Life on glass houses: sponge stalk communities in the deep sea

Photographs of the deep-sea floor often show organisms attached to biogenic structures that protrude from the soft bottom. In particular, the stalks o f glass sponges (hexactinellids) provide hard substrata and act as habitat islands for deep-sea fauna. The primary objectives of this study were to de termine the abundance of glass sponge "stalks" at an abyssal station in the NE Pacific, to identify the fauna associated with stalks, and to compare t he distribution patterns of epifaunal taxa both horizontally and vertically . Densities of stalks and large epifauna were estimated from analysis of si milar to9 km of photographic transects taken in 1994-1995 at station hi (34 degrees 45'N; 123 degrees 00'W; 4,100 m depth) off California, USA. At lea st 87% of the stalks were the spicule columns of live or dead hexactinellid s in the genus Hyalonema (Gray, 1832). Stalks appeared to be distributed ra ndomly across the sea floor (density: 0.13 stalks m(-2)). A colonial zoanth id. Epizoanthus stellaris (Hertwig, 1888), inhabited 20% of the stalks and was the most commonly observed epifaunal organism, followed by other suspen sion feeders that generally were situated at the top of the structures. Thi rty-five stalk communities were collected in tube cores in 1994-1995 using the submersible "Alvin". A total of 139 taxa was associated with these hard -substratum habitats (another five species were observed only in photograph s). Although taxon richness was high, the species diversity of these commun ities was relatively low due to the dominance in percentage abundance of a foraminiferan, Cibicides lobatulus (Walker and Jakob, 1798), and a serpulid polychaete, Bathyvermilia sp. (Zibrowius, 1973). The relationship between number of taxa and surface area of the stalks yielded a slope (z-value) typ ical of islands with a low rate of immigration. Three-dimensional complexit y created by branching epifauna on the stalks provided more surface area an d a variety of cryptic microhabitats. Vertical zonation on the stalks appea red to be controlled by biological interactions among species, with solitar y fauna and certain functional groups of colonial organisms restricted by s heetlike colonial organisms that appeared to be dominant space competitors.