Term of Award

Fall 1998

Degree Name

Master of Science

Document Type and Release Option

Thesis (restricted to Georgia Southern)

Department

Department of Biology

Committee Chair

Daniel F. Gleason

Committee Member 1

Stephen P. Vives

Committee Member 2

Sophie B. George

Abstract

Larvae of echinoderm species Echinometra Iucunter, Lytechinus variegatus, and MelIita isometra were reared in up to five different salinities (25, 28, 31, 33, 34 and 37 ppt). Larvae were assessed at several developmental stages for differences in behavioral responses to short-term (less than 1 hour) exposure to lowered salinity, and for differences in morphology in response to long-term (throughout development) exposure to lowered salinity. Questions asked were 1) Do the echinoplutei of stenohaline intertidal echinoderms respond to salinity differences in a stratified water column in a manner that is adaptive for retention near or return to intertidal settlement sites? And 2) Do echinoplutei reared at lower salinities develop a longer and wider body thus allowing them to enhance buoyancy in a low-salinity environment'?

To answer the first question, larvae were assessed at several stages for differences in distribution patterns in single-salinity (rearing salinity) or salinity-stratified (rearing salinity and ± 3 ppt) water columns. Results show significant heterogeneity between distributions in single-salinity v. stratified columns. Regardless of rearing salinity or stage, larvae in stratified columns preferred to settle at rearing or lower salinity, avoiding depths containing higher salinity water in significantly greater numbers than expected. Data show that these larvae can detect salinity differences in the water columns and adjust depth to remain at a preferred salinity. Salinity-cued vertical swimming behavior may help larvae to remain out of low-salinity estuaries and close to suitable settlement sites.

The second question was answered by comparing the length and width of larvae reared at different salinities. Results show that Echinometra lucunter larvae were more likely to develop buoyancy-compensating morphologies during early larval development, while Mellita isometra larvae developed buoyancy-compensating morphologies during later stages. There were no significant differences in Lytechinus variegatus morphologies attributable to rearing salinity. The data show a correlation between buoyancy compensating morphologies and maternal investment in eggs. Buoyancy compensation in low salinity water may help larvae to remain within photic zones during developmental periods when feeding is critical.

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