Unpalatability of a Colonial Rotifer, Sinantherina socialis, to Small Zooplanktivorous Fishes

Abstract

Colonies of Sinantherina socialis (phylum Rotifera) form conspicuous masses that should make them extremely vulnerable to fishes capable of ingesting small live prey (<3 mm in length). To test this hypothesis we offered rotifer colonies as prey to 14 species of smallmouthed, predatory fishes, alternating with small cladocerans. Most rotifer colonies (71-100%), but few cladocerans (0-14%) were rejected after capture. Fish feeding behavior on rotifers followed a consistent pattern: locate, approach, attack, mouth repeatedly, and reject (spit out). Except for loss of a few individuals, rotifer colonies that had been repeatedly rejected remained undamaged. This is the first evidence for unpalatability of rotifers to zooplanktivorous fishes. Additional key words: fish feeding, Flosculariidae, Monogononta, predation, predatory defense, Rotifera, unpalatability, zooplanktivory Rotifers (about 100-1000 |jm) fall prey to a variety of tactile invertebrate predators such as other rotifers, insect larvae, and microand macrocrustaceans (Williamson 1983; Stemberger & Gilbert 1987). They also are an excellent food source for marine and freshwater fish larvae (Newrkla & Duncan 1984; Lubzens 1987; Lubzens et al. 1989). Larval fishes may feed extensively on rotifers (Landau et al. 1988), even when alternate prey are available (Hewitt & George 1987; Zale & Gregory 1990). Thus, rotifers are caught between the two millstones of tactile and visual predation; there is no refuge from zooplanktivorous fishes other than preserving a small body size, which maintains susceptibility to many tactile predators. One defense against tactile predators used by rotifers appears to be the formation of colonies, at least in those species that produce a gelatinous matrix (e.g., Conochilidae REMANE 1933). For those species the jelly mass provides a refuge into which the rotifers may retract when attacked (Gilbert 1980). However, colony formation in rotifers increases effective size by several fold, placing the colonies within a size range consumable by small zooplanktivorous fishes or their larvae: 0.8 to 4 mm (Siefert 1972; Crecco & Blake 1983; O'Brien & Evans 1991). Therefore, it seems plausible that colonial rotifers should be present in habitats where predation by tactile predators is more intense than by visual predators. Of course, colony formation may have other consequences, such as increasing the a To whom correspondence should be addressed. efficiency of water flow over a group of closely spaced individuals engaged in suspension feeding (see Wallace 1987 for a review). Of 8 rotifer genera with colonial species, the genus Sinantherina BORY DE ST. VINCENT 1826 (Monogononta, Flosculariidae) is particularly interesting because sinantherinids do not possess the gelatinous matrix that is characteristic of the family and many species form colonies so large and conspicuous that they are easily seen from above the water surface. For example, two sinantherinids produce pelagic juveniles with adult colonies that are sessile on aquatic plants: S. ariprepes EDMONDSON 1939 (Edmondson 1940), S. socialis (LINNAEUS 1758) (Surface 1906; Wallace 1987). A third sinantherinid is holoplanktonic in shallow waters, S. semibullata (THORPE 1889) (Canella 1952; Gunter & Knight 1978). As a consequence of these features, one might expect that sinantherinids would be extraordinarily vulnerable to fish predation, yet they are often present in habitats with small, predatory fishes (R. Wallace, unpubl. obs.). How do these colonial rotifers avoid extirpation by fishes? Kerfoot and his co-workers have demonstrated that many conspicuous aquatic arthropods (insects and hydracarinids) have chemical defenses (e.g., steroids) against predation by fishes (Kerfoot et al. 1980; Kerfoot 1982; Scrimshaw & Kerfoot 1987). Extrapolating from that work, one of us (Wallace 1987) suggested that colonies of S. socialis might possess chemical defenses rendering them unpalatable to zooplanktivorous fishes. Here we describe a test of a simpler hypothesis: This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:42:00 UTC All use subject to http://about.jstor.org/terms Felix, Stevens, & Wallace that colonies of S. socialis are unpalatable to predatory fishes and will be preyed upon at lower rates than will other small live prey.