Population structure of two deep-sea hydrothermal vent gastropods from the Juan de Fuca Ridge, NE Pacific

The gastropods Lepetodrilus fucensis and Depressigyra globulus are abundant faunal components of animal communities at deep-sea hydrothermal vents along the Juan de Fuca Ridge in the NE Pacific. The population structure and recruitment pattern of both species were studied using modal decomposition of length-frequency distributions. Gastropod populations were collected from Axial Volcano and Endeavour Segment in 2002 and 2003. Polymodal size-frequency distributions, particularly at Axial Volcano vent sites, suggest a discontinuous recruitment pattern for D. globulus. In contrast, there were no distinct peaks visible in the distributions of L. fucensis, suggesting a continuous recruitment pattern for this species. For both species, distributions were positively skewed towards the smaller length-classes, implying post-settlement mortality is high. However, variations in growth, due to short- and long-term variability in environmental conditions in the hydrothermal vent habitat, as well as biological interactions, may also be influencing the distribution and abundance of subsequent life-history stages. Using maximum shell lengths from populations of known ages, the growth rate of L. fucensis was estimated as 9.6 mu m day(-1), indicating adulthood would be reached in similar to 1 year. Our results suggest that, despite occupying the same habitat, abundance and population structure are regulated by different biotic and abiotic processes in L. fucensis and D. globulus.