Bull. Natl. Mus. Nat. Sci., Ser. A, 47(1), pp. 31–35, February 22, 2021
DOI: 10.50826/bnmnszool.47.1_31

A Record of the Rare Fileﬁsh, Thamnaconus garretti (Fowler, 1928),
Collected around Midway Island, Central Paciﬁc(Actinopterygii, Tetraodontiformes, Monacanthidae)
Keiichi Matsuura
Department of Zoology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan
E-mail: matsuura@kahaku.go.jp(Received 12 November 2020; accepted 23 December 2020)

Abstract Four specimens of the rare ﬁleﬁsh, Thamnaconus garretti (Fowler, 1928), were collected from the waters around Midway Island in the central Paciﬁc. Detailed examinations of these
specimens provide distinguishing characters of this ﬁleﬁsh, placing it in the genus Thamnaconus
Smith, 1949. Comparisons of the present specimens and previously reported specimens suggestthat sexual dimorphism occurs in this rare ﬁleﬁsh: the body is more elongate in males than females
and the dorsal proﬁle is slightly convex in males and straight in females.
Key words : Taxonomy, distribution, Monacanthidae, Midway Island, Thamnaconus.

Introduction
While examining ﬁleﬁsh collections of theMaizuru Fisheries Research Station, Kyoto University, I found four specimens of the rare ﬁleﬁsh, Thamnaconus garretti (Fowler, 1928). Thisﬁleﬁsh was previously known only from theholotype and additional 12 specimens collectedfrom the Hawaiian Islands (Fowler, 1928; Randall, 1975, 2007). The four newly discovered
specimens are described in detail below withcomments on the speciesʼ taxonomic position andsexual dimorphism.
Materials and Methods
Four specimens of Thamnaconus garretti werecollected by bottom trawl operated by the RV
Kaiyo-maru oﬀ Midway Island in the central
Paciﬁc. Methods for counts and measurements
follow Matsuura (1980). Institutional codes are
as follows: Bernice Pauahi Bishop Museum,Honolulu (BPBM) and Maizuru Fisheries© 2021 National Museum of Nature and Science

Research Station, Kyoto University, (FAKU).
Standard length and total length are abbreviatedas SL and TL, respectively. Radiographs wereused to count the number of vertebrae.
Thamnaconus garretti (Fowler, 1928)(Figs. 1–2)
Paramonacanthus garretti Fowler, 1928: 459, ﬁg. 78(Hawaiian Islands)
Thamnaconus garretti: Randall, 2007: 481, unnumbered
ﬁgure (Midway Island).

Specimens examined. BPBM 16264 (167 mmSL), southern edge of Nero Bank in 44 fathoms,
28 nautical miles southwest of Midway Island,baited ﬁsh trap (1.5 hours), P.J. Struhsaker,
aboard R/V David Starr Jordan, cruise 80, station 50, 26 August 1973; FAKU 109579 (94.4mm SL), 109580 (94.4 mm SL), 330 km northwest of Midway Island, 29°25′N, 179°00′E, September 1973; FAKU 109558 (90.1 mm SL), 1000
km northwest of Midway Island, 32°13′N,172°51.3′E, 9 September 1973; FAKU 125876(male, 167 mm SL), 400 km northwest of Mid-32K. Matsuura

Fig. 1. Thamnaconus garretti collected around Midway Island. A, FAKU 109580, 94.4 mm SL; B, FAKU
125876, 167 mm SL (male); C, BPBM 16264, 167 mm SL. A and B, preserved specimens, photographs by F.
Tashiro; C, defrosted specimen, photograph by J. E. Randall.

Thamnaconus garretti from Midway Island

Fig. 2.

33

X-ray photographs of Thamnaconus garretti. Top, FAKU 109580; bottom, FAKU 125876.

way Island, 30°15′N, 178°43.2′E, 330–710 m
depth, 2 October 1972.
Diagnosis. Distinguished from other species ofThamnaconus by the following combination ofcharacters: body behind eye covered with many
longitudinal, narrow dark stripes; pelvic ﬂap withmany narrow dark stripes running almost parallel
to postero-ventral edge of pelvic ﬂap; head withsimilar narrow dark stripes but at right angles tothose on pelvic ﬂap.

Description. Dorsal-ﬁn rays II+33–34; anal-ﬁnrays 31–33; pectoral-ﬁn rays 13; vertebrae
7+12=19. Body depth 2.9–3.1; head length 3.0–3.4; snout length 3.9–4.7; length of snout to ﬁrst
dorsal-ﬁn origin 3.0–3.3; length of snout to analﬁn origin 1.5–1.6; length of snout to end of pelvic ﬁn 1.5–1.7; length of second dorsal-ﬁn base
3.1–3.3; length of anal-ﬁn base 3.4–3.5; all inSL.
Body width 0.9–1.0; eye diameter 2.6–3.3;
interorbital width 2.9–3.5; depth of caudal

34

K. Matsuura

peduncle 3.2–3.7; length of caudal peduncle 2.4–2.8; gill opening length 3.3–3.8; length of ﬁrst
dorsal-ﬁn spine 1.2–1.3; length of longest dorsalﬁn ray 1.9–2.0; length of longest anal-ﬁn ray
1.7–2.2; length of longest pectoral-ﬁn ray 2.4–
2.8; length of caudal ﬁn 1.2–1.7; all in HL.
Body moderately elongate and laterally compressed; dorsal proﬁle of head straight or slightly
convex; gill opening below eye; ﬁrst dorsal-ﬁnspine above posterior half of eye; two rows of
15–21 downward directed spinules on its anterior
surface and one row of 10–20 downward
directed spinules on each postero-lateral surface;second dorsal-ﬁn spine short and feeble, embedded in ﬁn membrane; a shallow groove behindﬁrst dorsal-ﬁn spine on dorsal surface of body;
second dorsal and anal ﬁns elevated anteriorly;
pectoral ﬁn short and rounded; caudal ﬁn slightlyrounded; pelvic terminus immovable, composedof two pairs of encasing scales.
Color in alcohol: ground color of body lightbrown covered with many longitudinal, darkbrown stripes; pelvic ﬂap with many narrow dark
stripes running almost parallel to postero-ventraledge of pelvic ﬂap; head with similar narrowdark stripes but at right angles to those on pelvic
ﬂap; ﬁrst dorsal-ﬁn spine light brown; caudal ﬁndark brown; pectoral, second dorsal and anal ﬁns
pale.
Remarks. Fowler (1928) described Paramonacanthus garretti based on a single specimen (77mm SL, 94 mm TL) collected by Andrew Garrett
from the Hawaiian Islands. Randall (1975) redescribed this species and placed it in the genus
Pseudomonacanthus Bleeker, 1865 based on theholotype and 12 additional specimens (71–167mm SL) collected by trawl and ﬁsh trap at depthsof 67–84 m oﬀ Oʼahu Island and Nero Bank 28
nautical miles southwest of Midway Island.
Uchida and Uchiyama (1986) included this ﬁleﬁsh under the name of Pseudomonacanthus garretti in the list of ﬁshes collected by trawl at
depths of 31–187 m northwest of the Hawaiian
Islands. However, this species is diﬀerent frommembers of Pseudomonacanthus in having thesecond dorsal and anal ﬁns elevated anteriorly.

Pseudomonacanthus is characterized by the following combination of characters: moderately
elongate body; gill opening mostly before middleof eye; ﬁrst dorsal-ﬁn spine over posterior half ofeye, the spine with four rows of spinules (posterolateral ones larger than anterior); no deep
groove on dorsal surface of body for reception ofthis spine when depressed; pelvic terminus
immovable, composed of two pairs of encasing
scales; and second dorsal and anal ﬁns not elevated anteriorly (Fraser-Brunner,1940; Hutchins,1977; Randall, 1975). Randall (2007) removed
this ﬁleﬁsh from Pseudomonacanthus to Thamnaconus Smith, 1949. He stated that he followed
the suggestion of Barry Hutchins about the classiﬁcation of this species, however, he did notindicate by which characters he placed this rareﬁleﬁsh in Thamnaconus.
Thamnaconus is distinguished from other genera of the Monacanthidae by the following combination of characters: ﬁrst dorsal-ﬁn spine witha row of laterally-directed barbs along each lateral edge; its origin over center or posterior halfof eye; shallow groove for receiving ﬁrst dorsalﬁn spine when depressed; second dorsal and anal
ﬁns elevated anteriorly; pelvic terminus immovable, composed of two pairs of encasing scales;
19 vertebrae (Matsuura, 1978; Hutchins, 1977,1986). All these characters are found in P. garretti, therefore reasonably classifying it within
Thamnaconus.
Examination of the gonads of FAKU specimens showed that FAKU 125876 (167 mm SL)
is male but it was not possible to determine thesex of the other three specimens because of thepoor condition of the gonads. Randall (1975)
said “the gonads of the adults were small, andmicroscopic examination was necessary to determine that two are males and two females.” However, he did not mention whether he found sexualdimorphism and which specimens are males andfemales. The male, FAKU 125876 (167 mm SL),suggests that T. garretti has sexual dimorphismin the body and head shape. It has a more elongate body than BPBM 16264 (167 mm SL), andthe dorsal proﬁle of head is slightly convex in theThamnaconus garretti from Midway Island

FAKU specimen and straight in the BPBM ﬁsh.
Considering the same size of these specimens,
the diﬀerences of the body and head shape may
well be due to sexual dimorphism. This speculation is supported by the fact that the same sexualdimorphism in body and head shape wasreported in members of Paramonacanthus by
Hutchins (1997), Thamnaconus modestus (Günther, 1877) by Ebina (1932) and Yamada (2007),and T. hypargyreus (Coper, 1871) by Yamada(2007).
Acknowledgments
I am grateful to Yoshiaki Kai (FAKU) for
specimen loan. I thank John E. Randall andArnold Suzumoto for providing a photograph ofThamnaconus garretti (Fig. 1C) and collection
data, and Fumihito Tashiro (FAKU) for taking
photographs of FAKU specimens (Fig. 1A andB). Masanori Nakae of the National Museum ofNature and Science kindly took X-ray photographs of T. garretti. Helen K. Larson of theMuseum and Art Gallery of the Northern Territory provided helpful comments on the manuscript.
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