Abstract
Egg masses of two populations of the ascoglossan Costasiella with different developmental modes were examined. Costasiella ocellifera is an encapsulated developer; Costasiella sp. has an obligate plankto-trophic larval stage. Adults of the two populations were also separable by habitat, but could not be distinguished by external morphology. Comparison of capsule diameters and calorimetric analyses showed that C. ocellifera eggs have nearly eight times the capsule volume and 10 times the calories of eggs of its congener. Histochemical examination of extra-embryonic intra-capsular vesicles of C. ocellifera revealed the contents to be glycoprotein. No inclusions were visible in the capsular fluid of Costasiella sp. Egg masses from the two populations differed significantly in the amount of TCA-soluble carbohydrate, lipid, and NaOH-soluble protein per egg and per milligram dry weight of egg mass. These components were, however, similar when expressed on an ash-free dry-weight basis. The advantage of hatching directly onto a suitable food source that is temporally persistent, but patchily distributed might have provided the selective pressure to achieve extended intra-capsular development in C. ocellifera.
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