Morphological and ecological parallels between sublittoral and abyssal foraminiferal species in the NE Atlantic: a comparison of Stainforthia fusiformis and Stainforthia sp.

Dead specimens of a minute fusiform rotaliid foraminifer are common in the 28-63 mum fraction of multiple corer samples from a 4850 m-deep site on the Porcupine Abyssal Plain (PAP). Their test morphology is remarkably similar to small specimens of Stainforthia fusiformis (Williamson, 1858), a specie s which is well known from coastal settings (intertidal to outer shelf) aro und NW Europe and North America. A detailed comparison of the PAP form with typical individuals of S. fusiformis from Norwegian waters (55-203 m depth ), however, reveals slight but consistent morphological differences. The PA P specimens are smaller (test length 40-140 mum) than those from Norway (te st length 80-380 mum), the chambers tend to be rather less elongate, the de nsity of pores in the test wall is much lower, and there are differences in apertural features. We therefore conclude that the diminutive abyssal form is a distinct species, here referred to as Stainforthia sp. This interpret ation is consistent with increasing evidence for genetic differentiation in deep-sea organisms, particularly along bathymetric gradients. Stainforthia sp. was previously illustrated by Pawlowski as Fursenkoina sp. and appears to be widespread and abundant in the abyssal North. Atlantic (> 4000 m dep th). Stainforthia fusiformis, on the other hand, is most abundant in contin ental shelf and coastal settings. It extends onto the continental slope in the North Atlantic but has not been reported reliably from depths greater t han about 2500 m. We suggest that the striking morphological convergence between these two sp ecies reflects the adoption of similar ecological strategies in widely sepa rated habitats. Both are enrichment opportunists, a life-style which may ex plain the rather broad bathymetric range of Stainforthia fusiformis. This i s a dominant species in organically-enriched and sometimes extremely oxygen -depleted environments on the continental shelf, and is a rapid coloniser o f formerly azoic habitats. Live specimens of the abyssal form are typically found embedded within phytodetrital aggregates (organic material derived f rom primary production in the euphotic zone). It is presumably the availabi lity of these organic-rich microhabitats, which enables this species to sur vive in the otherwise oligotrophic deep sea. (C) 2001 Published by Elsevier Science Ltd.