MORPHOLOGICAL AND PHYSIOLOGICAL-RESPONSES OF ECHINODERM LARVAE TO NUTRITIVE SIGNALS

Echinoid larvae have been found previously to develop shorter arms as phytoplankton concentrations increase. In the present study, the skeletal dimensions of larvae of the sea urchins Lytechinus pictus and Strongylocentrotus purpuratus were measured after exposure to dissolved organic compounds in seawater. The presence of glucose or individual amino acids (at 1 or 2 mu M), a mixture of 16 different amino acids (100 nM each), or algal exudate resulted in larvae with shorter arms (by 12 to 88%), relative to larvae in seawater with no additions. Larvae exposed to mixtures of amino acids also had changes in the constituents of their internal free amino acid pools (as determined by HPLC). For another echinoid, Dendraster excentricus, amino acid transport (T, from 500 nM) by individual larvae (n = 47) scaled to arm length (L) as follows: T = 2.06L(0.81) (0.81 +/- 0.26, 95% confidence intervals). Mass (M) and metabolic rate (MR) did not scale in the same manner (as transport) to arm length for larvae of the echinoid Centrostephanus coronatus (MR = 148L(1.51); M = 1.7L(2.01)). These characteristics may scale to arm length independently from transport rate under certain conditions. Larvae of echinoderms respond morphologically to ''signals'' in their environment that may indicate the availability of dissolved and particulate nutrients. This in turn will have consequences for their ability to take up and metabolize these nutrients. The response may be mediated by surface receptors for dissolved organic compounds, or by changes in the sizes of intracellular substrate pools.