Daphnid morphology deters fish predators

Abstract

Spine and helmet production in zooplankton are thought to provide protection from invertebrate rather than vertebrate predators. We examined selectivity for Daphnia lumholtzi, a species that exhibits extreme cyclomorphosis with a large helmet and long tail spine (total length can exceed 5 mm), by juvenile bluegill (15-80 mm) in the laboratory and field. Bluegill consumed more D. pulex than D. lumholtzi when the species were presented alone. When the daphnids were offered together in equal numbers, bluegill selected against D. lumholtzi. Bluegill foraging behavior helped explain the observed nonrandom feeding. Bluegill capture efficiency foraging on D. pulex was high (85-100%) and handling times were low (usually too short to detect), whereas efficiencies were lower (40-96%) and handling times were longer (1-3 s) when foraging on D. lumholtzi, particularly for fish <50 mm. As they gained experience, bluegill <50 mm that oriented towards D. lumholtzi rejected them more often than striking. In addition, more D. lumholtzi were rejected and expelled than were D. pulex. From these experiments, we conclude that larger bluegill (>50 mm) are able to forage more successfully on D. lumholtzi than smaller fish. Selectivity by bluegill collected from a reservoir infested with D. lumholtzi verified our laboratory conclusions. Smaller bluegill selected against D. lumholtzi, whereas it was a preferred diet item for bluegill >50 mm. These results show that the morphology of D. lumholtzi interferes with predation by small planktivorous fish, posing foraging constraints for these fish more similar to those of piscivores, where handling time is important, than to those of planktivores, where prey density is of primary importance.

First record of Daphnia lumholtzi (Sars, 1885), exotic cladoceran, in São Paulo State (Brazil)

“…Resistance to predation might be a determinant factor in the successful invasion of this species because the carapace spine and helmet of D. lumholtzi protect against invertebrate predation (Kolar & Wahl, 1998), although this is still a matter of speculation because the real protective potential of such structures is not well known (Dzialowski et al, 2000). Many invaders are successful because, besides resistance to predation (Crawley, 1986), either native competitors are ineffective or other species do not take advantage of existing opportunities.…”

Can Daphnia lumholtzi invade European lakes?

“…This length compares to 2-3.5 cm for bluegill sunfish used by Swaffar and O'Brien (1996), and 1-8 cm for bluegills used by Kolar and Wahl (1998). Kolar and Wahl (1998) found that bluegills with a length of up to 5 cm had difficulty in handling D. lumholtzi and learned to reject them. In contrast, bluegills with a length above 5 cm were able to ingest D. lumholtzi, although less efficiently than undefended D. pulex.…”

“…Some other clones are permanently defended: they carry these defenses even in the absence of predator cues. Swaffar and O'Brien (1996) and Kolar and Wahl (1998) conducted feeding experiments with bluegill sunfish (Le-pomis macrochirus) on D. lumholtzi compared to other Daphnia species. They showed that the spines of D. lumholtzi make it difficult for juvenile sunfish to consume these defended Daphnia and strongly increase their handling time.…”

Different effects of planktonic invertebrate predators and fish on the plankton community in experimental mesocosms

“…This concept suggests that the promotion of efficient planktonic herbivores through decreased biomass of planktivores by piscivores can reduce the biomass of phytoplankton. This general pattern can be modified by altered life histories or other interactions among particular species such as defence mechanisms in prey (Jeffries, 1988;Kolar and Wahl, 1998;Straile and Halbich, 2000). These findings are widely used during food web manipulation with the ultimate goal to improve water quality management (i.e., biomanipulation techniques) (Berg et al, 1997;Beklioglu, 1999;Ha et al, 2013).…”