In situ evidence for pre-capture qualitative selection in the tropical bivalve Lithophaga simplex

Few feeding studies have been performed on tropical bivalves, and in situ feeding studies are lacking altogether. We investigated retention efficiencies for natural particles in the coralboring tropical mytilid Lithophaga simplex. Using the in situ InEx technique (Yahel et al. 2005; Limnol Oceanogr Methods 3:46-58) SCUBA divers collected samples from the water inhaled and exhaled by undisturbed bivalves at the coral reef of Eilat (Gulf of Aqaba). Particle retention efficiencies were determined using flow cytometry analysis of the paired water samples. The photosynthetic bacterium Synechococcus (0.9 ± 0.1 μm) and larger eukaryotic algae (1 to 10 μm) were preferentially retained by the bivalve with removal efficiencies of up to 90% (1996 to 2000: averages of 69 ± 14% and 60 ± 17%, respectively, n = 74 individual bivalves). The minute photosynthetic bacterium Prochlorococcus (0.4 ± 0.1 μm) was also moderately retained (41 ± 19%). Only a small proportion of the non-photosynthetic bacteria (0.3 ± 0.1 μm) were retained (5 ± 18%, median of 9%), despite their numerical dominance in the plankton and considerable size overlap with Prochlorococcus. Sizeindependent preferential retention was also observed within particle types: (1) L. simplex more efficiently retained Prochlorococcus and picoeukaryotic algal cells with higher chlorophyll content and (2) the small fraction of non-photosynthetic bacteria retained did not differ in size, but had higher nucleic acid content (compared to the inhaled population) an indicator for viable and active bacteria. We conclude that particle retention is not strictly size-dependent in L. simplex, and probably involves other cell attributes such as cell surface properties and/or motility.