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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276 AMER. MALAC. BULL. 5(2) (1987)
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Fig. 23. Comparison of teeth of calciphilic species, showing Halimeda-spur (A, B, C) with caulerpivorous species (D) of Elysia. A = E. papulosa; B = E. flava; C = E. tuca; D = E. subornata. A, B, and E are from Clark, 1984; C is from Jensen and Clark, 1986. cuneata with especially dense ascoglossan feeding tracks (probably of a Bosellia). Caribbean elysiids that feed primarily on calcified algae (Elysia tuca, E. flava Verrill, E. papulosa, E. patina Marcus) often have a spurlike tip on the radular tooth (Fig. 23), while those that feed primarily on less-calcified algae (£. subornata, E. ornata (Swainson), E. sp. "AF") have teeth with a broad tip. This "Halimeda spur" appears necessary to pierce the narrow utricles of Halimeda through the interutricular car- bonate matrix. The high densities noted for stiligerids, particularly in high latitudes, suggest that feeding effort is lower, and con- sequently growth and reproductive output are higher, for species feeding on septate, low-ash algae. Unfortunately, we have no data for high-latitude algal ash levels, but the biology of ascoglossans that eat high-ash foods suggest that feeding effort can constrain life history patterns. The transient, irrup- tive cycles of stiligerids (Clark, 1975) are probably unsupport- able on algae of high ash content or siphonaceous structure because of lower feeding rates. Thus far, we have observed no examples of such cycles on siphonalean algae. Biomass ratios above 1% often lead to massive destruction of algal food resources in high latitude populations (Clark, 1975), but this overgrazing apparently does not occur on siphonalean algae. Kleptoplastid retention is apparently absent among the Conchoidea, is relatively common among ascoglossans that feed upon high-ash algae (Elysiidae), and uncommon among those feeding upon low-ash algae (e.g. Stiligeridae). The energetic benefit of kleptoplastid maintenance would be greatest in species whose energy intake is limited by algal resistance to feeding. Indeed, noting the very low densities of reef populations, retention of kleptoplastids might be the only energetically feasible way that most ascoglossans can maintain populations in reef environments. RELATIONSHIP OF ALGAL MORPHOLOGY AND PHYSIOLOGY TO ASCOGLOSSAN DIET: Plastid morphology has been identified as one factor limiting the occurrence of kleptoplastids. Apparently, only "robust" plastids, generally spheroid in shape and usually occurring in coenocytic (siphonaceous) algae (Hinde and Smith, 1974), are able to survive ingestion and phagocytosis by ascoglossans. Plastids of septate algae (Chaetomorpha, Cladophora) used as ascoglossan foods are in contrast parietal, netlike, or fragmented in shape (Prescott, 1968), fragile, and break during ingestion. The functional basis for the robust nature of siphonalean plastids has not been de- fined. Their shape and size, however, are convergent with those of erythrocytes among a range of animal species, and we suggest that the shape and robustness of such plastids represent necessary adaptations to shear forces resulting from fluid transport in the cytoplasm of coenocytic algae, or, alternatively, that cytoplasmic streaming creates a less- controlled, less predictable intracellular environment that re- quires resistant plastid membranes. Cytoplasmic streaming movements occur in Siphon- ales (Dawes and Barilotti, 1969), and the observed rapid up- take and transport of sedimentary nutrients by such algae (Williams, 1984) would seem to require large scale circulatory movements of cytoplasm (macrocyclosis). Further, this would explain the ecological dominance of siphonalean algae in oligotrophic environments, as sediments represent a nutrient sink and source of nutrient fixation unavailable to algae that lack rhizoidal uptake and coenocytic structure. Thus, the siphonalean algae often occupy a sediment-extractive niche similar to that of seagrasses, and several dominant siphonalean genera normally co-occur with seagrasses (Taylor, 1960). The xanthophyte genus Vaucheria is also sediment-associated, siphonaceous in structure, and sup- ports Septoplasty (Graves ef a/., 1979). The simpler, less-robust plastid membrane of non- siphonalean chlorophytes could also represent adaptation to higher external nutrient levels, in that membrane simplifica- tion would facilitate exchange of nutrients and permit higher plastid metabolic rates in situations where nutrient availability is relatively non-limiting (high latitude, eutrophic or meso- trophic habitats during vernal nutrient peaks). The growth strategy of siphonalean algae involves a

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