Development in the Floating World: Defenses of Eggs and Embryos Against Damage from UV Radiation

Eggs and embryos of many aquatic organisms develop in the watercolumn and can experience ultraviolet radiation with potentiallydeleterious effects. This is especially vexing for floatingembryos that develop in the surface or neuston layer. Radiationdamage can be a particular problem for these embryos since thecell division cycle during the cleavage period is quite shortand often these cycles do not have mitotic checkpoints to insurefaithful transmission of DNA to the daughter cells. This couldresult in cell division with unrepaired DNA in the blastomeres,which could impact embryogenesis and the transmission of thegenome through the germ line. Described strategies to restrictradiation damage include mechanisms to limit oxidative damageand the use of sunscreens such as the mycosporines to curb radiationto sensitive targets. We describe a particularly ingenious useof sunscreens in the tunicate embryo, the use of extra-embryoniccells to shield the embryo from potentially harmful UV-A andUV-B radiation. We also raise questions regarding the natureof UV damage to embryos (is it DNA or also protein) and thecharacteristics of DNA repair in such embryos. It is likelythat unique mechanisms are present in floating embryos thatdevelop in this air-water interface to assure that cell andgenomic integrity are maintained in this challenging environment.     

Asexual Reproduction, Population Structure, and Genotype-Environment Interactions in Sea Anemones

Our studies of sea anemones reveal that asexual reproductioncan lead to the amplification of particularly successful genotypesPopulations of Haliplanella luciae studied to date are characterizedbv exclusively asexual reproduction and typically are dominatedby one or a few clones A field translocation experiment suggeststhat this population structure may result from differentialmortality among colonizing clones most of which are not preadaptedto local conditions Asexual reproduction by survivors leadsto extensive multiplication of one or a few genotypes Metridiumsenile reproduces sexually and asexually and we ofler evidencethat there is significantly less vegetative proliferation butlarger individual body size in areas of low tidal current velocitythan in areas of moderate to high velocities This may indicatethat small individuals (produced asexually) are at a particularfeeding disadvantage in slowly moving water leading to an emphasison maintaining individual body sizeat the expenseol asexualleproduction Individuals heterozygous fora phosphoglucose isomerase(PG1) locus appear to be more successful than homozygotes inmaximizing body size independent of current regime and in maintaininglarge clone biomass in low velocity habitat Members of heterozygoteclones are significantly more dispeised some clonemates beingseparated by 9 meters or more and are overrepresented in thelow velocity habitat Selection against (small) homozygotes activechoice of habitat, and passive differential dispersal of larvaeand adult anemones may all contribute to this pattern.     

Effects of ultraviolet radiation on corals and other coral reef organisms

The discovery of the importance of solar ultraviolet radiation (UVR) as a factor affecting the biology of coral reefs dates only to about 1980. Interest has heightened during the past five years owing to the demonstration of loss of stratospheric ozone through human activities. We have only begun to document gross, qualitative effects of UVR on coral reef organisms, usually in experiments comparing the biological response to the presence or absence of UVR through the use of UV-cutoff filters, or to varying levels of UVR in transplantation studies. Most such studies have not distinguished between the effects of UVA (320–400 nm) and those of UVB (290–320 nm), although in the context of global change involving stratospheric ozone loss, it is the latter wavelengths that are relevant. To date we have been addressing physiological and ecological questions, not yet attempting to evaluate quantitatively the impact of forecast increases in solar UVB penetration. Interacting and synergistic effects of UVR with increased temperature, pollutants, sedimentation, visible light, etc. have scarcely been studied but will be essential to understanding and predicting the fate of coral reefs under conditions of global change. Here we comprehensively review the effects of UVR on corals and other reef macroorganisms, mindful that although much is known of proximal effects, little of this knowledge is directly useful in making long-term predictions regarding the health of coral reefs. We conclude that even small anthropogenic increases in UVB levels will have sublethal physiological manifestations in corals and other reef organisms, but that this will have relatively small impact on the distribution of reef corals and coral reefs, perhaps affecting their minimum depths of occurrence.     

Calorimetric Studies of Behavior, Metabolism and Energetics of Sessile Intertidal Animals

SYNOPSIS: Behaviors to conserve water during intertidal exposureat the same time impair respiratory gas exchange, so that observedresponses to emersion may reflect compromises between theseincompatible needs. Behavioral isolation of the tissues fromair results in the complete or partial reliance on anoxic energymetabolism, which is most reliably measured directly as heatdissipation. Combined direct calorimetry and indirect calorimetry(respirometry) enable the partitioning of total metabolic heatdissipation into its aerobic and anoxic components, which mayvary according to physical and biological factors. The musselMytilus edulis is tolerant of anoxia and saves water and energyduring aerial exposure in its rocky intertidal habitat by closingits shell valves and becoming largely anoxic. Like most suspensionfeeders in this habitat, its compensation for reduced feedingtime involves energy conservation; there is little evidencefor energy supplementation such as increases in feeding rateor absorption efficiency. Ammonia production continues duringaerial exposure and is involved in acid-base balance in thehemolymph and mantle cavity fluid. Infaunal cockles (Cardiumedule) and mussels (Geukensia demissa) gape their shell valves,remain largely aerobic and have high rates of heat dissipationduring intertidal exposure, a response which appears relatedto the lower desiccation potential and exploitation of richertrophic resources in their soft-sediment habitats. The variableexpansion of the symbiotic sea anemone Anthopleura elegantissimareflects interaction among the responses to desiccation, irradianceand continued photosynthesis by its zooxanthellae during exposureto air.