Icosteoidei: Development and Relationships 



A. C. Matarese, E. G. Stevens and W. Watson 



THE suborder, Icosteoidei, consists of one family, Icosteidae, 

 with a monotypic species Icosteus aenigmaticus (Nelson, 

 1976). Adults inhabit the epi- and bathypelagic areas of the 

 North Pacific Ocean from southern California to Japan. Adults 

 may exceed 2 m in length and have dark flaccid bodies; a char- 

 acteristic implied in the common name of the species, raglish 

 (Hart, 1973). 



The systematic position of this group and its designation as 

 an order or suborder is not well established. Greenwood et al. 

 ( 1 966) consider it a suborder of Perciformes while Gosline (1971) 

 elevates it to an order, Icosteiformes, a probable perciform de- 

 rivative. 



Development 



The only early life history data previously published is a brief 

 description of the egg (Allen, 1968). Icosteus aenigmaticus eggs 

 are commonly collected in ichthyoplankton surveys off the Pa- 

 cific coast of North America [California Cooperative Oceanic 

 Fisheries Investigations (CalCOFI) and Northwest and Alaska 

 Fisheries Center (NWAFC)], but larvae (mostly preflexion) are 

 infrequently found and a complete size series from hatching to 

 transformation is not presently available. Larvae may move 

 offshore or into deeper waters. The first published description 

 and illustration of the larvae from pre- to postflexion stages are 

 provided here, based on National Marine Fisheries Service 

 (NMFS) collections. Although /. aenigmaticus ]uven\\e^ undergo 

 a marked transformation to the adult stage, little information 

 is available concerning this change (Hart, 1973). 



Eggs 



The pelagic egg of /. aenigmaticus ranges in diameter from 

 2.8 to 3. 1 mm (Fig. 305). A large, sometimes irregular, oil glob- 

 ule with a diameter of 0.42 to 0.60 mm is present. The oil globule 

 usually decreases in size with development. The chorion is 

 smooth, sometimes amber or rose colored. Early stage egg yolks 

 are frequently opaque, although later stages have a clear, un- 

 segmented yolk. During the middle stage of development, em- 

 bryos have pigment along the dorsal body as well as on the yolk 

 and oil globule. Late stage embryos have functional mouths, 

 pectoral buds, and very wide finfolds. Scattered pigment occurs 

 on the eyes, snout, jaws, and dorsal head. The dorsal surface of 

 the gut is pigmented. Along the dorsal and anal finfolds, three 

 or four clusters of melanophores appear at each distal edge. 

 Melanophores also appear above and below the tail in the caudal 

 finfold. An irregular double row of melanophores extends the 

 length of the dorsal body margin. A few mediolateral spots 

 appear anteriorly. Occasionally, pigment occurs along the ven- 

 tral body margin. 



Larvae 



Morphology.— Nevj\y hatched larvae of/, aenigmaticus are 6.5 

 mm NL; yolk material may persist until larvae are 10 mm. 

 Flexion begins at about 1 1 mm and is complete at about 1 7 

 mm SL. The size at transformation is not known, but fin de- 



velopment is almost complete by 28 mm. The body, surrounded 

 by a wide finfold, is very soft. Preflexion larvae have small heads 

 with rounded snouts and long tapering bodies (Fig. 306). Dor- 

 sal and ventral finfolds are wider than the body. During flexion 

 the body thickens and becomes more robust, especially ante- 

 riorly. Postflexion larvae have a robust head and gut and a 

 tapering trunk (Fig. 306). Preanal length is less than 50% body 

 length. A series of preopercular spines appears during late flex- 



Pigment. — New\y hatched larvae of /. aenigmaticus display es- 

 sentially the same eye, head, gut, body, and finfold pigment as 

 the embryos. With increasing size the head and gut usually 

 become increasingly covered with discrete spots. Dorsal body 

 margin pigment is present throughout larval development, while 

 the amount of lateral and ventral body margin pigment varies 

 and is relatively sparse. The characteristic embryonic caudal 

 pigment persists in the developing larvae, becoming less prom- 

 inent but remaining as scattered melanophores on the hypural 

 margin and fin ray bases. In general, postflexion larvae are less 

 pigmented except on the head. Pelvic and pectoral fin bases and 

 pelvic rays acquire melanophores during postflexion. 



Meristics. — Icosteus aenigmaticus larvae have the following ver- 

 tebral and fin ray counts: Vert. 66-68; D 55; A 39; Pec. 21; Pel. 

 1,4; and C 9 -(- 8 = 17 (NWAFC files). These counts conform 



Fig. 305. Egg of Icosteus aenigmaticus: 2.8 mm, drawn by H. Orr. 



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