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Abstract

Sponges play an important ecological function in many benthic habitats.
They filter large volumes of water, retain suspended particles with high
efficiency, and process dissolved compounds. Nevertheless, the factors
that regulate sponge pumping rate and its relation to environmental
factors have been rarely studied. We examined, in situ, the variation of
pumping rates for five Mediterranean sponge species and its relationship
to temperature, particulate food abundance and sponge size over two
annual cycles. Surprisingly, temperature and food concentration had only
a small effect on pumping rates, and the seasonal variation of pumping
rates was small (1.9-2.5 folds). Sponge size was the main determinant of
the specific pumping rate (pumping normalized to sponge volume or mass).
Within the natural size distribution of each species, the
volume-specific pumping rate [PRV, ml min(-1) (cm sponge)(-3)] decreased
(up to 33 folds) with the increase in sponge volume (V, cm(3)),
conforming to an allometric power function (PRV = aV(b)) with negative
exponents. The strong dependence of the size-specific pumping rate on
the sponge size suggests that the simplistic use of this value to
categorize sponge species and predict their activity may be misleading.
For example, for small specimens, size-specific pumping rates of the two
low-microbial-abundance (LMA) species (allometric exponent b of -0.2 and
-0.3) were similar to those of two of the high-microbial-abundance (HMA)
species (b of -0.5 and -0.7). However, for larger specimens,
size-specific pumping rates were markedly different. Our results suggest
that the pumping rate of the sponges we studied can be approximated
using the measured allometric constants alone in conjunction with
surveys of sponge abundance and size distribution. This information is
essential for the quantification of in situ feeding and respiration
rates and for estimates of the magnitude of sponge-mediated energy and
nutrient fluxes at the community level. Further work is required to
establish if and to what extent the low seasonal effect and the strong
size dependency of pumping rate can be generalized to other sponges and
habitats.