Wax-ester reserves facilitate dispersal of hydrothermal vent shrimps

Late zoeal and early postlarval stages of vent shnmp (Rimcans exoculata, Chorocans chacei and Alvlnocarls rnarkensls), sampled from the water column above the Broken Spur vent site (mid-Atlantic Ridge), contained large amounts of bright orange coloured lipid in their thoracic and abdominal regions. The lipid compnsed 75 to 8 2 % wax ester, containing predominantly 16:O and 18:l fatty alcohols. In contrast, wax esters were found to be absent in samples of adult vent shrimp (R. exoculata and A. markensis) Substantial wax-ester reserves are a recognised adaptation to a prolonged bathypelagic, planktotrophic existence in deep-sea organisms where food is scarce and they have to survive extended periods without feeding This characterist~c is clearly of benefit to larval vent shnmp, which appear to be the main dispersal phase in the llfe history of these organisms, whose adult stages inhabit a stnkingly ephemeral environment whlch is discontinuous in both space and time. K E Y WORDS: Hydrothermal vent shrimps . Larval dispersal. Wax esters Although various aspects of the ecology and the trophic dynamics of vent shrlmp have been addressed (Van Dover 1995), fundamental questions, such as the mechanism of their population dispersal, remain unresolved. To address this issue we analysed the lipids (which can give valuable insights into the life history strategies of marine organisms; Sargent & Henderson 1986) of late zoeal stage and early postlarval stage bresiliid shrimp (16 to 22 mm) sampled from between 200 and 1000 m above the seabed in a hydrothermally active segment of the mid-Atlantic Ridge. Surfaceoperated nets (RMT1+8) were trawled within the Broken Spur segment (latitude 29" 10 'N) during RRS 'Charles Darwin' cruise 95 (chief scientist: B. J. Murton) in August 1995 (Hernng 1996). The shrimp larvae (Rimicaris exocula ta, Chorocaris chacei and Alvinocans markensis; Dixon & Dixon 1996) all contained substantial orange-pigmented lipid reserves in their thoracic region and in a vacuole located beneath the anterior part of the abdomen. High performance thin layer chromatography indicated that these lipld vacoules comprised 75 to 82 % wax esters. Wax esters were found to be absent in specimens of adult R. exoculata and A. markensis (no samples of adult C. chacei were avadable for analysis). Genetic analysis of adult Rimicaris exoculata from the TAG and Broken Spur vent sites has indicated low genetic diversity (Creasey et al. 1996), suggesting an effective mechanism of dispersal, although at which stage, larval or adult, dispersal occurs in these organisms was unclear. As distances between vent fields can be considerable (hundreds of kilometres), this implies a considerable period of time spent away from the vent food source. A characteristic of hydrothermal vents is the high temporal and spatial instability linked to variations in magmatic heat convection and tectonic activity along the ridge axis (Tunnicliffe 1991). Due to thelr nutritional dependence on vent-derived materials, vent organisms live under the constant threat of extinction, at least on a local scale, unless they are able to colonise new vents as these become active and before old vents die. It has been suggested that the cold ambient bottom waters associated with the deep ocean (2 to 3°C) could lower metabolic and developmental rates, enabling the pelagic larvae of vent organisms to remain In the plankton for prolonged periods (Lutz et al. 1984). However, the presence of wax esters in marine zooplankton is characteristic of organisms which inhabit environments subject to marked fluctuatlons in food availability and enables organisms to survive considerable periods without feeding (Lee & 0 Inter-Research 1997 Resale of full article not permitted 290 Mar Ecol Prog Ser 146289-290, 1997 Table 1. Rimicaris ex-oculata, Alvinocaris markinses and Acknowledgements. We thank Dr M. Segonzac, IFREMER, Chorocaris chacei. Fatty acid and fatty alcohol composition Brest, for provld~ng specimens of adult shnmp and Dr M (S$ weight) of total lipid from late zoeal stage and early Samt-Laurent. Museum of Pans, for identifying the larval post-larval stage bresiliid shrimp collected from the water colstages. This work was funded in part by 2 NERC BRIDGE umn above the Broken Spur hydrothermal vent, mid-Atlantic grants (GST/02/1125 and GST/02/1143) and by the EURidge. Only mean (SE) values of major components are MAST 3 programme (AMORES: PL950040). D.P. was supshown for clarity, n 8 ported by an NERC-PRIME grant (GST/02/1075). This IS PRIME contribution number 16 Fatty acid Fatty alcohol 16:O 7 3 (0.8) 47.7 (1.6) 16:l(n-7) 21 0 (2.0) 18:0 1.7 (0.2) 13.5 (1 5) 18:l(n-9) 24.1 (1.5) 18.8 (0.5) 18:l(n-7) 8.8 (0.7) 18.1 (0.7) 18:2(n-6) 2.7 (0.04) 20:4(n-6) 3.0 (0.4) 20:5(n-3) 6.7 (0.8) 22:6(n-3) 12.9 (2.1)