CHAPTER II 



LARVAE OF SEA STARS 

 (MORPHOLOGY, PHYSIOLOGY, BEHAVIOR) 



EARLY DEVELOPMENT 



Egg 



Eggs of sea stars have a diameter ranging from 100 to 3,540 |a,m (see 

 Table on p. 105). From an egg of minute diameter containing very little yolk, 

 a definitive sea star develops after passing through larval stages of dipleurula, 

 bipinnaria, and usually branchiolaria, with subsequent metamorphosis. Larger 

 eggs bypass the stages of dipleurula and bipinnaria and often, in such cases, 

 a lecithotrophic modified brachiolaria develops, followed by metamorphosis 

 into a definitive sea star. In many species, larval stages are eliminated alto- 

 gether and development is direct {Pteraster militaris) (Kaufman, 1968; and 

 others). Sea star eggs with little yolk are often colorless, almost transparent. 

 Eggs containing many nutritive substances may be yellow, pink, orange, or 

 brown. Ovulation and shedding of eggs usually occur immediately before 

 metaphase I maturation division. In sea water oocytes are capable of "matu- 

 ration" without fertilization, i.e. proceeding from prophase I maturation divi- 

 sion (with intact germinal vesicle) to metaphase I, and then separating polar 

 bodies I and II. With rare exceptions, the eggs of sea stars are shed directly 

 in water. 



Egg membrane : Each egg is enclosed in a thin hyaline membrane sur- 

 rounding a vitalline and an exterior jellylike membrane. The jellylike mem- 

 brane is 5 — 20 |im thick. In most species it is apparently present only up to 

 the beginning of cleavage, of soft consistency, and destroyed when eggs are 

 passed through a capron net. In some species the jellylike membrane is 

 relatively thick and sticky, and hence a batch of eggs can adhere to the 

 substrate. 



