Reference: Bio/. Bull 196: 224-244. (June 1999) Intracapsular Feeding by Embryos of the Gastropod Genus Littorina A. L. MORAN 1 Oregon Institute of Marine Biology. University of Oregon, Charleston, Oregon 97420 Abstract. Many gastropod species develop within egg capsules within which larvae are provided with extraemhry-onic nutrients. Species with encapsulated development fre-quently have transitory embryonic organs, such as "larval kidneys." that may represent specializations for consump-tion of intracapsular nutrition. Larvae of Littorina species with nonplanktonic, encapsulated development consume in-tracapsular albumen, but they lack obvious morphological modifications for albumen consumption. To determine the mechanism and location of protein uptake, larvae of seven species of Littorina (L. keenae, L. littorea, L. plena. L. saxatilis, L. scutitlata. L sitkana. L subrotundata) were exposed to solutions of either fluorescently labeled protein (FITC-bovine serum albumen) or ferritin. Under fluores-cence microscopy, larvae of all species with encapsulated, nonplanktonic development displayed strong regional affin-ity for FITC in the ciliated cells of the velum, whereas hatched larvae of planktotrophic Littorina species did not. Transmission electron microscopy of epithelial cells of non-planktotrophic veligers exposed to ferritin supported the interpretation that localized affinity for labeled protein in-dicated endocytotic protein uptake. Planktotrophic Littorina and Littorina with encapsulated, nonplanktonic develop-ment were shown to share equivalent velar width/larval length ratios during early embryonic development, whereas a literature search suggested that in other nonplanktotrophic prosobranchs the velum is relatively smaller than in plank-totrophs. Retention of a large velum in Littorina that de-velop entirely within egg capsules may facilitate feeding on intracapsular protein, in the absence of specialized assimi-lative organs found in other species with encapsulated de-velopment. Received 9 September 1997; accepted 12 February 1999. 1 Present address: Friday Harbor Laboratories, University of Washing-ton, 620 University Road, Friday Harbor. WA 98250. Email: moran fhl.washington.edu Introduction Marine invertebrates exhibit a remarkable variety of re-productive and developmental modes both within and among taxa, and this variation provides a powerful compar-ative means of studying the integration of development, life history, and evolution. One of the best-known dichotomies in invertebrate development is between species with larvae that must feed in the plankton to grow and attain metamor-phic competence (planktotrophic), and species that reach competence without feeding in the plankton (nonplanktotro-phic) (Thorson, 1946; Jablonski and Lutz, 1983; Strath-mann, 1985). Another important distinction can be made between species with entirely planktonic development, and species that spend all or part of development in egg capsules (planktonic. encapsulated nonplanktonic, and mixed devel-opment, respectively) (Thorson, 1946; Pechenik, 1979; Per-ron, 1981). Larvae of many species with encapsulated, nonplanktonic development are morphologically similar to larvae of re-lated planktotrophic species, and retain structures that pre-sumably had an ancestral role in larval swimming and feeding. For example, larvae of most gastropod molluscs with nonplanktonic, encapsulated development possess a velum (e.g., Fretter and Graham, 1962; Buckland-Nicks et ul.. 1973; Strathmann, 1978; Hadfield and laea. 1989), which is the primary larval structure that planktotrophic molluscan larvae use in swimming and food collection (Strathmann and Leise, 1979). The velum may be smaller in species that lack free-living larvae than in planktotrophic species (Jagersten, 1972; Webber, 1977; Rivest, 1983; but see Hadfield and laea, 1989), and it may have different patterns of ciliation (e.g., Lyons and Spight, 1973: Hadfield and laea, 1989). These changes in gastropod larval mor-phology have been interpreted as the loss of complex, ancestral planktotrophic features due to relaxation of stabi-lizing selection, and as functional modifications that en-229
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