Supply of gastropod larvae to hydrothermal vents reflects transport from local larval sources

Variation in larval supply to disjunct marine populations can provide insight into larval transport and delivery mechanisms, especially when compared with observations of physical transport. Daily variability in larval supply at two mussel-dominated hydrothermal vents, East Wall and Choo Choo, near 9u509N, East Pacific Rise, was quantified concurrently with hydrodynamic observations to investigate local dispersal processes. The magnitude and temporal variation in supply differed between the two vent sites despite their close proximity, 1.6 km. Larval supply was relatively high and uninterrupted at East Wall compared to low and episodic at Choo Choo. Observed variation in larval supply was compared to predictions based on advective transport from larval sources at neighboring vents. Variation in larval supply at Choo Choo correlated with along-axis southward currents, consistent with larval transport from a northern larval source. Larval supply to East Wall appeared to be independent of current velocities, suggesting that larvae came from multiple sources north of and south of and possibly including East Wall. Transport of larvae from discrete local sources can explain differences in larval supply to vent communities, even on spatial scales of kilometers. Understanding the biological and physical transport mechanisms involved in the dispersal, retention, and delivery of larvae is essential to the study of marine benthic population dynamics and structure, gene flow, and connectivity. Larval supply contributes to shaping population and community structure and dynamics and the strength of postsettlement benthic interactions: competition, facilitation, and predation (Gaines and Roughgarden 1985; Menge 1991; Underwood and Keough 2001). The timing and spatial extent of larval supply to benthic habitats can be governed by physical processes. Recent research has focused on larval delivery on tidal scales (Roegner 2000; Underwood and Keough 2001), especially in regard to bay–ocean exchange. However, larval delivery has also been correlated with wind-driven and mesoscale events, such as wind reversals (Epifanio 1995; Bertness et al. 1996), storms (Reed et al. 1988; Etherington and Eggleston 2000), upwelling relaxation (Roughgarden et al. 1988; Shanks and Brink 2005), and eddies (Rodrı́guez et al. 2004; Sponaugle et al. 2005). In many of these studies, correlations between observed physical transport and larval supply have provided insight into larval transport and delivery mechanisms. Disturbed and dynamic communities, such as hydrothermal vents, may be particularly sensitive to variation in larval supply. Hydrothermal vents along the East Pacific Rise (EPR) undergo successional processes (Shank et al. 1998) reset by eruptive and/or tectonic events on a decadal time scale (Fornari and Embley 1995; Tolstoy et al. 2006). Initial colonization of nascent vents after a disruptive event will be determined, in part, by the availability and delivery of larvae. Larval supply in conjunction with the environment and benthic interactions determine the community composition and the successional sequence. While environmental effects (Shank et al. 1998; Luther et al. 2001) and benthic interactions (Mullineaux et al. 2003) have been studied on the EPR, the role of larval supply remains largely unknown because of infrequent larval collections and a poor understanding of larval dispersal and delivery. Larval transport and recruitment processes at vents along the EPR have been inferred from only a few physical and biological studies. Physical observations suggest that passive advective transport in currents would carry larvae tens to hundreds of kilometers within a 30-d larval life span (Chevaldonné et al. 1997; Marsh et al. 2001). Population genetic studies also suggest extensive dispersal over 1 Corresponding author ([email protected]).