Long-term and seasonal changes in the life-history biology of the abyssal holothurian Pseudostichopus aemulatus from the Porcupine Abyssal Plain (North-East Atlantic)

Abstract

Long-term interannual variations in the benthic community structure are well-known from abyssal plains in the North Atlantic and North Pacific, where rapid responses to changes in the environment by first-order opportunists modify overall species composition. To increase our knowledge of the long-term variations of deep-sea benthic communities in the North-East Atlantic, we studied the response of a second-order opportunist echinoderm, the holothurian Pseudostichopus aemulatus, before, during and after the “Amperima Event” – a four-year episode (1996–1999) where the first-order opportunistic holothurian, Amperima rosea, increased dramatically in density. We used a collection of 14 sampling times of deep-sea community assemblage over a sixteen-year period to understand the temporal variation (i.e., interannual and seasonal) in population structure (i.e., density and individual sizes) and reproduction (i.e., mean oocyte sizes, sex ratios, gonadal indices, and fecundity) in P. aemulatus. Samples were collected from the Porcupine Abyssal Plain (PAP) in the NE Atlantic Ocean at a depth of ca. 4850 m between 1989 and 2005 with a semi-balloon otter trawl (OTSB14) and an epibenthic sledge (BN1.5/C) at varying seasons. We measured individual body size, dissected organisms to prepare histological slides of their gonads, and measured the Feret oocyte diameter in females. We found changes in the life-history biology of P. aemulatus during the “Amperima Event”. Population density increased gradually from September 1997 until April 1999, when P. aemulatus reached a peak in density coinciding with the smallest mean body size observed (1998 and 1999) suggesting recruitment of juveniles. In 1997, before these changes in population structure, mean oocyte sizes were at their largest, suggesting a strong reproductive effort with individuals either preparing to spawn or spawning. This reproductive effort was minimal in the years after 1999, when we found the largest proportions of previtellogenic oocytes in individuals that were probably allocating their energy into growth rather than reproduction given their smaller size. The presence of gametes of all sizes across seasons and a broad range of individual sizes in various sampling times suggests no seasonal reproductive patterns. We suggest that feeding guild differences between various holothurian species explain the diverse life-history responses observed among holothurians competing for resources in a food-poor environment.