KENDALL ET AL.: ELH STAGES AND CHARACTERS 



17 



Table 4. 



Some Contributions in Which Ontogenetic Characters have been used to Examine Systematic Relationships (Updated from 



Ahlstrom and Moser, 1981). 



The Yolk-sac Larval Stage 



At hatching, larvae can be at various states of developmenl, 

 dependent to a large degree on the size of the yolk (Fig. 7). 

 Larvae from eggs with small yolks are less developed at hatching 

 than those that hatch from eggs with larger yolks. Since the bulk 

 of maiine fish spawn eggs that are about I mm in diameter and 

 have a narrow perivitelline space, the yolk is only slightly less 

 than I mm. Larvae from such eggs generally lack a functional 

 mouth, eye pigment, and differentiated fins. They possess a large 

 yolk sac relative to the size of the lai~va which supplies nour- 

 ishment while the larvae develop to become self-feeding. Newly 

 hatched larvae from demersal eggs are generally further ad- 



vanced in development than lai^ae from pelagic eggs of com- 

 parable size (Fig. 8). In these and other fish with large eggs, 

 hatching may be delayed until the yolk sac is absorbed and the 

 larvae are ready to feed at hatching, having bypassed the yolk- 

 sac larval stage. The delayed absorption of yolk reaches an ex- 

 treme in fishes such as salmonines in which the yolk-sac larva 

 transforms directly into a juvenile; Hubbs (1943) proposed the 

 term "alevin" be applied to this yolk-sac larval stage. 



At hatching, locomotion and orientation of most yolk-sac 

 larvae are aided by a continuous median finfold (dorsal, caudal, 

 anal) and larval pectoral fins. During egg development, many 

 fish embryos develop melanophores that originate in the neural 



