| Pelagic marine environments are highly dynamic and pose grave threats to small-bodied planktonic organisms. Nonetheless, more than 70% of benthic marine animals possess a planktonic larval stage that can last for days, weeks or years. Larvae experience unstable food supplies, threats from predator attack, and variable physical conditions. Using sea urchins---a model organism for developmental biologists since the mid-nineteenth century---I studied the effects of each of these three broad categories of environmental stress on larval development. The effect of food supply on larval development was investigated at two extremes: super abundant food and no food. When larvae of S. purpuratus, and S. droebachiensis were cultured with a dense, mixed-algal diet, larval cloning---a previously unrecognized phenomenon among echinoids---could be induced. Conversely, when S. purpuratus larvae were starved they appeared to reabsorb their guts to sustain the rest of the larval body until feeding resumed and they recovered. The effects of sub-lethal predator attack were investigated by fracturing the feeding arms of S. droebachiensis larvae. Fractured arms were repaired in three different ways: (1) degeneration followed by regeneration, (2) autotomy followed by regeneration, or (3) re-alignment and fusion of the fractured ends of the skeletal rod. All repairs were completed within 48 hours and neither the incidence, nor frequency of fractures affected the time to metamorphosis. Finally, the effects of variable oceanographic conditions on early development, the rates of fertilization, hatching, and gastrulation under different temperature and salinity combinations were studied in three congeneric species of sea urchins from a sympatric population. Embryos of Strongylocentrotus franciscanus tolerated a broader salinity range than embryos of S. droebachiensis and S. purpuratus , and all embryos tolerated a broader salinity range at lower temperatures. Overall, the great regenerative capacity of echinoid larvae makes them extremely resilient to many forms of environmental stress. Moreover, the developmental plasticity of echinoid larvae is significantly greater than previously believed, and may have been key to their evolutionary persistence for half a billion years. |
