Placobdella picta

source: researchgate

Abstract. Our study investigates the amphibian infestation rates of the parasitic leech Placobdella picta in a number of ponds

from Arkansas. We also note differences in the sucker attachment on the host from what was previously published. The infestation

rates reflected large populations of P. picta at some ponds; whereas, we could find no evidence of it in other locations. Diagnosis

of picking injuries was easy and occurred on as many as 50% of some species. It also appears that this leech demonstrates host

and size preference in its tadpole prey. This study adds interesting details to the paucid database of life history information on P.

picta.

Introduction

Understanding the details of an organism’s life history

is important for understanding its role in the ecosystem

(Bury, 2006; McCallum and McCallum, 2006). Despite

their pivotal role in ecosystem processes, we know

little about the life histories of most leeches, including

Placobdella picta (Verrill, 1872). This aquatic species

specializes in parasitizing amphibians (Barta and

Sawyer, 1990), which as a group are on a global

extinction trend (McCallum, 2007) and susceptible

to climate change (Johnson and McKenzie, 2009;

McCallum, 2010). Placobdella picta attacks at least

12 host species including the Wood Frog (Lithobates

sylvaticus, McAllister et al., 1995), Bullfrog (Lithobates

catesbeianus; Tse, Barta and Desser, 1986; Barta and

Sawyer, 1990), Spotted Salamander (Ambystoma

maculatum), Blue-spotted Salamander (Ambystoma

laterale), Jefferson Salamander (Ambystoma

jeffersonianum), Triploid Hybrids (A. laterale x A.

jeffersonianum; Tse, Barta and Desser, 1986), American

Toad (Anaxyrus americanus; Martin and Desser, 1991),

Red-spotted Newt (Notophthalmus viridescens; Mock

and Gill, 1984; Brigler, Lohraff and Adams, 2001;

McAllister, Jamieson and Sidall, 2008), and the Green

Frog (Lithobates clamitans; Barta and Desser, 1984),

and other species (Barta and Desser, 1989; Watermolen,

1996; Bolek and Janovy, 2005). Placobdella picta is a

vector of several amphibian blood-parasites (Woo and

Bogart, 1986; Martin and Desser, 1991) and can affect

tadpole fitness traits (Bervin and Boltz, 2001). Their role

as amphibian specialists combined with their service as

a vector of amphibian pathogens, and the tendency for

many amphibians to return to their oviposition sites

(Whitford, 1966; Trauth, McCallum and Jordan, 2006)

stokes speculation that this species is an important

regulator of tadpole populations (Berven and Boltz,

2001). As a vector, P. picta could play an important role

in amphibian declines.

Descriptions exist of the blood-feeding behaviour and

resulting feeding wounds of P. picta, but those authors

were uncertain if their observations were isolated, host-

specific, or locality specific behaviours (Barta and

Sawyer, 1990). Below is an abbreviated description

of their (Barta and Sawyer, 1990) observations.

Placobdella picta finds its host by detecting vibrations

from swimming amphibians. In response, P. picta

reaches its posterior sucker from the substrate, ready to

attach to its host upon contact. As the leech anchors its

posterior sucker on the host tadpoles begin to thrash.

Next it uses the anterior sucker to tear a portion of skin

from the host (termed picking). Then the leech uses its

proboscis to probe the newly formed wound for blood.

Once feeding starts, it releases the posterior sucker

from the host and the leech dangles free by its attached

anterior sucker.

Northeast Arkansas is at the intersection of the Ozark

Plateau, Crowley’s Ridge, and the Mississippi Delta.

The Ozark habitats comprise upland hardwood forests

interspersed with cattle grazing, cold-water streams,

and upland ponds. Crowley’s Ridge has similar habitats,

although urbanization of this geographic province is

rapid. The Mississippi Delta is lowland floodplain

habitat comprised of lowland hardwood forests and

is dominated by agronomic pursuits including rice

and cotton farming. Further, this region of Arkansas

is known for instances of amphibian die-offs (Trauth,

McCallum, Cartwright, 2000) and other population

problems (Wheeler et al., 2002; McCallum and Trauth,

2003, McCallum and Trauth, 2002).

Our study refines previous descriptions of P. picta

feeding behaviour, and documents an apparent prey

size preference by this parasite. Additionally, we

provide important information on the affliction rates

of amphibians in selected ponds from the Ozarks

(Randolph, Sharp and Stone Counties) and Crowleys

Ridge (Craighead and Greene Counties) in Arkansas.

Materials and Methods

We collected amphibians by hand and dip-net from fishless

ponds in the Ozark National Forest (n = 5; Stone County, Ar-

kansas), Crowley’s Ridge (n = 4; Greene and Craighead Coun-

ty, Arkansas) and on private lands in the eastern Ozarks (n = 3;

Randolph and Sharp Counties, Arkansas). After collection, we

examined amphibians for leeches and signs of leech attack. We

relaxed leeches in a weak 1% ethanol solution, fixed them in 10%

buffered formalin, and then deposited them in the Invertebrate

Zoology collections of the National Museum of Natural History,

Smithsonian Institution.

We identified picking marks (signs of leech attack), tabulated

and compared incidences among ponds to determine the proporti-

on of amphibians attacked by leaches in each setting. Amphibians

having fresh picking marks were housed in filtered aquaria to de-

termine the time required for picking marks to heal and to infer

how long these injuries remained evident.

Results

Three ponds harboured amphibian populations in-

fested by leeches, whereas the other ponds showed no

evidence of parasitism. Ponds with leeches were located

near the Randolph Co. – Sharp Co. line in Arkansas.

Although no attempt was made to directly estimate the

population levels of P. picta, this leech appeared to be

very common in the ponds and we recovered it by dip-

netting nearly as frequently as amphibians.

All P. picta held on to their amphibian hosts with both

suckers (Fig. 1) before and during blood-feeding. He-

matoma (Fig. 2) resulted from repeated attachment and

release of the anterior sucker. Leech blood-feeding or

“picking” injuries faded in a stepwise pattern and disap-

peared after 2-3 days at 22°C. Fresh marks appeared as

bright-red hematoma. After one day these began to fade

to brown and then gradually transformed into a white

mark slightly smaller than the original injury (Fig. 2).

We determined the prevalence of P. picta infestation

and frequency of leech “picking” injuries throughout the

study (Table 1). Both anuran larvae (60/340, 17.65%)

and adult caudates (7/63, 11.11%) showed signs of lee-

ch attacks. Picking marks occurred on tadpoles of L.

catesbeianus (18.3%, 62/339) and L. clamitans (54.5%,

5/10). Adult A. maculatum (1/2, 50%) and N. viride-

scens (6/61, 9.8%) had signs of leech-picking. Picking

injuries occurred in 100% (5/5) of 2yr L. clamitans tad-

poles; whereas, none (0/5) of 1yr L. clamitans had these

signs and no L. clamitans had leeches attached. Only

0.5% (2/340) of L. catesbeianus tadpoles had leeches

attached (these leeches released before they could be

photographed). Still, 18.2% (62/340) had obvious signs

of “picking injuries”. Frogs had higher infestation pre-

valence than tadpoles, and hosts had a higher number

of leech feeding injuries than would be indicated by the

number of leeches we found attached to them. Although

we collected several hundred Acris crepitans, Hyla ci-

nerea, and L. catesbeianus tadpoles at the Mammoth

Spring National Fish Hatchery (Randolph Co., Arkan-

sas), no leeches were associated with specimens from

that location.

Discussion

Although reports exist for P. picta attacks on N. v.

louisianensis (Briggler, Lohraff and Adams, 2001;

McAllister, Jamieson and Sidall, 2008), this study

represents the first known account of widespread P. picta

blood-feeding across an amphibian community. Except

for observations of Placobdella cryptobranchii on the

Ozark Hellbender (Cryptobranchus alleganiensis;

Benjamin Wheeler, pers. comm.; Moser et al. 2008),

this is the largest known frequency of leech infestation

in Ozark amphibian populations.

Barta and Sawyer (1990) suggested that P. picta

released its posterior sucker during feeding. We did not

observe this. All leeches used both suckers both prior

to and during blood feeding. This minor difference in

observed behaviour may reflect the prey involved or

geographical variation.

The difference between the infestation rates of L.

clamitans and L. catesbeianus suggests that anti-

predator defences by L. catesbeianus may also resist

leeches. Lithobates catesbeianus tadpoles secrete

compounds that are distasteful to fish (Evenson and

Kruse, 1982) and their nutritional qualities cause fish

to lose weight (Evenson and Kruse, 1982), suggesting

that these tadpoles have potent chemical defences.

Alternatively, L. catesbeianus may have more evidence

of leech attacks because they are better able to deal

with the stress associated with this insult than does L.

clamitans. In fact, L. catesbeianus has a well-developed

antioxidant stress response, suggesting it can better

survive stressors such as injuries associated with leech

attack than can other species (Jones et al., 2010). Little

is known about the stress response in L. clamitans. The

role of the chemical defences of tadpoles in repelling

leeches such as P. picta and the ability of amphibians

to deal with this stressor are interesting ecological

questions in need of further inquiry.

Host size preference has implications for the evolution

of parasitism in this leech. Generally, parasites are on

an evolutionary pathway to mutualism (Weeks et al.,

2007 and citations within). Natural selection should

benefit leeches that cause the least harm to their host.

Otherwise, hosts capable of resisting parasites should

proliferate in the population due to selective pressure

exerted by harmful parasites. Leech picking injuries

appear similar in size regardless of tadpole size

(anecdotal observation). Also, a leech should remove

up to the same volume of blood whether the tadpole

is small or large. Small tadpoles should have smaller

blood volumes than large tadpoles. Therefore, smaller

tadpoles should experience more distress from tissue

damage and blood loss from leech parasitism than

would a larger tadpole. Many questions remain about

how these parasites discern hosts and host size.

As a side note, Barta and Sawyer (1990) suggested

that P. picta released its posterior sucker during feeding.

The leeches we observed attached to their host by both

suckers, suggesting that either they recently attacked the

tadpoles/newts, or there is variation in this behaviour.

The variation in leech attachment requires more

investigation to pinpoint its relevance.

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Amphibian infestation and host size preference by Placobdella picta