File:American malacological bulletin (1987) (18130079766).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 205 terns of development known in this group: metamorphic-direct developers (a shelled, veliger stage occurs within the egg mass) and ametamorphic-direct developers (a shell and most other vestiges of the veliger are lacking in their ontogeny) (see Bonar, 1978, for a discussion of these two modes). The ametamorphic direct developers are indicated by filled triangles in figure 13; resulting juveniles are clearly larger. These can be considered the most evolved in the direction of direct development. We conclude that the presence of a larval shell sets a maximum size limit on opisthobranch juveniles, a limit that doesn't exist, at least at such a small size, among prosobranchs. The great spread in juvenile sizes and the apparent relationship between juvenile size and developmental mode might be due to mis-interpretation of existing data since dif- ferent types of opisthobranchs have different length-to-weight ratios; only good weight measurements of newly meta- morphosed juveniles would resolve this problem. On the other hand, juvenile length could provide a valid measure of size as it relates to predator avoidance. For some species, factors unrelated to selection for juvenile size might relate develop- mental mode to different aspects of their biology; one is adult size (Menge, 1975; Strathmann and Strathmann, 1982). Arguments relating adult size to developmental mode have been devoted to brooding, a habit unknown for opistho- branchs (with a possible exception cited by Rose and Hoegh- Guldberg, 1982). The arguments assume that small animals, having an absolute (and small) limit on the amount of energy available for reproduction must take the "safer" path of direct development, by-passing the plankton as a source of nutri- tion and dispersal to avoid it as a great source of develop- mental mortality. Table 4 lists directly developing species for which we could find data on adult lengths as well as juvenile lengths. It is clear that both large and small species produce large eggs that develop directly into hatching benthic juvenile stages. Still, reduction in adult size may have driven selec- tion for direct development in some species. Another possibility, alluded to above, is that certain ecological conditions could predicate different "best sizes" after metamorphosis. This hypothesis defies clear testing, but at least among specialized feeding groups (e.g. sponge feeders; hydrozoan feeders; bryozoan feeders) no generaliza- tions about optimal juvenile sizes emerge from our data set. A wide range of juvenile sizes occur among all such groups, as they do among taxa which tend to have similar dietary habits (e.g. Sacoglossa). We propose the following hypothesis to explain the observations delineated above. Post-settlement mortality is size dependent; the larger the juvenile size, the greater the freedom from predation by one or more common groups of micro-carnivores (mainly small worms and crustaceans; e.g. Highsmith, 1982). Juvenile mortality is least among opisthobranch species with ametamorphic-direct develop- ment because the hatching juveniles of these species are suf- 400 350 - 300 - CD £ 250 M 200 - 150 in 100 -I 50 type II larval shells y=1 23.60+1.01 x r=0.67
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rval shells .60+1 .1 6x 88 40 60 80 100 120 140 160 Egg Diameter (pm) 1 80 200 220 240 Fig. 9. Larval-shell length at hatching vs. egg diameter in the Nudibranchia. □Type I larval shells (n = 75); A,Type II larval shells (n = 28).

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