Sponge diversity and community composition in Irish bathyal coral reefs

Sponge diversity and community composition in bathyal cold
water coral reefs (CWRs) were examined at 500-900 m depth
on the southeastern slopes of Rockall Bank and the northwestern
slope of Porcupine Bank, to the west of Ireland in 2004 and 2005
with boxcores. A total of 104 boxcore samples, supplemented
with 10 trawl/dredge attempts, were analyzed for the presence
and abundance of sponges, using microscopical examination of
(sub)samples of collected coral branches, and semi-quantitative
macroscopic examination. Approximate minimum size of identifi
ed and counted sponge individuals was 1 mm. Literature data
were added to the Porcupine Bank results to compensate for a
less intensive sampling program in that location. Species richness
and abundance were determined at local (sample diversity,
pooled-sample diversity, local reef diversity), between-reef
(diversity of two reef areas at 15 km distance), and regional
scales (diversity of three reef areas over a distance of 200 km).
Abiotic and biotic parameters including depth, the presence and
cover of live coral, dead coral and sand, local reef, and orientation
towards the nearest reef mound summit, were included in a
constrained ordination technique (RDA); a Monte Carlo forward
selection procedure was used to obtain signifi cant predictors of
variation in composition. The results of this analysis were compared
with unconstrained ordination (PCA) and cluster analysis.
The presence of live coral, depth and the local reefs C1 and C3
proved to be signifi cant predictors of variation in sponge composition.
The PCA and cluster analysis confi rmed these results.
Sample species richness was consistently heterogeneous from
zero species and individuals up to 57 species and 90 individuals
per (boxcore) sample. Species richness of local reefs determined
from pooled samples showed the three localities studied to have
similar species richness, namely 105-122 species in each location.
Species richness was highest in samples with relatively low live
coral cover. As in the RDA, live coral presence and depth appeared
to be responsible for most of the variation observed in
the cluster results. Cluster analysis of Bray-Curtis dissimilarity
values of the pooled samples of all three reef localities using
presence / absence data of all available samples indicated that
distance appeared to structure the composition of the sponge
assemblages of the three reef mound areas, but much less so
within and among local reefs. Bathyal reefs of the regions to the
west of Ireland were found to have a combined sponge species
richness of 191 species, exceeding the richness of individual reef
mound areas by c. 38-45%. Sponge presence in CWRs is
clearly structured and controlled by biotic and abiotic factors.
In particular, live coral presence appears a signifi cant predictor
of CWR sponge composition and diversity.