Evidence for episodic recruitment in a small opheliid polychaete species from the abyssal NE Atlantic

The abundance and size spectra of an infaunal opheliid polychaete species was followed over a two year period (September 1996-October 1998) in meiofaunal-(32 mum-1 mm) and macrofaunal -(> 250 mum) samples collected at an abyssal site (4850 m depth) in the NE Atlantic. The site, situated on the Porcupine Abyssal Plain (PAP), is characterised by the episodic deposition of aggregated phytodetritus. The response of the fauna to this seasonal food supply was addressed by time-series sampling within the MAST-III BENGAL programme. In autumn 1996, small opheliid juveniles (mean length: 281 mum in September and 254 mum in October) were sampled only in the meiofauna samples. In March 1997, juvenile specimens of the opheliid, which were on average nearly twice as large (mean length: 480 mum) as those collected in 1996 were sampled in both both meio- and macrofaunal samples. The occurrence of only small juvenile individuals in 1996 suggests that a synchronous recruitment event had taken place earlier during that year. Small juveniles (mean length: 252 mum) were also abundant in a sample collected at the PAP site in May 1991, immediately following the deposition of a pulse of phytodetritus. The opheliid population structure in 1997 and 1998 indicates the slow progression of the settled cohort, possibly supplemented by a further, but relatively minor recruitment event in March 1998. Size spectra analysis implies that either growth was slow or that immigration of larger juveniles had augmented the population. The PAP opheliid may be an opportunist, which waits for optimal conditions before converting its slowly accumulated energy into reproduction. In addition, this species can apparently maintain a stable pool of developing juveniles if the organic pulse fails to materialise. The present study also shows that a more holistic approach is necessary to investigate the life cycles of some organisms, which lie close to the boundary between the meiofauna and macrofauna. (C) 2001 Elsevier Science Ltd. All rights reserved.