Abstract
The salinity tolerance of a species has been shown to differ among ages and in different stages of the life cycle. The sea urchin Evechinus chloroticus (Val.) is a common grazer of the intertidal and subtidal areas around New Zealand, including Doubtful Sound. A persistent surface layer of low salinity in Doubtful Sound overlies deep basins in which adult E. chloroticus live. The planktotrophic larvae of E. chloroticus are the main dispersive stage of this species.
Three questions were posed in this study: do E. chloroticus larvae vertically distribute themselves differently in Doubtful Sound due to the Low Salinity Layer (LSL); what affect does low salinity have on E. chloroticus larval development; and do embryos and adult E. chloroticus from geographically separated populations within Doubtful Sound have differing tolerances to low salinity?
The first question was investigated through examination of the vertical distribution of Evechinus chloroticus larvae in Doubtful Sound. The second question was explored through a series of experiments analyzing the development of E. chloroticus embryos and larvae reared in salinities ranging from 5-35‰. Resolving the third question required the observation of the embryonic development of E. chloroticus in salinities from 5-35‰, as well as monitoring the reactions of E. chloroticus adults placed in diluted seawater (25-34.6‰). The purpose of this study was to investigate whether E. chloroticus was tolerant to reduced salinities as the environment in which certain populations live (Fiordland, Doubtful Sound) contains low salinity conditions which may be detrimental to survival and development.
Larvae of Evechinus chloroticus were never found within the LSL. Larvae were found in abundance from 6-8m depth, a .layer close to the halocline. Experimentally, development of E. chloroticus was incomplete in salinities lower than 27.5‰. Embryos lysed in salinities similar to the LSL indicating extreme sensitivity at this critical life-stage. Larval growth experiments revealed that even higher salinities (27.5‰ and 30‰) were detrimental. Adult E. chloroticus were less tolerant of low salinities than their larvae. Prolonged exposure to 25‰ and 27.5‰ caused mortality of all adults. The location from which a subject was either taken (adult experiments) or derived (egg experiments) was not a significant factor in its salinity tolerance. The later stages of the E. chloroticus larvae are proposed to be the most tolerant to reduced salinities, lending themselves well to dispersal by means of the LSL.