The Early Life History of the Rockfish, Sebastes Thompsoni (Scorpaenidae), in the Sea of Japan
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Japan. J. Ichthyol. 魚 類 学 雑 誌 41(4): 385-396, 1995 41(4): 385-396, 1 9 9 5 The Early Life History of the Rockfish, Sebastes thompsoni (Scorpaenidae), in the Sea of Japan Toru Nagasawa1 and Tokimasa Kobayash i 2 JapanSea National Fisheries Research Institute, 1-5939-22 Suido-cho, 1Niigata 951, Ja p a n 2NationalResearch Institute of FisheriesScience , 2-12-4 Fukuura,Kanazawa- ku , Yokohama,Kanagawa 236, Jap a n (ReceivedJune 10,1994; in revisedform October4, 1994;accepted November 28, 1994 ) Abstract Larvaland juvenile stages of the rockfish, Sebastes thompsoni, are described and illustratedwi t h particularreference to the developmentof headspines. Notochord flexion occurred between 4.6 and7.8 mm SL. Transformationoccurred between 16 and 22mmSL. Preflexionand flexionlarvae of S. thompsoniwere distinguished from related species by pigmentationof the dorsaland ventralmidlines of the tail. Larvaeand juveniles occurred in the coastalwater around Honshu Island in the Seaof Japa n wherethey were under the influence of theTsushima warm current. Thetiming of theirhabitat shift fr o m a pelagiclife style to an associationwith drifting seaweed and fooditems of larvaeand juveniles are als o describedand discussed . The genus Sebastes is one of the important compo- April 1987 (surface tows and oblique tows), May nents of the shelf and slope fish fauna of temperate 1987 (both tows), April 1988 (both tows), May 1988 and subarctic regions (Boehlert and Yamada, 1991), (both tows), April 1989 (both tows), May 1989 with about 18 species being distributed in the Sea of (both tows), April 1990 (both tows), May 1990 Japan (Nagasawa, 1991). Among these species, (both tows), March 1991 (oblique tows), April 1991 Sebastes thompsoni is an important resource for com- (oblique tows), May 1991 (oblique tows), March mercial fisheries in the coastal region of Honshu 1992 (oblique tows), April 1992 (oblique tows), Island. Although preextrusion larvae and juveniles May 1992 (oblique tows). The fish associated with of S. thompsoni have been described briefly (Shio- drifting seaweed were collected with large dip nets in gaki, 1988) and pelagic juveniles associated with June 1989. Samples were mainly taken during the drifting seaweeds reported (Ikehara, 1977; Yamada, period from March to June, in the area between Sado 1980), the early life history is poorly known. The Strait and Tsushima Strait. All samples were fixed aim of this study is to describe a developmental series with 5% buffered formalin sea water, fish larvae and of S. thompsoni with particular attention to the de- juveniles being subsequently sorted and preserved in velopment of head spines, horizontal distribution, 5% buffered formalin solution. Our approach to and the habitat shift from a free pelagic life style to identification, methods for making counts, measure- an association with drifting seaweed. Food items of ments and terminology for developmental stages larvae and juveniles are also described and discussed. follow Richardson and Laroche (1979) and Naga- sawa (1993). Terminology of head spination follows Moser and Ahlstrom (1978). For observations of Materials and Methods general development and head spines, specimens were stained lightly with saianin 5-R or cleared and Specimens examined in this study were came from stained following Dingerkus and Uhler (1977), re- the collections in the Japan Sea National Fisheries spectively. Observations, measurements, and illus- Research Institute. Samples were mainly obtained trations were made with the aid of a camera lucida with 130cm ring nets (130RN) (surface tows) and and micrometer attached to a binocular dissecting 70cm bongo nets or 80cm ring nets (oblique tows, microscope. For gut contents analysis, the entire gut from 100m deep to surface), from 1986 to 1992: was removed from each specimen and placed in a April 1986 (surface tows), May 1986 (surface tows), drop of lactic acid. The gut was then teasted apart •\3 8 5 •\ T. Nagasawa & T. Kobayashi by probes to release any organisms, which were during transformation and pelagic juvenile stages. identified and counted using a phase contrast micro- Relative head length increased markedly during the scope. early larval stages, but decreased gradually during the transformation and pelagic juvenile stages. Rel- ative eye diameter with respect to SL was 10-12% Results and almost constant during the larval and pelagic juvenile stages. However, relative eye diameter with Description of development respect to head length was variable and it decreasing markedly during the larval stages. Distinguishing features.-Preextrusion larvae of Fin development.-Pectoral fins were already pres- Sebastes thompsoni are characterized by three rows ent in preflexion larvae and the rayed portions are of melanophores along both the dorsal and the ven- fan-shaped membranes (Fig. 1A). The rays appear tral midlines of the tail, and also a few melanophores in flexion larvae as small as 5.2mm SL and post- on the lateral midline (Shiogaki, 1988). These char- flexion larvae of 11.9mm and larger have the full acters are also observed in preflexion and early complements of 15-17 (mainly 16) pectoral rays. flexion larvae. Larvae of S. inermis are similar to Pelvic fins are not observed in preflexion larvae, those of S. thompsoni (Kojima, 1988), but observa- but appeared in flexion larvae as small as 5.5mm SL tions of reared larvae of S. inermis revealed only a (Fig. 1A, B). In flexion larvae smaller than 7.9mm single row of melanophores along both the dorsal SL, the pelvic fins were visible only as fleshy buds and ventral midline of the tail. This characters is (Fig. 1B). Rays began to differentiate in larvae of therefore useful for separation of larvae of the two 8mm SL or larger. Postflexion larvae larger than species. Late flexion and postflexion larvae of S. 11mm had the full compliment of I, 5 pelvic rays thompsoni are characterized by a distinct row of (Fig. 1C, D). melanophores along the lateral midline. The bodies Caudal fin rays were not observed in preflexion of transforming larvae and early pelagic juveniles of larvae, although the finfold was well-developed, S. thompsoni are pigmented uniformly and the com- being continuous with the dorsal and anal finfolds bination of fin ray counts are useful for identifica- (Fig. 1A). Rays appeared in flexion larvae larger tion. Late pelagic juveniles have already five pigment than 4.7mm SL and in those larger than 6mm SL, bands as in the adults. Fin ray counts are as follows: serving to support the caudal finfold (Fig. 1B). Slits D; XIII, 13-15, A; III, 7, P1; 15-17, P2; I, 5. in the fin fold between the caudal peduncle and General development and morphology.-In the dorsal or anal regions appeared in postflexion larvae study material, the smallest larva (4.30mm in stan- 9.1mm SL and larger. The finfold of the caudal dard length [SL]) was regarded as a newborn pre- peduncle region disappeared in postflexion larvae flexion larva. Preflexion larvae had large heads, larger than 11mm (Fig. 1C, D). Branching of the large well-formed eyes and functional jaws (Fig. caudal rays occurred in postflexion larvae larger than 1A). The gut was short and bulbous, posteriorly 14 mm (Fig. 1E), and the full compliment of 7+7 with a short posteroventral orientation. Yolk and an principle caudal rays being established in pelagic oil globule were observed in the anterior region of juveniles larger than 30mm SL. the liver in some specimens. Notochord flexion Dorsal and anal fin rays were not observed in occurred between 4.6 and 7.8mm SL. Transforma- flexion larvae, but correspondig fin folds were well- tion from postflexion larvae to pelagic juveniles developed and continuous with the caudal portion. included a structural change of the dorsal and anal In flexion larvae larger than 4.6mm SL, dorsal and fin "prespines" to sharp, hard spines (Laroche and anal fin bases appeared and portions of the soft ray Richardson, 1981), occurring between 16 and 22mm pterygiophores of both fins started to differentiate SL. (Fig. 1B). In flexion larvae larger than 7.6mm, Allometry of some body parts are shown in Figure some soft rays appeared in both fins. In postflexion 2. Relative body depth increased markedly in the larva of 9.1mm, some dorsal spine buds were ob- early larval stages (4.3-16mm SL), less so in trans- served (Fig. 1C). In larger postflexion larvae, the forming larvae and pelagic juveniles (Figs. 1, 2). The finfold between the anus and first anal spine became relative snout to anus distance also increased mark- smaller, disappearing by 11mm SL. In postflexion edly during the early larval stages, but gradually larvae larger than 11.3mm SL, the total dorsal and •\3 8 6•\ Early Life History of Sebastes thompsoni A' A B C D E F G Fig. 1. Developmental series of Sebastes thompsoni. A) Preflexion larva 4.3mm NL; A') pigmentation of the dorsal midline of specimen A; B) flexion larva 6.4mm SL; C) postflexion larva 9.1mm SL; D) postflexion larva 11.9mm SL; E) postflexion larva 16.1mm SL; F) transforming larva 18.2mm SL; G) pelagic juvenile 28.1mm SL. •\8 7•\ T. Nagasawa & T. Kobayashi on the dorsal and ventral midlines of tail from the A 16th or 17th to 21-22nd myomeres, in adition to a few (usually 1 or 3) melanophores on the lateral midline of the tail. In flexion larvae, some melanophores appeared on the tip of the upper and lower jaws, with melano- phores being added to the dorsal surface of the brain, B resulting in its being almost covered in larvae larger than 6.1mm SL (Fig. 1B). The pigmented region on the dorsal midline extended anteriorly and post- eriorly from the 11-14th to 20-23rd myomeres. The ventral midline was also covered with melanophores.