File:American malacological bulletin (1987) (17535996723).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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HADFIELD AND MILLER: OPISTHOBRANCH DEVELOPMENT 207 of this species, like most sea hares, produce hundreds of millions of offspring; its success is indicated by its distribu- tion throughout tropical and subtropical seas of the world, and even into temperate regions such as Japan. This hypothesis predicts a great reduction in numbers of offspring in the shift from metamorphic to ametamorphic direct develop- ment. However, life-time fecundity data are not sufficiently abundant to test this prediction. What determines the developmental mode of any in- dividual opisthobranch species? Assuredly, there is no single answer. Given the preponderance of species with plankto- trophic larvae (more than 70% of all opisthobranchs), we assume that this is the primitive mode for the group, an assumption strengthened by the unlikelihood of evolution from direct development to larviparous development (Strath- mann, 1978b). Thus the evolutionary direction will be toward lecithotrophic-planktic development and from there to direct development. The most evolved forms, in terms of this life history adaptation, will be those with ametamorphic direct development. It is probable that the selective pressures leading away from planktotrophic development have not been the same across all opisthobranch species. Selection can occur at any life-history stage. If mor- tality is too great in the pelagic phase, that phase can be reduced or eliminated. For example, it is possible that direct development evolved in some species in response to a brief and unreliable polar phytoplankton season, as suggested by Thorson (1950). Intense predation on early juveniles could have selected for increased size, which we have shown to be limited by pelagic development. Thus direct development evolves. Finally, any process that restricts adult size could also limit fecundity and thus influence the evolution of lecithotrophic or direct development. In some cases, a predatory opisthobranch could have adapted to a relatively short-lived prey (e.g. some hydrozoans) by itself becoming short-lived in order to grow to maturity and reproduce before the prey is exhausted. The adaptation will almost certainly include a considerable reduction in predator size, and thus, fecundity. Under these conditions, larger, lecithotrophic eggs will be favored for reasons discussed above. If prey are not too patchy, pelagic larvae could be dispensable, and the reduced fecundity related to small size will be further com- pensated by the production of still larger, directly developed offspring with a concomitant reduction in both larval and juvenile mortality. In other cases, competition could have restricted the growth of a species and thus reduced its reproductive output to the point where it could not successive- ly replace itself via a larviparous mode (an argument made 1.1 1 - 0.9 - 0.8 -
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y=0.269 + 0.001 x r=0.54 Egg Diameter Qjm) Fig. 13. Length of post-metamorphic juveniles vs. egg diameter. <species with planktotrophic larvae (n = 21); □, species with lecithotrophic larvae (n = 11); A, species with metamorphic direct development (n = 14); ▲, species with ametamorhpic direct development (n = 8). The vertical dashed lines emphasize the egg-size limits of species with the planktotrophic and lecithotrophic development. The horizontal dashed line indicates the upper limit of juvenile length for most species which have a larval shell in their development.

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