File:American malacological bulletin (1987) (17533807494).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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26 AMER. MALAC. BULL. 5(1) (1987) barely above background, apparently because of their small body size. DISCUSSION Experimental results support the idea that Pisidium casertanum and P. conventus are interstitial suspension feeders, utilizing bacteria and perhaps other small particles suspended in the interstitial water (Efford and Tsumura, 1973; Holopainen and Hanski, 1979). Our observations confirm those of Meier-Brook (1969) that water is being pumped out of the burrow, but not necessarily that overlying water is being pulled into the sediment (Fig. 2). Because it is a blind burrow, the only direct source of nutrient is the surrounding sediment. Similarly, larval lamprey appear to obtain particles suspended in water just above the substratum and also from pore water within sediment (Moore and Mallatt, 1980; Mallatt, 1982). Water is pumped into the sandy sediment, and exhalant water is extruded into the sediment around the burrow. Like am- mocoetes, Pisidium spp. appear to pump water extremely slowly, and are able to filter very concentrated suspensions of interstitial bacteria. Once filtered, ingested bacteria are ab- sorbed efficiently. The reciprocal labelling experiments indicate that Pisidium casertanum and P. conventus preferentially ingest interstitial bacteria from a sediment suspension, but that a substantial fraction of absorbed bacteria came from sediment ingestion. In trials 1 and 2, animals incorporated label more dramatically from 3H-thymidine labelled bacteria than from 14C-glucose labelled cells, indicating an asymmetry in labelling protocol. These results are at variance with gut observations of field collected animals, as mineral particles were rarely observed in the alimentary tract. One of the big- gest problems with these experiments is the destruction of sediment texture. Certainly, Pisidium spp. ingest sedimen- tary particles under laboratory conditions, which in this case consisted of rather severe disruption of sediment texture. We suspect that sediment ingestion in these experiments was an artifact of this disruption. Many Pisidium species typically live in loose, flocculent sediment with a high water content and a coarse pore structure. In Lake Pa'ajarvi, water content varied from 60 to 90%, increasing with water depth (I. Bergstrom, unpubl.). Because stomachs and midguts of animals in nature were invariably empty, and gut contents rarely included mineral grains, we doubt whether sediment ingestion is significant under natural conditions. Animals could be induced to feed upon dense sediment suspensions, but at a very low rate of ingestion. We saw no evidence of particle collection by the foot, as described by Mitropolskij (1966, 1970) for P. casertanum. Animals were offered a variety of particles, including sediments, fluorescent particles, char- coal powder, and flour. Particles did collect at the pedal gape, this appeared to result from crawling behavior. Sediment par- ticles drawn into the mantle cavity by the inhalent current and are transported to the large labial palps. The palps appear to be efficient in rejecting sedimentary particles. On first inspection, there appear to be several prob- lems with the postulated interstitial suspension feeding
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Fig. 5. Ingestion/absorption of interstitial and particle-bound bacteria. Relative enrichment of interstitial bacteria in the diet was calculated as: 3H: 14C incorporated/3!-!: i«C sediment mixture (where 3H was used to label interstitial bacteria), or 14C:3H incorporated/14C:3H, sedi- ment suspension (14C labelling of interstitial bacteria). Differences among trials is described in the text. mechanism. One might expect that sediment particles would be drawn in with the inhalant current. More important is the question: Is the density of interstitial bacteria high enough to support such feeding? With regard to the first problem, sediment particles taken into the mantle cavity to be selec- tively rejected as pseudofeces. Selective rejection of mineral grains has been demonstrated in several species of suspension-feeding bivalves (Kiarboe era/., 1980; Bricelj and Malouf, 1984). The amount of sediment taken into the man- tle cavity may depend upon the water velocity caused by pumping, and the texture of the sediment. Because of the small size of most Pisidium spp., the absolute pumping rate (ml x hr"1) and the water velocity is very low, even though weight-specific pumping rates may not be particularly low, In aquaria, the measured pumping rate of 0.6 ml x hr"1 for P. casertanum maintained a water velocity of only .006 cm x sec"1 into the burrow. We have used weight-specific respiration rates (Holo- painen and Ranta, 1977) and measured interstitial abundance of bacteria to calculate the volume of interstitial water that would have to be filtered to meet metabolic demands. We assume that the carbon content of bacteria is 2 x 10"13 g C

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