MOSER: DEVELOPMENT AND DISTRIBUTION OF SEBASTOLOBUS 



DESCRIPTION OF DEVELOPMENT 



Distinguishing Features 



Early Sebastolobus larvae (up to 6 mm) can be 

 distinguished from those of all other genera of 

 eastern Pacific Scorpaenidae on the basis of pig- 

 mentation. Sebastolobus larvae of this size range 

 are unique in having two large melanistic 

 blotches about midway along the tail, one at the 

 dorsal midUne and one at the ventral midline 

 (Figure lA, B). These are sometimes expanded to 

 form a solid band on the tail. Early larvae of all 

 other eastern Pacific scorpaenid genera have a 

 series of melanophores along the ventral midline 

 of the tail, and in some species of Sebastes an 

 opposing row is present at the dorsal midline. The 

 large tail blotches of Sebastolobus disappear in 

 larvae between 5.0 and 6.5 mm. Soon after the 

 loss of these large tail blotches in Sebastolobus 

 the larvae develop prominent crestlike parietal 

 ridges that terminate in double spines, the pos- 

 terior (nuchal) spine being longer and more prom- 

 inent than the anterior (parietal) spine (Figure 

 IF). Of the other eastern Pacific scorpaenid gen- 

 era, only the larvae of Scorpaenodes have 

 parietal ridges and spines like Sebastolobus. If 

 two spines are present on the parietal ridges of 

 other genera, the anterior spine is always longer 

 and more prominent than the posterior. Sebas- 

 tolobus larvae may be distinguished from those of 

 Scorpaenodes on the basis of a melanistic shield, 

 which covers the dorsolateral surface of the gut in 

 the former and is absent in the latter. Larvae of" 

 Sebastolobus smaller than 10.0 mm could not be 

 distinguished to species. Larvae larger than this 

 can be identified to species by a combination of 

 characters. The pectoral fins of S. altivelis larvae 

 are relatively longer and are wider at the base 

 than in S. alascanus. Also, larvae of S. altivelis 

 are deeper bodied than those of S. alascanus. 

 Details of the structui-al and pigmentary charac- 

 ters that differentiate the larvae and juveniles of 

 the two species are given below. 



General Morphology 



Sebastolobus larvae hatch and are freed from 

 their transparent gelatinous egg masses at about 

 2.6 mm (Pearcy, 1962). The smallest larvae in our 

 plankton collections are about 3.0 mm long and 

 still have the elliptical yolk sac with a posteriorly 

 positioned oil droplet (Figure lA). When the lar- 

 vae reach approximately 3.5 mm, the yolk sac has 



been resorbed and the jaws and feeding apparatus 

 are well formed (Figure IB). Flexion of the 

 notochord begins in larvae about 6.0 mm long and 

 is completed in larvae about 7.5 mm (Figure IE, 

 F). Larvae larger than 10.0 mm and pelagic and 

 benthic juveniles can be identified to species on 

 the basis of characters mentioned above (Figures 

 2-7). In both species, transformation into the 

 pelagic juvenile stage is initiated within the size 

 range of 14.0 to 20.0 mm. All specimens larger 

 than 20.0 mm have some juvenile pigmentation. 

 The largest pelagic juvenile of S. alascanus 

 encountered was 27.2 mm, whereas the smallest 

 benthic juvenile was 22.5 mm. In contrast, in the 

 protracted pelagic juvenile stage of S. altivelis, 

 individuals may attain 56.0 mm in length. The 

 smallest benthic juvenile of this species in the 

 collection was 42.0 mm. Pelagic juveniles of S. 

 macrochir are similar in shape and pigmentation 

 to those of S. altivelis. Although the largest 

 pelagic specimen available was only 30 mm long, 

 it is probable that pelagic juveniles of S. mac- 

 rochir grow larger than this and have a pro- 

 tracted mid- water life as in S. altivelis. 



Relative body depth (maximum body depth/ 

 standard length) changes markedly during de- 

 velopment and is an important taxonomic 

 character in the pelagic juveniles (Figure 8). It is 

 15% at the beginning of the larval period, almost 

 doubles by the onset of notochord flexion, and av- 

 erages 28% during flexion. It increases further to 

 35% in larvae 7.5 to 10.0 mm. Relative body 

 depth remains at about this percentage in larvae 

 and pelagic juveniles of S. alascanus but de- 

 creases sharply to an average of 25% (range of 22 

 to 32%) in benthic juveniles. Late-stage larvae 

 and pelagic juveniles of S. altivelis are much 

 more robust and deeper bodied, averaging 41% 

 (range of 36 to 45%) in the 10- to 40-mm size 

 range. Pelagic juveniles 40 to 50 mm long begin 

 to show a decrease in body depth (mean 38%; 

 range of 36 to 41% ), 50- to 55-mm specimens show 

 a further decrease (mean 35%; range of 34 to 

 37%) and the decrease is precipitous in benthic 

 juveniles (mean 28%; range of 27 to 30%). Body 

 depth in 21- to 30-mm pelagic juveniles of S. 

 macrochir averaged 37.5% of the body length 

 (range of 34 to 42%). 



The gut has an unusual shape in early larvae up 

 to about 5.5 mm. The narrow, dorsally positioned 

 esophagus runs posteriad from the head for some 

 distance before entering the principal mass of the 

 gut. The effect of this is to produce a space between 



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