RESEARCH ARTICLE E. Ramirez-Llodra Æ W. D. K. Reid Æ D. S. M. Billett Long-term changes in reproductive patterns of the holothurian Oneirophanta mutabilis from the Porcupine Abyssal Plain Received: 30 April 2004 / Accepted: 7 September 2004 / Published online: 14 October 2004 Ó Springer-Verlag 2004 Abstract Time-series studies (1989–2002) in the NE Atlantic have shown large-scale changes in the compo- sition and structure of the benthic community on the Porcupine Abyssal Plain. Radical changes in the abun- dance of some species in 1996 led to a significant shift in the way in which organic matter was reprocessed at the seabed. This article examines the reproductive processes of the holothurian Oneirophanta mutabilis collected during the time series. The reproductive biology of O. mutabilis is reviewed. No males were evident in any of the samples. The sex ratio (females: ‘no gender’) was biased significantly towards no-gender individuals. The maximum egg size was 650 lm and there was no evi- dence of synchrony in reproduction. Significant changes in the oocyte-size distribution and the fecundity of O. mutabilis were noted through time, coinciding with the time of greatest faunal change in the benthic community. There was an increase in the proportion of previtello- genic oocytes and a decrease in the proportion of mature vitellogenic oocytes in 1997 and 1998, in parallel with a significant decrease in fecundity of the post-1996 sam- ples. Samples from 2002 showed a reversal in the reproductive trends, with an increase in the proportion of mature vitellogenic oocytes and fecundity. The results are discussed in relation to large changes in abundance of the epibenthos on the Porcupine Abyssal Plain. It is suggested that the superabundance of certain megafa- unal species on the abyssal seafloor affected the avail- ability of trophic resources for O. mutabilis, leading to the changes in its reproductive effort. Introduction With the exclusion of chemosynthetic ecosystems, such as hydrothermal vents and cold seeps, the deep-sea benthic fauna depends ultimately on the input of sur- face-produced organic matter. Under areas of high surface production, large, seasonal fluxes of organic matter couple the surface water and deep-sea benthic ecosystems (Billett et al. 1983; Lampitt 1985; Rice et al. 1986, 1991, 1994; Thiel et al. 1990; Smith et al. 1994; Smith et al. 1996; Baldwin et al. 1998; Beaulieu and Smith 1998; Lampitt et al. 2001; Beaulieu 2002). Because the rapid sinking of phytodetritus prevents its complete utilisation by upper-ocean pelagic grazers, the arrival of this organic matter provides deep-sea communities with seasonal, high-quality food resources (Gooday and Turley 1990; Beaulieu and Smith 1998; Ginger et al. 2000; Billett et al. 2001; Ramirez-Llodra et al. 2002; Wigham et al. 2003a; Ruhl and Smith 2004). Over the last few decades, the northeast Atlantic abyssal plains have been the focus of several multidis- ciplinary research programmes (Roe et al. 1987; Sibuet et al. 1993; Rice et al. 1994; Thurston et al. 1994; Billett and Rice 2001). In particular, the Porcupine Abyssal Plain was studied by the BENGAL project (high-reso- lution temporal and spatial study of the BENthic biol- ogy and Geochemistry of a north-eastern Atlantic abyssal Locality; Billett and Rice 2001). This project showed that the vertical flux of organic matter to the Porcupine Abyssal Plain seafloor varies on seasonal and inter-annual time scales, in terms of both quantity and quality (Witbaard et al. 2000, 2001; Kiriakoulakis et al. 2001; Lampitt et al. 2001). In addition, major long-term changes in the structure and composition of benthic invertebrate megafauna were documented (Billett et al. 2001). The megabenthos of the Porcupine Abyssal Plain comprises 17 major taxa, with holothurians dominating the invertebrate megafauna in abundance (81%) and biomass (91%). Actiniarians, annelids, asteroids, and Communicated by J.P. Thorpe, Port Erin E. Ramirez-Llodra (&) Æ W. D. K. Reid Æ D. S. M. Billett Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, UK E-mail: ezr@soc.soton.ac.uk Fax: +44-2380-593052 Marine Biology (2005) 146: 683–693 DOI 10.1007/s00227-004-1470-z