Larval dispersal between hydrothermal vent habitats

Abstract

Hydrothermal vents are isolated, short-term habitats that support unique biotic assemblages with relatively high biomass utilizing an unusual energy source. How these communities establish themselves and maintain species identity despite their isolation and impermanence is a significant question in vent ecology. Planktonic larval forms provide a dispersive life stage for many species, but the exact mechanism and controls of dispersal are unknown. A description of larval distribution patterns, for which reliable identification of vent larvae is necessary, is critical to understanding larval dispersal. In gastropod species, the retention of the larval shell on the adult allows identification without the necessity of culturing larvae or using species-specific molecular probes, both techniques that are still difficult with vent organisms. At one site (9°N, East Pacific Rise), eleven species of vent gastropod larvae were identified from the water column up to 200 m off the seafloor. The species-specific level of these identifications is important, as there may be species-level differences in species distributions that influence dispersal. Finding vent larvae high in the water column above the source populations suggests that some mechanism must exist to raise larvae off the sea floor. Literature on related species indicates that larval behaviors such as swimming or buoyancy control could not be responsible for such significant vertical movement over a reasonable time period. Passive movement via entrainment into buoyant plumes rising from smoker vents provides a pathway for larvae to get high above the bottom, and has implications for larval dispersal because vertical shear in flows above vents can cause trajectories in the plume to deviate considerably from those along the seafloor. The distance that vent communities extend around smokers is limited, so 28-97% of the larvae produced may be entrained into the rising plume. Buoyant plumes have the potential to transport a substantial proportion of the larvae produced by hydrothermal vent communities. Larvae may follow several different paths in dispersing between vents. Near-bottom flows tend to be topographically constrained to parallel the ridge crest, and advection in near-bottom currents is one potential larval dispersal mechanism. Entrainment into a buoyant plume and lateral advection hundreds of meters above the sea floor is a feasible alternative or additional dispersal mechanism. However, in deep-sea habitats it is difficult to directly test these hypotheses. Ecological modeling offers an alternative to experimental approaches to hypothesis testing. By creating a realistic model simulation, insight can be gained into which factors: vent spacing and instability (geological); flow regime (physical); or fecundity, larval mortality, and adult maturation time (biological), most strongly influence the patterns of species distributions along mid-ocean ridges. The results of model simulations suggest"that long distance dispersal (as might be provided by plume flow) is vital to long-term persistence of vent populations, and that fecundity and larval mortality interact with habitat spacing and vent lifespan to influence the stability of the overall population. Larval transport, establishment of vent communities and specie$ persistence are key factors in hydrothermal vent ecology. This thesis examines the potential for larval dispersal and species survival under various flow regimes, and identifies and observes the distribution of vent larvae in the water column. Its significance lies in the potential to further understanding of endemic species dispersal in a patchy, ephemeral habitat.