Thamnaconus garretti ( Fowler, 1928 )

Nagappan, Nagappan, Dcruz, Thomson Mariadasan, Loganathan, Archana, Gupte, Shreyas H, Ali, Rajab, Karthik, Prasanna, Mani, Bhuvaneswari & Moulvi, S M M, 2021, A Record of the Rare Filefish, Thamnaconus garretti (Fowler, 1928), Collected around Midway Island, Central Pacific (Actinopterygii, Tetraodontiformes, Monacanthidae), Bulletin of the National Museum of Nature and Science. Series A, Zoology 47 (1), pp. 31-35 : 31-34

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 mm SL), 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 (94.4 mm 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 Midway Island , 30°15′N, 178°43.2′E, 330–710 m depth, 2 October 1972 .

Diagnosis. Distinguished from other species of Thamnaconus by the following combination of characters: body behind eye covered with many longitudinal, narrow dark stripes; pelvic flap with many narrow dark stripes running almost parallel to postero-ventral edge of pelvic flap; head with similar narrow dark stripes but at right angles to those on pelvic flap.

Description. Dorsal-fin rays II+33–34; anal-fin rays 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 anal-fin 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 in SL.

Body width 0.9–1.0; eye diameter 2.6–3.3; interorbital width 2.9–3.5; depth of caudal 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 dorsal-fin 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-fin spine 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 behind first dorsal-fin spine on dorsal surface of body; second dorsal and anal fins elevated anteriorly; pectoral fin short and rounded; caudal fin slightly rounded; pelvic terminus immovable, composed of two pairs of encasing scales.

Color in alcohol: ground color of body light brown covered with many longitudinal, dark brown stripes; pelvic flap with many narrow dark stripes running almost parallel to postero-ventral edge of pelvic flap; head with similar narrow dark stripes but at right angles to those on pelvic flap; first dorsal-fin spine light brown; caudal fin dark brown; pectoral, second dorsal and anal fins pale.

Remarks. Fowler (1928) described Paramonacanthus garretti based on a single specimen (77 mm 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 the holotype and 12 additional specimens (71–167 mm SL) collected by trawl and fish trap at depths of 67–84 m off O`ahu 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 from members of Pseudomonacanthus in having the second dorsal and anal fins elevated anteriorly. Pseudomonacanthus is characterized by the following combination of characters: moderately elongate body; gill opening mostly before middle of eye; first dorsal-fin spine over posterior half of eye, the spine with four rows of spinules (posterolateral ones larger than anterior); no deep groove on dorsal surface of body for reception of this 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 not indicate by which characters he placed this rare filefish in Thamnaconus .

Thamnaconus is distinguished from other genera of the Monacanthidae by the following combination of characters: first dorsal-fin spine with a row of laterally-directed barbs along each lateral edge; its origin over center or posterior half of eye; shallow groove for receiving first dorsal-fin 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 the sex of the other three specimens because of the poor condition of the gonads. Randall (1975) said l the gonads of the adults were small, and microscopic examination was necessary to determine that two are males and two females. z However, he did not mention whether he found sexual dimorphism and which specimens are males and females. The male, FAKU 125876 (167 mm SL), suggests that T. garretti has sexual dimorphism in the body and head shape. It has a more elongate body than BPBM 16264 (167 mm SL), and the dorsal profile of head is slightly convex in the 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 sexual dimorphism in body and head shape was reported 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).

Figures

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.

Fig. 2. X-ray photographs of Thamnaconus garretti. Top, FAKU 109580; bottom, FAKU 125876.