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. Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
˚ and Riisgard, 1992; Cloern, 1996. Lemmens et al. 1996a estimated the biomass and
filtering capacity of several benthic groups in seagrass meadows of Western Australia, and concluded that ascidians and polychaetes are the two groups of macrofauna that
contribute most to community filtering capacity. These groups have been studied further Clapin, 1996; Lemmens et al., 1996b; Lemmens and Petersen, in prep.. Because of
their minute size, epifauna such as bryozoans, spirobids, and cirripeds have often been neglected in community analysis assessing the flux between the water column and
benthic communities. However, Lemmens et al. 1996a found that small epifaunal suspension-feeders also contribute considerably to the total filtering capacity and
especially bryozoans may be of significance.
Literature concerning the role of bryozoans in an ecological perspective is very limited. In the few early works by Menon 1974 and Bullivant 1968 experiments were
performed in order to measure bryozoan filtration rates at different temperatures and different food concentrations. They obtained filtration rates measured as the clearance
i.e. the reduction in the number of algae in a beaker containing a colony. In several later studies e.g. Strathmann, 1982; Best and Thorpe, 1986, 1994; Sanderson et al., 1994;
˚ ´
Riisgard and Manrıquez, 1997 pumping rates have been estimated from particle velocity within the lophophores and the cross-sectional area of the lophophores. Best and
Thorpe 1983 calculated feeding rates by timing the frequency of pharynx emptying, assuming equal size of the bolus formed before emptying, and estimating the volume
and number of cells the bolus contains.
In order to estimate the clearance capacity of bryozoans in ecological terms, individual clearance rate and population densities are needed, but also morphological
and life cycle variations within the bryozoan group is important; all species are polymorphic to some extent, implying that some zooids within a colony might be
specialized as e.g. some sort of defence organs avicularia or vibracularia, spinozooids
´ or involved in reproduction, forming distinct male female zooids or ovicells Silen,
1977. Because of this partitioning, only a part of the colony will contain actively feeding zooids. Also, in bryozoans, the feeding zooids perform a cycle of polypide
regression brown body formation and regeneration, which is dependent on food availability Bayer et al., 1994. Due to this cycling, an even lesser part of the colony
may be actively feeding at any time and this must be addressed when estimating colony clearance capacity. In the present study clearance rate of a bryozoan species common to
the seagrass meadows of the southern hemisphere is measured in order to elucidate bryozoan colony clearance as a function of temperature. Emphasis is on bryozoan
clearance capacity and its ecological implications.
2. Materials and methods
2.1. Location and species Electra bellula Hincks is an encrusting bryozoan species known throughout tropical
and subtropical zones. Different varieties have been described; and the animals in the present study are of the variety multicornis. The zooids of E
. bellula are approximately
D . Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
287
300 mm long and 250 mm wide with a lophophore of about 300 mm high and 250 mm in diameter with 10–12 tentacles.
Electra bellula was collected in Amphibolis sp. seagrass beds in Marmion lagoon, Perth, Western Australia in a depth of 4–6 m of water. E
. bellula live as epifauna on the leaves of the Amphibolis sp. but are more commonly found on the epiphyte Dictyopteris
sp. Only colonies on Dictyopteris sp. were used. Under dissecting stereomicroscope, a piece of leaf containing one colony of E
. bellula was cleaned of other epifauna and attached to a slide with cyanoacrylate glue.
The yearly water temperature amplitude in the region is between 16 and 248C e.g. Prata, 1989. During the study period February–March 1997 the water temperature
ranged between 22 and 248C. Animals attached to slides were placed in holding tanks at 228C prior to experiments.
2.2. Experiments
˚ Clearance experiments were measured as described by Petersen and Riisgard 1992.
Twelve hours prior to experiments one or two colonies were placed in 250-ml beakers containing filtered 1 mm seawater in thermo-constant baths at 2260.28C. Con-
centrations of the food source Rhodomonas sp. were measured using a Coulter Multisizer II particle counter with a 70-mm orifice. In the present study the formula used
˚ by Petersen and Riisgard 1992 was modified so as to give the area specific clearance
rates F : F 5 V ln C C At, where A is the area of the colony,t is time, V is the
t
volume of the beaker and C and C the algae cell concentration at times 0 and t,
t
respectively. In order to achieve maximum clearance rates, experiments were replicated 3 to 8 times with each beaker. For each beaker, the 2–3 replicates with the highest
clearance rates were regarded as the colony potential, i.e. the maximum clearance rate F
. This procedure excludes replicates in which feeding activity was lowered due to
max
stress caused by handling or other disturbances in execution of the experiments. Returning of the subsamples after counting the algae cells on the Coulter Counter may
influence the clearance rates of the bryozoan colonies. A change in zooid behaviour was noticed by Bullivant 1968, when subsamples from algae concentration analysis were
returned to the beaker. The phenomenon has later been discussed see Best and Thorpe, 1994, and may be due to the electric charge through the water during analysis. Such
effects were also observed in this study in preliminary experiments, hence, subsamples were not returned. Instead an equal volume containing approximately the same algae
concentrations were added. The difference in cell concentrations between the experimen- tal beaker and the volume added was taken into account when calculating the clearance
rates.
The effects of water temperature were studied in a temperature range of 16 to 248C, with 28C intervals i.e. 16, 18, 20, 22 and 248C. Before experiments, the temperature
was altered by two degrees every 2 days to allow the organisms to acclimatize. Initial
21
algal cell concentrations varied between 1500 and 3000 cells ml . Sampling intervals
varied between 40 min at 168C, 30 min at 18 and 208C, and 20 min at 248C. Colony clearance rates are related to both the total and the active area of the colonies. Here, the
total areas are defined as the extent of the calcified part. The leaves of Dictyopteris sp.
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. Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
are flat making accurate estimates of the areas of the encrusting E . bellula colonies
possible. Photos of the colonies were taken with a Nikonos V 35-mm lens mounted with a 1:1 extension tube, using a flash directly behind the colonies. Measurements of colony
areas were obtained on scanned pictures, using a computer. The guts of the active zooids could easily be distinguished by their red colour, resulting from the captured
Rhodomonas sp. From this, the specific areas were estimated. To relate area to weight, areas of 36 colonies were measured. Under dissecting microscope these colonies were
carefully scraped off the Dictyopteris sp. and weighed. Dry weight was measured after . 48 h in 60–708C, and ash-free dry weight after ashing at 4758C for a minimum of 4 h.
3. Results
