Population biology, pre-settlement processes and recruitment in the New Zealand sea urchin, Evechinus chloroticus Valenciennes (Echinoidea: Echinometridae)

Abstract

The sea urchin, Evechinus chloroticus Val. (Echinoidea: Echinometridae) is widely distributed throughout New Zealand and is an important invertebrate grazer within kelp forests. E. chloroticus has a planktotrophic larva with a pelagic existence ranging from 21 to 60 days.

Three questions were asked in this study. How do pre-settlement processes affect the distribution and abundance of E. chloroticus larvae? How does the supply of competent larvae influence recruitment in E. chloroticus? Lastly, what is the importance of recruitment in regulating populations of E. chloroticus (that is, are populations recruitment-limited or resource-limited)?

The first question was investigated by examining reproductive cycles (gamete production), larval distribution and transport, and the growth and mortality of E. chloroticus larvae. To address the second question, the abundance of competent E. chloroticus larvae was quantified using artificial settlement samplers, which were later correlated with recruitment patterns. Thirdly, the importance of recruitment in regulating population dynamics in this species was quantified by examining rates of recruitment, and post-settlement growth and mortality in E. chloroticus populations. The basis of this study was to compare the population biology and pre-settlement processes in two E. chloroticus populations that differed both in latitude and in the hydrography of the area in which they occurred, (Doubtful Sound, Fiordland and Tory Channel, Marlborough Sounds).

Recruitment of Evechinus chloroticus was found to be greater in Doubtful Sound and the population was not recruitment-limited over the three-year period, however the Tory Channel population was recruitment-limited over the study period. This difference was attributed to a greater supply and settlement of competent larvae in Doubtful Sound compared to Tory Channel. The greater supply of larvae into the Doubtful Sound population may, in turn, be the result of restricted larval transport in the fiord, which removes much of the variability in larval supply associated with long-range larval transport. The Tory Channel may be an example of an open population. Inter-annual differences in recruitment in each population may be due to variability in the number of larvae reaching competency, a consequence of inter-annual differences in the rate of larval development and/or mortality.