Morphological defenses induced in situ by the invertebrate predator Chaoborus: comparison of responses between Daphnia pulex and D. rosea

Abstract

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gräfenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term "maximum size for neckteeth formation" (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures.

Population dynamics of Chaoborus flavicans and Daphnia spp.: effects on a zooplankton community in a volcanic eutrophic lake with naturally high metal concentrations (L. Monticchio Grande, Southern Italy)

“…The presence of Chaoborus strongly reduces the chances of individuals reaching maturity (Sell 2000(Sell , 2006. A slight increase of the total population density should have been observed in August due to the absence of a high predation rate.…”

Reciprocity in predator–prey interactions: exposure to defended prey and predation risk affects intermediate predator life history and morphology

“…They may take the form of novel physical structures, as in daphnids (Sell 2000;Tollrian 1993), or via changes in existing morphology, such as tail depth and colour in anuran larvae (Relyea and Werner 2000;Van Buskirk and Relyea 1998). Such changes in morphology are predicted to come with costs, and this cost-benefit scenario generates less predictable life history outcomes.…”

Formation of morphological defences in response to YOY perch and invertebrate predation in two Daphnia species coexisting in a mesotrophic lake

“…Induction of morphological defences in Daphnia populations by either fish or invertebrate predators is well studied under experimental conditions (Spaak & Boersma, 1997;Kolar & Walh, 1998;Sell, 2000) but more rarely in field studies (Havel & Dodson 1985;Lindholm, 2002). Several experimental studies showed that Daphnia respond to fish kairomones by increasing tail spine length (Tollrian, 1994;Spaak & Boersma, 1997), whereas elongation of the helmet or formation of a neck tooth are common responses of Daphnia to invertebrate predators (mainly Chaoborus) (Tollrian, 1993;Brancelj et al, 1996;Sell, 2000).…”