal

portion; lower margin of rostrum with teeth; second pair of legs longer than first pair;

known only from "Squirrel Chimney," Alachua County, Florida P. cummingi Chace

Distal free portion of shorter ramus of lateral antennular flagellum longer than fused basal

portion; lower margin of rostrum without teeth; second pair of legs equal in length to

first pair 4

4. Telson tapered posteriorly with two pair of spines on posterior margin; known from Ezell's

Cave and subterranean waters of San Marcos, Hays County, Texas P. antrorum Benedict

Posterior margin of telson widened, with eight to twelve spines on posterior margin; known

only from Ezell's Cave, San Marcos, Texas P. holthuisi, new species

5. Appendix masculina with three or four apical setae 6

Appendix masculina with five or six apical setae 7

6. Appendix masculina with three apical setae P. kadiakensis Rathbun

Appendix masculina with four apical setae P. paludosus (Gibbes)

7. Appendix masculina with five apical setae 8

Appendix masculina with six apical setae 9

8. Posterior pair of dorsal telson spines situated midway between anterior pair and posterior

margin of telson; known only from La Media Luna area southwest of Rioverde, San Luis

Potosf, Mexico P. lindsayi Villalobos and Hobbs

Posterior pair (seldom only one) of dorsal telson spines situated on or in close juxtaposition

to posterior margin of telson; known only from small spring-fed river 15 miles west of

Ciudad Valles, San Luis Postosf, Mexico P. mexicanus, new spedes

9. Upper or lateral antennular flagellum with rami fused for 10 to 16 joints; known only from

water systems in and flowing from Cuatro Cienegas basin west of Monclova, Coahuila,

Mexico P> suttkusi Smalley

Upper or lateral antennular flagellum with rami fused for 19 to 29 joints; known only from

spring-fed rivers of Hays and Comal counties of central Texas

P. texanus, new species

Systematic Relationships of Marine and monetes of the southeastern United States and

Freshwater PaUtemonetes Mexico poses a perplexing problem. At first consideration one might expect the freshwater PalaeThe systematic relationship which exists be- monetes to be of polyphyletic origin from the

tween the marine and freshwater species of Palae- marine or estuarine stocks which had invaded the
10 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

different river systems. There seems to be evidence

that the freshwater species instead constitute a

monophyletic assemblage.

Sollaud (1923a) divided members of the subfamily Palaemoninae into two groups: those with

small eggs and those with large eggs. He also presented evidence based on larval appendage morphology (p. 595) to substantiate the separation of

the two groups. An examination of Sollaud's separation of the species of Palaemonetes, based on

egg size, which was made possible only by use of

Holthuis' (1950:9-11) list of synonyms, reveals a

distinct separation of marine and freshwater species. Those listed in the small egg grouping are

P. vulgaris (Say) and P. varians (Leach) (as P.

varians occidentalis). Both of these are brackishwater species. The second group, those with large

eggs, is somewhat longer and includes: P. antennarius (H. Milne Edwards) (as P. varians lacustris), P. tonkinensis (Sollaud) (as Coutierella

tonkinensis), P. mesopotamicus Pesta (as P.

varians mesopotamicus), P. sinensis (Sollaud) (as

P. (Allocaris) sinensis), and P. mesogenitor Sollaud (as P. punicus). All of these are freshwater

species. The discussion of poecilogony by Gurney

(1942:64) and his rejection of its existence within

the genus Palaemonetes appears to substantiate

Sollaud's separation of species based on egg size

and number.

Holthuis (1949:88, 1952:201) has shown that

the two North American assemblages can usually

be separated on the basis of upper antennular

flagellum morphology. The marine species have

the distal free part of the upper antennular flagellum longer than the basal fused part while the

freshwater species have the free part shorter than

the fused part. The only exceptions are the aberrent subterranean species, P. antrorum and P.

holthuisi, which resemble the marine species

instead of the freshwater forms.

A comparative examination of the adults of

available marine and freshwater species did not

reveal any additional taxonomic characters which

might serve to distinguish these two groups. The

larvae, however, do show some significant differences. There is available a sizable number of excellent larval studies on most of the North American species of Palaemonetes. Among the marine

species which have larval studies available are P.

vulgaris (Say) (Faxon, 1879), P. pugio Holthuis

(Broad, 1957), and P. intermedium Holthuis

(Hubschman and Broad, 1974). Among the freshwater species studied are P. paludosus (Gibbes)

(Dobkin, 1963), P. kadiakensis Rathbun (Broad

and Hubschman, 1963), and P. cummingi Chace

(Dobkin, 1971.)

As noted in Hubschman and Broad's (1974:

102, Table II) summary of differences of larvae of

North American species of Palaemonetes, the marine larvae could be distinguished from the freshwater larvae on the basis of the antennal scale of

the Form I larvae and the postlarval telson

morphology. The acquisition of the initial larval

stages of the remaining species of freshwater Palaemonetes was undertaken.

Form I larvae were present in the collections of

P. antrorum that were trapped from the flowing

water of the artesian well at the Aquatic Station

(site of old Fish Hatchery) on the campus of

Southwest Texas State University, San Marcos,

Texas. The antennal scale (Figure 4c), like the

previously described freshwater forms, is not segmented. Adult gravid females of P. suttkusi, P.

texanus, and P. mexicanus were collected from

their respective type-localities and returned to the

laboratory. The females were retained until their

eggs hatched. The antennal scales of each of the

Form I larvae are shown in Figure 4. None are

segmented. Larvae of P. holthuisi could not be

obtained. Several attempts to maintain gravid female P. lindsayi in the laboratory were unsuccessful but examinations of late embryos from preserved specimens did not reveal the presence of

any segmentation in the antennal scale.

The unsegmented antennal scales of the Form

I larvae as well as the production of large eggs of

the known species of North American freshwater

Palaemonetes together give evidence for their

monophyletic origin. A separate systematic position from the marine species seems warranted in

light of the importance of larval differences as

noted by Brooks (1886:15) and Gurney (1942:

13); however, their exact relationship to the rest

of the world's freshwater Palaemonetes remains

uncertain due to the sparsity of information

available on them.

Palaemonetes argentinus Nobili, a South American freshwater species known from South Brazil

to Argentina (Holthuis, 1952:226), is an unusual

freshwater Palaemonetes. The larval development
NUMBER 228 11

FIGURE 4.—Comparison of antennas of Form I zoeas: a, Palaemonetes texanus n. sp.; b, Palaemonetes mexicanus n. sp.;

c, Palaemonetes antrorum Benedict; d, Palaemonetes suttkusi

Smalley. (Scale 0J> mm.)

was studied by Boschi (1960). The first larval

stage has the segmented antennal scale common

to the North American marine species. In addition the adults of this species have antennular

flagellum morphology similar to the marine species, as well as ovigerous females bearing eggs

which "are rather numerous and small" (Holthuis,

1952:226). Based on these few characteristics it

appears that P. argentinus is most closely related

to the marine species and probably represents an

independent invasion of freshwater, distinct at

least from that of the North American freshwater

species. It also indicates that the characters which

relate together the freshwater species are not

merely adaptations to freshwater.

The freshwater P. antennarius (as P. varians

lacustris) of Europe was noted by Sollaud

(1923a: 571) to possess the nonsegmented antennal scale in the subparva (first free form of Palaemonetes) or Form I zoea. This species was also

included by Sollaud in the group bearing large

eggs. Palaemonetes varians (Leach) (as P. varians

occidentalis), a marine species as noted by Sollaud

(1923a: 551), possesses the segmented antennal

scale in the first stage. This species along with

the North American marine species P. vulgaris

(Say) bears small eggs. It then appears that, based

on larval antennal scale morphology and type of

eggs produced, P. varians exemplifying the European marine species is most closely related to the

North American marine species, while P. antennarius exemplifying the freshwater species of

lands around the Mediterranean is related to the

North American freshwater species. Shen (1939:

181) noted that the Form I zoea of P. sinensis

possesses the nonsegmented antennal scale. We

can then hypothesize that due to its production

of large eggs and larval antennal scale morphology

that P. sinensis may also be closely related to

the North American freshwater species. Sollaud

(1923b:4) noted the difficulty and inability to

satisfactorily explain the geographical distribution

of the freshwater Palaemonetes. This subject will

be further discussed in the section on zoogeographical considerations.

Habitat preferences based on different underlying physiological abilities also appear to separate the marine species from the freshwater species. Panikkar (1941) discussed the evolutionary

significance of varying osmotic behaviors in the

Palaemoninae and notes Kemp's (1925:315) suggestion of polyphletic origin in the genus Palaemonetes. Parry (1957:421) indicates the distinction of P. antennarius (freshwater) from P.

varians (marine) on the basis of physiological

evidence. Dobkin and Manning (1964) compared

the osmoregulatory differences existing between

P. paludosus (freshwater) and P. intermedius

(marine).

It appears that the North American marine

and freshwater species of Palaemonetes represent

distinct and separate evolutionary lineages. This

is based on larval antennal scale morphology, size

and number of eggs produced, and physiological

differences associated with habitat preference.

These differences are clear and distinct and suggest that the freshwater species warrant separate

generic status. This would involve resurrection of

the earlier synonym used by Stimpson (1871).
12 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Such a step is postponed at this time due to a lack

of knowledge of the worldwide marine and freshwater species. This appears particularly justified

in light of the questionable status of the genus

Palaemonetes with relation to Palaemon as reviewed by Chace (1972).

Reexamination of the Subgenus Alaocaris Holthuis

and Affinities within the North American

Freshwater Species

At the time of the revision of the family Palaemonidae by Holthuis (1952) only three species of

freshwater Palaemonetes were known from the

southeastern United States and Mexico. Four

other blind cave species from Cuba (P. calcis

Rathbun, P. eigenmanni Hay, P. gibarensis Chace,

arid P. inermis Chace) were removed to the genus

Troglocubanus. This was done primarily on the

lack of a branchiostegal spine in those species

assigned to Troglocubanus. Holthuis also erected

the subgenus Alaocaris to receive the aberrant subterranean species P. antrorum.

Three species of freshwater Palaemonetes have

been described from the United States and Mexico

since 1952. These are P. cummingi Chace 1954,

P. suttkusi Smalley 1964, and P. lindsayi Villalobos

and Hobbs 1974. This study adds three additional species: P. holthuisi, P. texanus, and P.

mexicanus. Even though P. cummingi shares several characteristics with P. antrorum, Chace

(1954) indicated that it was more closely related

to members of the typical subgenus Palaemonetes.

Smalley (1962) believed P. suttkusi to be allied

to the subgenus Palaemonetes even though he

noted that it lacked a movable exopod spine, one

of four characters used by Holthuis (1949, 1952)

in separating Alaocaris from Palaemonetes. Smalley (1964) also indicated a probable relationship

between P. paludosus, P. kadiakensis, P. suttkusi,

and P. cummingi.

Flemming (1969) in agreement with Smalley

retained P. suttkusi in the subgenus Palaemonetes.

In their description of P. lindsayi, Villalobos and

Hobbs (1974) consider that of the six known

freshwater species of the genus, all except P. antrorum comprise a closely allied group. They also

believe that P. lindsayi is more closely related to

P. suttkusi and P. paludosus than to other congeners. All authors to date have chosen to place

P. antrorum aside and have somewhat justifiably

ignored its relationships because of its somewhat

different morphological characters.

The separation by Holthuis (1949, 1952) of P.

(Alaocaris) antrorum as a subgenus distinct from

the typical subgenus Palaemonetes was based on

the following four characters: eyes strongly degenerated and without pigment, first and second pair

of pereiopods similar in relative size, absence of

teeth of the lower rostral margin, and the absence

of a "movable spine at the inner side of the final

tooth of the external exopod of the uropod."

Species of the subgenus Palaemonetes have the

opposite condition of these characters.

Our current knowledge of the known freshwater species of the United States and Mexico

demands a reevaluation of each of the above

characters. The degenerated and depigmented

eye of P. antrorum is a character shared with P.

cummingi, which is considered to be in the typical

subgenus Palaemonetes by Chace (1954), Smalley

(1964), and Villalobos and Hobbs (1974). Anophthalmy (reduction of the eyes) is considered to be

regressive and frequently encountered in cavernicoles (Vandel, 1965:20). Anophthalmy in Palaemonetes certainly appears to be a result of selection pressures of the subterranean environment.

It is therefore considered to be a regressive character and in accordance with Mayr (1969:222)

should be given low taxonomic weight. The use

of degenerated eyes therefore no longer appears

tenable as a character separating Alaocaris from

Palaemonetes at the subgeneric level.

The second character is the relative similarity

in size of the first and second pereiopods of P.

antrorum as opposed to the more elongate second

pereiopods of the typical subgenus. It should be

noted that P. holthuisi is similar to P. antrorum

with respect to this character. The solution to this

morphological difference is not readily understood

by simple examination of preserved specimens.

Observations of living specimens of both P. antrorum and P. holthuisi were made in the laboratory

in aquaria containing water and natural substrate

from Ezell's Cave. Both of these species feed in

a manner somewhat dissimilar from observed epigean forms. Instead of remaining somewhat

crouched with the second pair of chela extended

or moving about in search of food as is found in
NUMBER 228 13

the surface species, P. antrorum and P. holthuisi

suspend themselves high above the substrate on

the last three pair of legs and spend long periods

of time picking up particles of bottom debris with

the first two pair of legs and sorting the debris in

an apparent modified form of feeding. The substrate immediately below the mouth region is

thoroughly examined before the animal moves

slowly about to search a new area for potential

food particles.

The similarity in size and shape of the two pair

of pereiopods provides what appears to be a

specialized form of feeding, which is modified for

living in the subterranean environment in which

these species are found. Mayr (1969:223) states:

"Characters associated with shifts in the food

niche are particularly susceptible to a rapid

attainment of conspicuous differences." The similarity of the first two pair of pereiopods of P.

atrorum appears to be a narrow specialization for

feeding in the subterranean habitat, and according to Mayr (1969:223) characters associated with

narrow specializations should be accorded low

taxonomic weight.

The third character is the absence of teeth on

the ventral margin of the rostrum of P. antrorum.

This is a character also shared with P. holthuisi.

The assessment of taxonomic weight to this character is a somewhat difficult task due to the sparsity of subterranean species of Palaemonetes;

however an examination of the subterranean

species found in the subfamily Palaemoninae reveals some rather strong correlations (Table 1).

According to Vandel (1965:422), the condition of

the eye of each of the species may be used as a

relative indicator of the length of time each has

existed in the subterranean environment: "The

very ancient cavernicoles have usually very regressed eyes while the troglophiles, or the recent

cavernicoles possess normal or slightly reduced

eyes." Vandel (1965:422) also quotes Verhoeff

(1930) as stating that "the state of regression of

the eye generally depends on phylogenetic age."

Table 1 is included to illustrate the existence

of a strong correlation between degeneration of

the eye and loss of the lower rostral dentition.

There is a general evolutionary trend among the

subterranean species of the subfamily PalaemonTable 1.—Comparison of eye condition to lower rostral dentition

in species of subterranean Palaemonids

Species

Macrobrachium cavernicola (Kemp)

Macrobrachium villalobosi Hobbs

Creaseria morleyi (Creaser)

Palaemonetes cummingi Chace

Palaemonetes antrorum Benedict

Palaemonetes holthuisi, new species

Troglocubanus gibarensis (Chace)

Troglocubanus calcis (Rathbun)

Troglocubanus eigenmanni (Hay)

Troglocubanus inermis (Chace)

Troglocubanus jamaicensis Holthuis

Troglocubanus perezfarfanteae Villalobos

Eye condition

cornea strongly reduced with pigment and

optic elements

without facets—without trace of pigment

or tinted with diffuse reddish granules in

degenerate corneal area

strongly reduced and possesses no pigment

unpigmcnted, but the small hemispherical

cornea is distinguished from the much

broader stalk

cornea entirely degenerated and shows no

pigment

cornea entirely degenerated

entirely without pigment

cornea strongly degenerated and possesses

no pigment

cornea strongly degenerated and possesses

no pigment

cornea strongly reduced and possesses no

pigment

cornea strongly degenerated and possesses

no pigment

eyes reduced and cornea without pigment

Dentition of ventral

rostral margin

one to three, usually two

one to three, usually two

one to three

three teeth

none

none

none

upper and lower margin

unarmed

none

none

none

upper and lower margin

unarmed
14 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

inae to lose the lower rostral teeth in a fashion

similar to the degeneration of the eye. It appears

reasonable that reduction and eventual loss of the

ventral rostral teeth is a specialization associated

with life in the subterranean environment. The

absence of teeth on the ventral margin of the rostrum of P. antrorum is therefore considered a

regressive character and according to Mayr (1969:

222) should have low taxonomic weight. More

recently Holthuis (1955:43) no longer uses this

character to separate the two subgenera.

The position taken by previous authors of

closely allying the various epigean species to one

another on the basis of the above three characters

now appears weakened. The use of the fourth

character, the movable exopod spines, has been

noted by all authors but it was never really used to

determine a close relationship between any two

species. No one, for example, has proposed a close

relationship between P. antrorum and P. suttkusi

on the basis of similarity in lacking the movable

exopod spine. While the first three of Holthuis'

characters appear to be narrow specializations to

the subterranean environment, it appears impossible at this time to assign any particular functional or adaptive significance to the presence or

absence of the movable exopod spine.

There are authors who might maintain that

this would be the most important taxonomic

character of the four. Sachs (1906:82) states that

"those characters of organisms are shown to be of

the greatest value for classification, which are

entirely or in a very great measure independent

of the functions of the organs." Wilmott (1950)

also places taxonomic importance to those characters of no functional value. Darwin (1859:414)

concludes: "The less any part of the organization

is concerned with special habits, the more important it becomes for classification." Mayr (1969:

221) states: "Of high weight are characters that

are not functionally correlated. .. ." It appears that

of the four characters used by Holthuis to separate

the subgenus Alaocaris from Palaemonetes, only

the use of the movable exopod spine withstands

close scrutinization. In my opinion the presence

or the absence of the movable exopod spine is the

only remaining character upon which the separation of the two subgenera could possibly be maintained. It is therefore pertinent that this character

is extremely variable in the epigean P. texanus

and P. mexicanus. Also some specimens of P. kadiakensis collected from the San Saba River near

Ft. McKavett in Menard County, Texas, as well

as from the South Concho River near Cristoval

in Tom Green County, Texas, have two movable

exopod spines on one side, with the opposite exopod being normal. The frequency of this abnormality occurs in only about ten percent of the

specimens examined.

Three of the nine species of North American

freshwater species of Palaemonetes exhibit different degrees of variation with respect to diversity of the movable exopod spine. Because of this

high degree of variability and in accordance with

Mayr (1969:222), this character like the other

three must be accorded low taxonomic weight. I

believe that it is now no longer tenable to maintain the classification of P. antrorum as a separate

and distinct subgenus.

The movable exopod spine, as variable as it

may be, is still an important character in determining species affinities within the freshwater

forms found in the United States and Mexico. An

examination of the geographical distribution of

those species having the movable exopod spine

(P. paludosus, P. kadiakensis, and P. cummingi)

reveals that they are generally found from south

central Texas eastward. Those species lacking the

movable exopod spine (P. antrorum, P. holthuisi,

P. suttkusi, and P. lindsayi) are found from central

Texas southwest into Mexico. Palaemonetes texanus, which exhibits mixed movable exopod spination, is found in central Texas in areas where the

spined and spineless zones overlap. Palaemonetes

mexicanus is found due south of central Texas in

a tributary of the Rio Panuco only about 100

miles from the coast. It should be noted that the

Sierra Madre Oriental is found between the distributional area of P. mexicanus and that of P.

lindsayi, even though both are associated with

the headwaters of the Rio Panuco.

To account for the current distributions of the

spined and spineless conditions, one must consider the possible origins of the freshwater Palaemonetes of the United States and Mexico and

their past distributional routes. As suggested

earlier, it is unlikely that individual species or

species groups have arisen polyphyletically from

marine and brackish water stocks. This contention

is supported by the similarities of antennal scales
NUMBER 228 15

of the Form I larvae of the freshwater species in

contrast to the brackish water species. This

strongly indicates that the freshwater species of

the United States and Mexico represent a monophyletic group.

The somewhat odd distribution of the presence

of movable exopod spines in the eastern species

and the absence of movable exopod spines in the

southwestern species, with the intermediate or

mixed condition found in between, could represent any of a number of past occurrences. Possible

solutions to the explanation of this distribution

pattern include clinal variation, separate dispersal

pathways, hybridization following temporary isolation, isolated founder populations, and possible

isolation due to advancement of epeiric seas of

the central United States such as those proposed

by Schuchert (1935, pi. 4-10) from the Lower

Cretaceous through the Eocene.

Based on the condition of the movable exopod

spine of the uropod and geographical distributions, the nine currently known North American

species may be separated into three groupings of

relatively close affinities. The first of these includes P. kadiakensis, P. paludosus, and P. cummingi. The second includes P. antrorum, P. holthuisi, P. suttkusi, and P. lindsayi. The third

group contains P. texanus and P. mexicanus.

Salinity Tolerance and Related Dispersal Abilities

of Palaemonetes kadiakensis Rathbun

INTRODUCTION.—In any zoogeographical analysis

the salient question that must be answered is:

Can the animal disperse across apparent barriers?

In the case of many freshwater species this resolves

into whether or not barriers of marine water can

be crossed. Since some species of Palaemonetes are

estuarine, it would appear at first glance that

Palaemonetes cross full marine barriers with ease.

This hypothesis is challenged by the demonstration that North American and European freshwater Palaemonetes appear to constitute a monophyletic taxon.

The present study is designed to ascertain the

possible long distance dispersal capabilities of

Palaemonetes kadiakensis Rathbun. This species

along with its close relative P. paludosus (Gibbes)

is found widespread in freshwater systems of the

southeastern United States. Few of these systems

are directly connected; instead most of them flow

into the Gulf of Mexico or Atlantic Ocean. The

maintenance of genetic continuity among what

appear to be separated populations is apparently

being maintained as shown by definite conspecificity of the populations. Ross (1974:240) maintains that coastal flooding is one means of dispersal by which many freshwater organisms become

geographically widespread. Tiwari (1955b: 236)

proposed dispersal through a marine agency to

account for the widespread distribution of the

genus Palaemon; he also notes salinity reduction

of littoral areas due to coastal flooding during

the rainy season. The tolerance to salinity of the

freshwater shrimp P. kadiakensis is necessary to

understand the role played by brackish waters

connecting these freshwater systems as possible

geographical barriers or, conversely, as dispersal

pathways.

Nagabhushanam (1961) tested the lower salinity

tolerance of the brackish water species P. vulgaris.

Salinity effects on larval forms have been studied

for the marine species P. varians (Le Roux, 1970)

and P. vulgaris (Sandifer, 1973). Parry (1957:

421) noted the physiological separation in osmoregulatory ability between P. varians (marine)

and P. antennarius (freshwater). Maguire (1961)

studied the effects of increased salinity on the

freshwater species P. kadiakensis and P. paludosus.

Although he did not report survival rates at normal marine salinities over prolonged time periods,

his results led him to state: "Neither species

should theoretically have much difficulty in adjusting to higher salt concentrations." Dobkin and

Manning (1964) compared the abilities of freshwater P. paludosus and estuarine P. intermedius

to osmoregulate at varying salinities and concluded

that P. paludosus was unable to regulate at salinities above 20 ppt. With this conflict in the literature it becomes necessary to establish the salinity

tolerance of a freshwater Palaemonetes before

attempting to generalize about zoogeography.

PROCEDURE.—The salinities in each of seven onegallon aquaria were adjusted by mixing Instant

Ocean and distilled water to give a salinity series

of 5 ppt, 10 ppt, 15 ppt, 20 ppt, 25 ppt, 30 ppt,

and 36 ppt. The salinities were analyzed and adjusted through the use of an optical refractometer

(American Optical Company No. 474). Approxi-
16

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

mately 100 specimens of P. kadiakensis were obtained from the drainage area associated with

the Little Brazos River. These specimens were

held in the laboratory overnight in a two-gallon

aquarium.

Acclimation was begun 12 hours prior to the

start of the test. The first test group of 10 specimens was taken from the stock tank and placed in

the 5-ppt tank for a two-hour time period. At the

end of two hours, the first group of 10 specimens

was removed and placed in the 10-ppt tank. A

second group of 10 specimens was then selected

and placed in the 5-ppt tank. This process was

repeated until each of the seven tanks contained

a group of 10 test animals, each of which had

been progressively acclimated at all lower salinities

for two hours, prior to the arrival of the animals

in the final test tank. Observations were made at

three-hour intervals for the first 24 hours, and at

each succeeding 12-hour time period thereafter;

the number of surviving individuals was recorded.

RESULTS AND DISCUSSION.—The results of this

study are expressed in Figure 5. One notes that

the salinity ranges may be divided into three general groupings. The first of these is one of zero

mortality and includes salinities of 5 ppt, 10 ppt,

and 15 ppt. The second is that of limited mortality

and includes salinities of 20 ppt and 25 ppt. The

third group is that of high or total mortality and

includes 30 ppt and 36 ppt. Test animals were

observed to feed normally in salinities of 15 ppt

and below; limited feeding took place in salinities

of 20 and 25 ppt, while no specimens were observed to feed in salinities of 30 and 36 ppt. Prior

to expiration, specimens were usually observed to

become opague white in color. This was accompanied by decreased activity and was interpreted

as a visible indicator of stress endured by the

organism. Serventy (1938:55) noted the same

color change in Palaemonetes australis Dakin

under similar laboratory experimentation. To

mimic a return to freshwater, several salinitystressed specimens were removed from the test

and placed directly into control tanks with no

deacclimation; most recovered from the osmotic

shock within a two- to four-hour time span.

0-15ppt, control

0 i'2 TA 36 7i 66^0 /5 84 98 168 120 132 144 156 168

Time in hours

FIGURE 5.—Survival rate of Palaemonetes kadiakensis Rathbun at varying salinities.
NUMBER 228 17

Larval stages of P. kadiakensis are far less tolerant of higher salinities than adults. Similar testing as described above was run on newly hatched

zoea. Immobility of zoeas occurred in 15 ppt and

above during acclimation. At the end of 12 hours

100 percent mortality had occurred in all salinities

of 15 ppt and above. No mortality occurred at 5

and 10 ppt.

While the results of this test indicate a high

vagility of P. kadiakensis with respect to limited

movements along the coastline, the results appear

to have broader and more far-reaching implications with respect to dispersal abilities of freshwater Palaemonetes on a worldwide scale. While

it would seem that P. kadiakensis could under

favorable circumstances disperse over relatively

short distances, it would appear that large

stretches of open ocean would certainly represent

a geographical barrier. Upon examination of the

world distribution of freshwater species of Palaemonetes (Figure 6), one notes that species such

as P. australis Dakin in southwestern Australia,

P. sinensis (Sollaud) in China, P. antennarius

(H. Milne Edwards) in Italy, P. ivonicus Holthuis

in South America, and P. kadiakensis in North

America present an unusual disjunct distribution.

This becomes a compelling zoogeographical problem if these species follow the trend shown by P.

kadiakensis (in this study) and P. paludosus (Dobkin and Manning, 1964) of an inability to osmoregulate in salinities of the 25 to 36 ppt range.

The solution to this problem may involve widespread polyphyly, limited polyphyly, or monophyly among the world's freshwater species of

Palaemonetes. As indicated earlier in the discussion of relationships of marine and freshwater

species, P. antennarius of Italy as well as P.

sinensis of China are similar in several respects

to the North American species, while P. argentinus of South America is similar to the marine

species. It appears from this limited information

that the freshwater species of Palaemonetes of the

world arose by monophyly or limited polyphyly.

Several disjunct species such as those in China

FIGURE 6.—World distribution of freshwater Palaemonetes: 1, Palaemonetes paludosus (Gibbes);

2, P. kadiakensis Rathbun; 3, P. cummingi Chace; 4, P. antrorum Benedict; 5, P. holthuisi n. sp.;

6, P. texanus n. sp.; 7, P. suttkusi Smalley; 8, P. lindsayi Villalobos & Hobbs; 9, P. mexicanus

n. sp.; 10, P. carteri Gordon; 11, P. ivonicus Holthuis; 12, P. argentinus Nobili; 13, P. mesogenitor

Sollaud; 14, P. antennarius (H. Milne Edwards); 15, P. mesopotamicus Pesta; 16, P. sinensis

(Sollaud); 17, P. tonkinensis (Sollaud); 18, P. australis Dakin; 19, P. turcorum Holthuis.
18

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

and Australia still pose a zoogeographical dispersal

problem. This subject will be further discussed in

the section on zoogeographical considerations.

Zoogeographical and Geological Considerations

There is a general consensus in the literature

that members of the family Palaemonidae are of

very recent origin, disperse readily, and as a result

have little or no value in the investigation of

past geographical history (Ortmann, 1891:744,

1902:274; Hart, 1961:79). Riek (1959:254) considers the Palaemonids to be very recent arrivals

to Australia. Tiwari (1955a: 189, 1955b:238)

places the origin of the genus Palaemon during

the Pleistocene or post-Miocene, while Sollaud

(1923b:4) maintains that the immigration of

Palaemonids into freshwater is of recent occurrence. Coutiere (1900) speculated that the genus

Macrobrachium (as Palaemon) might date from

as early as Miocene times. Chace and Hobbs

(1969:22) state that members of the genus Troglocubanus "should perhaps be looked upon as

relicts of a stock of the family that reached the

Antilles comparatively early, perhaps as early as

the Miocene."

Glaessner (1969:R452-54) places the genus

Propalaemon in the lower Oligocene of Europe

and Palaemon, which is closely related to Palaemonetes, as questionably present in the Oligocene

of Europe. Harland (1967:556) and Glaessner

(1969:R452) date the section Caridea from the

Jurassic to Recent. Thompson (1965:321) gives

evidence to support an ancient origin of the superfamily Palaemonoidea and states that it "has

been found in large numbers in Upper Jurassic

formations" but fails to mention any substantiating geological reference. Villalobos (1971:1) tentatively places the origin of the genus Troglocubanus

as somewhere between the Upper Cretaceous and

Eocene.

The current distributional patterns of species

of Palaemonetes and Macrobrachium pose some

patterns that may indicate an evolutionary succession in these members of the family Palaemonidae

which has taken place in North America, and the

evidence demands consideration of a much earlier

time as the probable entry date of the ancestral

Palaemonetes and Macrobrachium stocks into the

Gulf of Mexico-Caribbean area. The various species of Macrobrachium found along the Atlantic

and Pacific sides of Mexico and Central America

pose a unique distributional pattern among the

American freshwater Palaemonids. They provide

interesting subject matter for both taxonomists as

well as zoogeographers. Early workers such as

Ortmann (1891, 1902) and Coutiere (1900) believed that identical species were found in both

eastern and western watersheds. Ortmann (1891:

747, 1902:274) maintained that the highlands

between the watersheds pose no barrier to the

distribution of certain species of Macrobrachium

(as Palaemon). Villalobos (1967:1060) believes

this to hold true for M. olfersi. Coutiere (1900:

1268) in an effort to explain the distribution of

identical species on both Atlantic and Pacific

slopes proposed the possibility that dispersal to

the west occurred during the Miocene when the

straits of Panama were open.

More recently taxonomists have chosen to distinguish the eastern and western forms as distinct

and separate species. Holthuis (1952:11) lists

seven eastern species along with their seven

closely related western species. No single species

was listed as occurring on both sides of Central

America. Villalobos (1967) later states that Macrobrachium olfersi is found in both the watersheds

of the Atlantic and Pacific and further (p. 1060)

maintains that the peculiar topographical and

hydrological conditions of the Isthmus of Tehuantepec have permitted M. olfersi to extend its

distribution very close to the Pacific coast and

thinks this could have resulted in population of

the rivers of the Pacific watershed. He does not

specify as to what these peculiar topographical

and hydrological conditions are but states (p.

1058) that specimens have been encountered at

very high altitudes near the headwaters of various

river systems. Holthuis (1952:10, 100) notes the

relative ease with which some species of Macrobrachium could attain wide distribution but also

(pp. 93, 94, 102) points out the controversy involved in identifying specimens of M. olfersi, M.

faustinum, M. hancocki, and M. digueti.

In any case there are six to seven species pairs

of Macrobrachium separated by the dry land barrier of the central highland ridge of Central

America. These species pairs certainly appear in

all respects to conform to the definition of "gemi-
NUMBER 228 19

nate species" as coined by Jordan (1908) and later

used by Eckman (1953:30), Hallam (1973:97),

and Barnwell (1973:1332). Jordan (1908:75) defines geminate species as "twin species—each one

representing the other on opposite sides of some

form of barrier." The barrier in this study as well

as those mentioned above is the Panama Isthmus.

That these species of Macrobrachium are indeed

geminate forms appears an inevitable conclusion.

The alternate to this would be that each of the

seven eastern species had independently crossed

the Central American and Mexican highlands,

established itself in the western watersheds, severed

gene flow with the eastern forms, and diverged to

the point that each is now taxonomically distinguishable from the original eastern parental stock.

It is much more probable that seven original

species were continuous from east to west when

parts of Central America were submerged and

the waters of the Pacific and Gulf of MexicoCaribbean were contiguous. The development of

a continuous land bridge between North and

South America isolated these populations, which,

during the course of independent evolution and

speciation, has given rise to the species pairs or

geminate forms that are observed today.

If we accept the above hypothesis as reasonable,

then it becomes possible to establish a geological

time period when the genus Macrobrachium was

present in the Central American region. Jordan

(1908:76) originally dated the final closing of the

Pacific-Atlantic water connection as the end of

the Miocene. More recent authors such as Schuchert (1935:596), Mayr (1946:9, 1964:281), Elton

(1958:39), Lloyd (1963:%), Whitmore and Stewart (1965:185), Savage (1966:719, 1973:428),

Howell (1969:323), and Hallam (1973:93) largely

agree that this event took place during the

Pliocene. On the basis of current geographical

distributions and past geological history, it then

appears justifiable to place the genus Macrobrachium in the Gulf of Mexico-Caribbean area at

least as early as the Pliocene. This hypothesis is

further substantiated by the fossil record of Macrobrachium by Rathbun (1918:131) in the Oligocene of the Canal Zone, Panama. Woodring

(1973:457) later dates the locality of this record

(Emperador limestone) as early Miocene.

Excluding P. kadiakensis in Nuevo Leon, state

of northern Mexico (Creaser, 1932:333), because

of its usually wide distribution, there are currently

only three known species of freshwater Palaemonetes uniquely native to Mexico. The first of

these is P. suttkusi, which is found from an elevation of 498 meters (1635 ft) to about 740 meters

(2428 ft). Palaemonetes lindsayi is known only

from the La Media Luna area southwest of Rfoverde in the state of San Luis Potosf, Mexico, at an

elevation of 1021 meters (3350 ft). Palaemonetes

mexicanus is known only from a small spring-fed

river in the mountains of the Sierra Madre

Oriental just west of Valles in the state of San

Luis Potosi, at an elevation of approximately 152

meters (500 ft).

All of the known species found in Mexico are

associated with the headwaters of the Rio Grande

(Rio Bravo) and Rio Panuco. None have been

reported to date from southeastern Mexico or the

Pacific watershed of Mexico or Central America.

The field work conducted during this study reveals that the lowland rivers of eastern Mexico,

unlike those of the southeastern United States,

apparently lack freshwater Palaemonetes. The

three currently known species were found at elevations generally above 152 meters (500 ft) and

in clear, fast flowing, small streams which are fed

by large springs. In collecting specimens of each

of these three species, it was noted that they are

often found in rooted vegetation or in fibrous

roots of terrestrial plants which extend into the

water below the surface. This type of habitat is

generally restricted to the upper two to three feet

of the stream bank.

The larger slow-flowing rivers of eastern

Mexico, such as the Soto la Marina, Panuco, Moctezuma, Tamuin, Cotaxtla, Papaloapan, Rfo

Jaltepec, Rfo Tonala, Carrizal, Rio Tulija, and

Usumacinta, appear to contain habitats that would

sustain large populations of Palaemonetes. Rivers

of similar size and appearance in the United States,

such as the Mississippi, Sabine, Brazos, and Colorado, contain large populations. This, however, is

not the case for the Mexican rivers. I collected in

all of the above-mentioned Mexican rivers and no

freshwater Palaemonetes were found. Instead, all

of them had large populations of various species of

juvenile Macrobrachium: M. acanthurus (Wiegmann), M. olfersi (Wiegmann), and M. carcinus

(L.) were the most common. Holthuis (1952) and

Rodriguez de la Cruz (1965) list additional species
20 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

of Macrobrachium common to eastern Mexico. In

the field these juveniles can easily be mistaken for

Palaemonetes, since they are the same size and are

similarly found in shallow vegetated areas along

the water's edge.

It would then appear that juvenile Macrobrachium competitively exclude Palaemonetes from

the river systems of the eastern Mexican lowlands.

Johnson (1965:430) notes the importance of competition between species in determination of distributions of Malayan freshwater prawns but also

points out the difficulty of proving exclusion by

competition.

Laboratory studies were carried out to examine

any interactions between Macrobrachium and

Palaemonetes which might cast light on this apparent competitive exclusion. Several juvenile M.

olfersi were collected from the Rio Moctezuma

near Tamazunchale, San Luis Potosi and returned

to the laboratory. They were easily maintained

and readily fed on commercial tropical fish food.

The specimens were in the size range common to

Palaemonetes and could easily be mistaken for

them. These specimens readily fed on immature

larvae of P. kadiakensis. Sixty newly hatched zoea

were placed in the aquarium containing eight

immature M. olfersi; within 24 hours all the larvae

had been consumed. During daylight hours the

Macrobrachium actively pursued and fed on the

free-swimming zoeas.

Mortality of the zoea appeared highest during

the night. The zoeas came to rest on or very near

the substrate during dark periods; apparently

Macrobrachium had little difficulty in locating the

larvae by tactile or taste receptors and feeding

upon them. The Macrobrachium were also observed to exhibit strong territorial activities, exhibiting threat displays toward adult specimens

of the nonterritorial P. kadiakensis to the point

that the latter gave up their normal bottom

existence and were found clinging to the sides of

the aquarium or in floating vegetation. In addition, all gravid female P. kadiakensis which were

placed with the Macrobrachium for any length

of time were killed and their eggs eaten. All of

these observations were conducted in the presence

of an excess of the commerical fish food normally

used to feed the Macrobrachium, so it is not

interpreted as a result of some abnormal hunger

stress. The larval Macrobrachium are able, to a

large extent, to escape predation by their own

species since their larval development occurs in

or near estuarine waters along the coast. Hughes

and Richard (1973) report salinity-directed movements of larvae of M. acanthurus.

Many if not most species of Macrobrachium,

unlike the freshwater Palaemonetes, are tied to

the saline waters of the marine coastline for successful larval development and growth. This has

been demonstrated to hold true for M. acanthurus

(Choudhury, 1970:114, 1971b: 124) and M. carcinus (Lewis and Ward, 1965:138, Choudhury,

197la:52), which were found to be quite numerous in the eastern Mexican rivers. As the young

larvae begin to grow, they begin rheotaxic movements into the freshwater rivers. They are able to

move upstream only so far before attaining maturity. The adult stage of Macrobrachium does

not compete as strongly with adult Palaemonetes

since they are morphologically different and benthic in their habits. This has left the headwaters

of the larger Mexican rivers essentially free from

competition by Macrobrachium; therefore the isolated distributions of P. suttkusi, P. lindsayi, and

P. mexicanus appear to represent relict populations

surviving in the headwaters.

Possible physical factors which could be involved in restricting the further spread of Macrobrachium include waterflow, altitude, and temperature. Johnson (1965) discussed these, plus several

chemical factors, with respect to distributions of

freshwater prawns in Malaya. He (p. 420) notes

the apparent restriction of still-water species (reference primarily to Macrobrachium) to low levels:

no records above 500 ft. This is a striking similarity

to the three known endemic Mexican species of

freshwater Palaemonetes that are found at elevations of 500 ft or above.

Another physical factor which could possibly be

involved is temperature. Ortmann (1891) noted

the effect of temperature as a barrier to dispersal

of species of Macrobrachium (as Palaemon) as

well as limiting northern and southern distributions. Hedgpeth (1949:36) relates the possibility

of temperature restriction of Macrobrachium

ohione (Smith) in reference to Hutchins' (1947)

paper on temperature zonation and geographical

distribution. A striking similarity is seen in Australia in the apparent zonation of Macrobrachium

and the freshwater P. australis as shown by Bishop
NUMBER 228 21

(1967:116, fig. 4.3) to that which occurs in North

America. It is interesting to note that the distributional boundary between the two genera occurs

roughly along the 30 degree south latitude. The

30 degree north latitude also serves as a rough

line of division between the two genera in North

America as well as in the Mediterranean and southeast Asian areas. It appears evident that the distribution of freshwater Palaemonetes has been directly

influenced by the distribution of Macrobrachium.

If the above relationship is correct, the freshwater members of the genus Palaemonetes were

once afforded a much wider geographical range

than presently observed. Furthermore the freshwater Palaemonetes geologically preceded the

genus Macrobrachium in the Gulf of Mexico area.

The most striking difference noted between the

distributional patterns of the two genera in North

America is the apparent absence of geminate species in the freshwater Palaemonetes. Also the

freshwater Palaemonetes exhibit a basically temperate distribution while the genus Macrobrachium is predominately tropical. No freshwater

species of Palaemonetes are currently known to

occur naturally in the western watersheds of the

United States, Mexico, or Central America. Palaemonetes paludosus, unfortunately, has been intentionally introduced into California (Hayden and

Ringo, 1963; St. Amant, Hulquist, and Day, 1972)

and Mexico (St. Amant and Hulquist, 1969) by the

California Department of Fish and Game.

This absence of freshwater Palaemonetes from

the western watersheds may be variously interpreted: (1) the species may very well be present,

but unknown due to the lack of adequate collecting (the three known species of freshwater Palaemonetes in Mexico have been described only since

1964); (2) species may have been present at one

time in the western watersheds but are now

extinct; (3) species of freshwater Palaemonetes

are not now present and have never in the past

inhabited the western watersheds. If the latter is

the case then one is pressed to account for this

distribution. The only probable barrier which

could have prevented invasion of the west would

have been the presence of a contiguous land connection between North and South America such

as that which exists today. This would at first

appear to argue for a Pliocene or later invasion

from the east; however, arguments presented

earlier in this section indicate that the freshwater

Palaemonetes were probably present prior to the

invasion by Macrobrachium. Moreover, the genus

Macrobrachium was very probably well established

prior to the emergence of the current land connection, which came into being sometime during

the Pliocene. If a land bridge had existed during

some geological time period earlier than the Pliocene, then it might have restricted the freshwater

Palaemonetes to the Gulf of Mexico area and

prevented their invasion into the areas of the

now western watersheds.

A review of the literature quickly reveals that

there is general disagreement as to the existence

of an earlier than Pliocene land bridge. Lloyd

(1963:99) discredited the possible existence of an

Upper Cretaceous land bridge. Simpson (1965:

213) maintains the existence of either a land

bridge or fairly traversable stepping stones between North and South America sometime in the

late Mesozoic or early Cenozoic. Dickerson

(1917:215) states that the Panama Portal was

closed from Cretaceous to Eocene, and Vaughan

(1919:611-12) notes the absence of interoceanic

connections from the Cretaceous to the lower

Eocene. Berry (1918:634) states that land connection existed from Upper Cretaceous into early

Cenozoic time. Schuchert (1935, plates 7, 8, and

9) indicated a land connection from Upper Cretaceous to Middle Eocene, while Savage (1966:719,

1973:414) supports the existence of an intercontinental land bridge during the Palaeocene.

The existence of a late Mesozoic to early Cenozoic land connection between North and South

America could possibly account for the apparent

absence of freshwater species of Palaemonetes

from the rivers of the west coast if the genus is

that old. If these forms originated in the Gulf area

or moved in from some other region and were

established during the period of a continental

land connection between North and South America, then they would have been denied western

access until the submergence of the land barrier

in the Paleocene or Eocene. If the freshwater

Palaemonetes had already become extinct in the

areas of the Isthmus of Tehuantepec and southward due to competition by the more recent arriving Macrobrachium, they still would not have

been able to move westward during the period of

interoceanic connection which existed during
22

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

much of the middle Cenozoic. Macrobrachium

was, however, easily able to disperse westward

through the Central American waterways and gave

rise to widespread species populations which were

eventually isolated by the reemergence of the

land connection again in the Pliocene. These

divided populations eventually gave rise to the

geminate species observed today.

It appears justifiable then, in the light of the

previous discussions, to speculate about the series

of events which may have taken place in the Gulf

of Mexico with respect to the distributions of

Macrobrachium and Palaemonetes. While much

of the reconstruction is based on facts, there is a

great deal of conjecture which only future fossil

finds and collecting can support or refute.

1. Intercontinental land connections were present between North and South America during late

Mesozoic and/or early Cenozoic. Freshwater Palaemonetes originated in the Gulf-Caribbean area or

migrated into the area from some other eastern

origin. Dispersal and establishment of these forms

took place in the rivers associated with the Gulf

of Mexico.

2. The genus Macrobrachium entered the GulfCaribbean area. Immature forms provided strong

competition with already established forms of

freshwater Palaemonetes.

3. The freshwater Palaemonetes were totally or

virtually totally eliminated in the lowland river

systems in the area of Eastern Mexico and Central

America, leaving only relict populations inland

at upper elevations of the headwaters of the larger

and more permanent river systems.

4. Submergence of the continental land bridge

took place resulting in the connection of the

waters of the Gulf-Caribbean and Pacific, which

allowed dispersal of established species of Macrobrachium into the areas of the Pacific coastline.

The exact timing of this event is only of relative

importance. It possibly took place in the late

Paleocene or Eocene.

5. Reemergence of intercontinental land bridge

occurred during the Pliocene. This served to

split the widespread populations of at least six to

seven species of Macrobrachium, which have subsequently evolved into the geminate forms observed today.

While possible future discoveries of both living

and fossil material may tend to support or refute

the above reconstruction, it should be noted that

the uncovering of single exceptions to the above

sequence does not necessarily refute the entire

postulation. As an example: the finding of a

freshwater species of Palaemonetes in the Pacific

watershed would not invalidate the entire hypothesis. It is not mandatory that all freshwater

Palaemonetes south of the Isthmus of Tehuantepec

had to have been eliminated prior to the submergence of Central America in the early Cenozoic.

It just appears to have been this way in consideration of our current knowledge of the group.

The above reconstruction has been proposed

for a variety of different reasons. First, in consideration of the data available, I feel that the above

reconstruction quite likely reflects events which

did indeed take place. Second, a close scrutiny

of the proposal by other workers in the field

will surely follow. This could serve either to

strengthen it, point out weaknesses, or completely

discredit it. Thirdly, the postulation may serve

to stimulate work in invertebrate paleontology

and zoogeography, as well as to initiate an increase in field work oriented toward a more

thorough understanding of the present distribution of species of freshwater Palaemonetes. Lastly,

the postulation appears to have a direct bearing

on the worldwide distribution of freshwater species of Palaemonetes due to its support of a more

ancient origin of the genus.

It was earlier noted that P. antennarius of the

freshwaters of Italy is more similar to the North

American freshwater species than the more closely

geographically situated P. varians of European

brackish waters. This was based on larval antennal

scale morphology, type of eggs produced, and

physiological differences involved in osmoregulation. The possibility that P. antennarius and possibly other far-removed species of freshwater

Palaemonetes are of the same monophyletic origin as the freshwater species of the United States

and Mexico must be taken under consideration.

Johnson (1958:172) notes the need to consider

the degree of salt tolerance in any study of the

distribution of freshwater organisms. It has already been demonstrated that P. kadiakensis and

P. paludosus are incapable of crossing long

stretches of open ocean due to their inability to

osmoregulate at high salinities. Palaemonetes

antennarius exhibits some similarities to P. palu-
NUMBER 228 23

dosus (Dobkin and Manning, 1964:154) and also

may well be incapable of surviving at high salinities. This eliminates possible dispersal between

the larger land masses as they are currently

situated.

Alternate possibilities could include dispersal

over former land bridges such as those proposed

by Ortmann (1902) and others or may be the

result of the breakup and separation of a former

single land mass as proposed by current proponents of Wegener's (1915, 1966) hypothesis of

continental drift. Massive land bridges are currently discredited in general and not considered

as a major possibility. The possibility of a former

single land mass might provide the solution even

though the land masses need not have been in

direct contact with one another; only close juxtaposition would have been required to facilitate

dispersal. Due to the sparsity of available information on most of the world's species of freshwater Palaemonetes, it is not deemed wise at this

time to propose that the geographical distribution

of freshwater Palaemonetes is a result of continental drift.

The following, however, are proposed:

1. The freshwater Palaemonetes are much older

than previously thought, possibly dating from the

late Mesozoic or early Cenozoic.

2. The possibility exists that the widely disjunct species of freshwater Palaemonetes are

largely of monophyletic origin, even though exceptions such as P. argentinus do exist.

3. Dispersal of these disjunct species does not

appear likely to have occurred between the larger

land masses as they are currently situated because

of salinity and temperature barriers.

4. Dispersal does appear likely if land masses

were closer or connected by land bridges.

5. Widespread competitive exclusion of freshwater Palaemonetes from the middle latitudes has

occurred due to more recent and successful genera,

most notably that of Macrobrachium.

Johnson (1958:177) notes the lack of Palaemonetes in the region between northeast IndoChina and southwest Australia and attributes this

to an ecological limitation. Though there are exceptions such as P. carteri, most of the freshwater

species are found generally above or below the

30 degree lines of latitude, or if found between

these latitudes they are generally at higher elevations or long distances from the coast. Temperature, current flow, and salinity requirements of

larvae appear to be factors limiting the further

spread of Macrobrachium.
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