Origin and transport of larvae of the sea urchin Evechinus chloroticus (Echinodermata:Echinoidea) in a New Zealand fiord

Abstract

Larval transport was investigated in a New Zealand fiord by examining the reproductive cycles and larval distributions of the sea urchin Evechinus chloroticus Valenciennes, and the hydrography in Doubtful Sound over 2 sampling periods (SP). These were from 27 September 1993 to 24 March 1994 (SP1993/94) and 27 October 1994 to 11 May 1995 (SP1994/95). Reproductive cycles of E. chloroticus were monitored at 2-weekly or monthly intervals in 3 populations in the fiord, while the transport and development of larvae were examined by plankton sampling every 2 wk at 4 permanent stations inside the fiord, and 1 permanent station outside. To determine water exchange in Doubtful Sound, an S4 current meter was deployed at the entrance of the fiord at 15 m depth. The fiord has an estuarine circulation pattern with a thin low salinity surface layer flowing seaward, below which is a compensatory landward flow of seawater. A major, synchronous spawning of E. chloroticus was observed during SP1993/94 which produced a widespread, single cohort of larvae throughout the fiord. In SP1994/95. spawning was fractional and asynchronous and resulted in 2 cohorts of larvae during the summer. Larval densities were an order of magnitude higher during SP1993/94 (2743.0 larvae tow-') than in SP1994/95 (155.3 larvae tow-'). E. chloroticus larval distribution patterns indicate that there was a high level of larval retention within the fiord, consistent with the estuarine circulation pattern. During SP1993/94. average larval densit.ies 4 wk after spawning ranged from 65.3 to 461.0 larvae tow-' within the fiord. compared to an average of 2.0 larvae tow-' at the outer site. Similarly, during SP1994/95 larval densities were consistently higher within Doubtful Sound than at the outer site. Larvae completed development within 4 to 6 wk in Doubtful Sound. These results suggest that recruitment of E chloroticus In Doubtful Sound is by larvae that have originated, been retained and completed development within the flord Reduced larval exchange provides a mechanism for observed genetic differentiation of this populatlon. K E Y WORDS: Echinoid . Sea urchin . Larval transport . Closed populatlon . Recruitment . Estuarine circulation . Fiord . Larval biology . Evechinus cl~loroticus