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

Abstract

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.

Particle capture mechanisms in suspension-feeding invertebrates

“…However, note that the capture mechanism changes from ciliary downstream collecting in veliger larvae (see 'Ciliary downstream collecting') to cirri trapping during the postlarval development of most bivalves (e.g. Cannuel et al 2009) Finally, it should be emphasized that qualitative selection in suspension-feeding bivalves has been assumed to be exclusively post-capture, but in the tropical bivalve Lithophaga simplex studied by Yahel et al (2009), qualitative selection occurs prior to retention. This encourages a re-examination of what is meant by 'retention'.…”

Flow effects on benthic grazing on phytoplankton by a Caribbean reef

“…This seemingly paradoxical situation is thought to arise because coral reefs are efficient at trapping nutrients, bacteria, zooplankton, and phytoplankton from the surrounding waters (Ayuki 1995;Yahel et al 1998;Holbrèque et al 2006) and so can balance the nutrients required for primary production and their leakage offshore ). Because many reefs depend on suspended particles and dissolved nutrients trapped from the flowing water, their functioning may be regulated by the hydrodynamics.…”

Introduction

“…These studies yielded seminal information which would become at once a foundation and a prism through which we have continued to interpret the characteristics of bivalve feeding, and indeed the techniques are important tools in the panoply we use today. Adaptation of this approach to measurement in the field, either indirectly (via the biodeposition method, Cranford et al 1998, Iglesias et al 1998, Cranford & Hill 1999 or pumping from the field site to experimental chambers, MacDonald & Ward 1994) or directly (the InEx method, Yahel et al 2003Yahel et al , 2005Yahel et al , 2006Yahel et al , 2009 this Theme Section) has allowed us to extend these observations much closer to the natural habitat. Simultaneously, application of the physical principles of fluid mechanics has yielded important insights into the hydrodynamic principles of particle movement underlying suspension feeding in bivalves (Jørgensen 1981, 1982, 1983, Jør-gensen et al 1984, Nielsen et al 1993, Riisgård & Larsen 2001.…”

“…(3) Pre-capture selection (Yahel et al 2009). Qualitative selection in suspension-feeding bivalves has been assumed to be exclusively post-capture, but in the tropical species studied by Yahel et al 2009, qualitative selection is shown to occur prior to retention.…”

“…Qualitative selection in suspension-feeding bivalves has been assumed to be exclusively post-capture, but in the tropical species studied by Yahel et al 2009, qualitative selection is shown to occur prior to retention. This forces us to re-examine what is meant by 'retention' (certainly different mechanisms for different gill types and sub-types) and how qualitative selection might intervene in some of these scenarios.…”