Sexual reproduction in Epistomia bursaria (Bryozoa: Cheilostomata), an endozooidal brooder without polypide recycling



Epistomia bursaria L. is unusual amongst cheilostomes in that there is only one polypide generation per autozooid, together with endocoelomic embryonic brooding and larval viviparity. This combination, which gives the minimum possible fecundity of one larva per gynozooid is only known for one other cheilostome species, the majority of the rest being extracoelomic ovicellate brooders with polypide recycling. The greater fecundities exhibited by ovicellate and non-brooding recyclers may offset the smaller degree of protection afforded by these reproductive strategies.

Spermatogenesis and oogenesis are examined ultrastructurally. Whereas spermatogenesis is similar to that of other cheilostomes studied previously, oogenesis is very different. The egg at maturity is small and alecithal. Evidence suggests that endozooidal self-fertilization occurs. Whereas nutrient for spermatogenesis and oogenesis is probably intrazooidal in origin, that for embryogenesis is derived inter-zooidally.

It is suggested that the combination of endocoelomic brooding, larval viviparity and one polypide generation may be a primitive feature from which recycling and extracoelomic brooding may have evolved.