Temperature adaptations of embryos from intertidal and subtidal sand dollars (Dendraster excentricus, Eschscholtz)
Bingham, Brian L.
Johnson, Leland G.
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Animals living in the marine intertidal zone have a suite of adaptations that allow them to survive thermal stresses associated with low tides. We hypothesized that adjustments of adult organisms to their thermal environment might be reflected in the thermal tolerances of their developing offspring (i.e., adults that survive large temperature fluctuations might produce embryos that have similarly large tolerances). Comparison of the thermal biology of developing sand dollar embryos (Dendraster excentricus) which were offspring of adults living in intertidal or subtidal habitats revealed subtle differences in effects of temperature on survival and developmental rates. Inter-tidal and subtidal D. excentricus showed similar fertilization success at temperatures from 7 degree to 19 degree C and could develop from fertilized egg to pluteus larva at temperatures from 7 degree to 26 degree C. Within this common tolerance range, however, subtidal animals' offspring were more likely to develop normally at lower temperatures (8 degree - 16 degree C), while the intertidal animals' offspring were more likely to develop normally at higher temperatures (22 degree - 26 degree C). There were also significant differences between subtidal and intertidal Dendraster groups in the effects of temperature on developmental rates. In particular, cleavage rates of embryos from intertidal adults remained constant or increased as temperature was raised from 7 to 15 degree C. In contrast, cleavage rates of embryos from subtidal adults decreased with increasing temperature. Ecological observations suggest that these differences are due to physiological acclimatization effects rather than genetic differentiation of the groups. To contrast D. excentricus with a different species that is largely confined to subtidal depths, we also examined temperature tolerances of developing sea urchin embryos (Strongylocentrotus droebachiensis). We found that its temperature tolerance during early development was slightly narrower and shifted to lower temperatures than that of D. excentricus. This correlates well with differences in habitat and reproductive season of these species and may help explain their distributions and abundances