PARTICLE FEEDING IN NATURAL POPULATIONS OF THREE MARINE DEMOSPONGES

Abstract

1. Microscopic and chemical analysis of ambient and exhalant water samples collected in situ indicates that the net POC diet of three tropical Demospongiae, Mycale sp., Verongia gigantea and Tethya crypta, consists primarily (80.5%) of particulate (filterable) organic matter which is unresolvable by direct microscopy (URPOC). Microscopically resolvable particulate material (MPOC) accounts for only the remaining 19.5% of the POC diet of these sponges. 2. The three sponges retain available resolvable particulate material within the size range of 0.3-50 µ at high efficiencies—means are 79.0% by calculated carbon content and 82.0% by particle volume. 3. Major components of the MPOC diet and of the total POC diet of these sponges are respectively: unarmored cells 83%, 16.2%; armored cells 12.3%, 2.4%; and bacteria 4.6%, 0.9%. 4. Two functionally independent capture systems appear to be operative in all 3 species, accounting for a basic bimodal pattern of particle retention. A system involving particles between 5 and 50 µ is attributable to phagocytosis by cells lining the inhalant system. A second system involving particles of the bacterial size range (0.3-1 µ) involves capture at the choanocyte collar and ingestion by choanocytes. 5. The amoebocyte capture system is by necessity constantly functional and must accept all particles entering the ostia. A transport path by amoebocyte migration cycles is proposed to account for transport and release of intact armored plankton at significant rates. 6. Bacterial retention is high in all species, 94.8-96.9%, mean = 96.1% by cell number. This choanocyte capture system is fallible, but under normal environmental conditions retention rates are constant and independent of ambient concentration. 7. Sponges with high tissue density (Verongia) show apparent size selection of armored cells which is attributed to slow amoebocyte cycling. Sponges with low tissue density (Mycale) retain armored fractions at lower efficiencies without apparent size selection. High retention of armored cells by Tethya may be attributed to behavioral adaptation. 8. All three species effect a net production of microscopically resolvable detrital organic matter. 9. The previously unrecognized unresolvable fraction of particulate organic matter (URPOC) represents an available carbon source 7 times that of all resolvable planktonic material in Jamaican waters. The ability of sponges to capture this material, probably via the primitive choanocyte system, is responsible for continuing dominance of Porifera as the filter feeders of coral reef habitats.