Predation effects by birds on intertidal mole crab (Emerita analoga) fouled by macro-algae

HIDALGO F. J.; FIRSTATER F. N.; LOMOVASKY B. J.; GALLEGO P.; IRIBARNE O. O.

Concepción

Simposio; Censor Midterm Symposium; 2006

COPAS; Alfred Wegener Institute for Polar and Marine Research; National Oceanography Centre

Epibiosis is a facultative association of two organisms: the epibiont, who attaches to the surface of a living substratum during the sessile stage of its life cycle; and the basibiont, the living surface that provides support for the epibiont. Epibiosis can have important effects on their hosts, both negatives and positives, such as affording camouflage, increasing water resistance and drag, and repelling or attracting predators. Specifically in predator-prey interactions, the epibiont may affect the predator?s ability to localize, recognize or handle the prey, shifting the susceptibility of the prey to predation, either increasing (?shared doom?) or decreasing (?associational resistance?) prey mortality (Wahl 1989). An epibiontic association between green macroalgae and the mole crab Emerita analoga (Crustacea, Anomura, Hippidae) was recently detected in protected to moderately exposed sandy beaches of the Ancón bay, in central Peru (personal observation, Firstater et al., in prep.). E. analoga is a widespread and abundant decapod inhabiting exposed sandy beaches of the temperate west coast of America, and is a characteristic species of the swash zone (Contreras et al. 1999). Given its abundance, E. analoga plays an important role in the secondary production of sandy beaches, being the main prey of several bird species that use the intertidal zone for feeding. Previous surveys and observations pointed out that this epibiosis might suppose a problem for the intertidal population of E. analoga, by increasing the risk of predation by birds and hence its mortality rate. In this work we evaluated the predation effects by birds on E. analoga fouled by macroalgae in a sandy beach of central Peru. The particular objectives addressed were 1) to determine the epibiosis prevalence on the intertidal population; 2) to calculate and compare predation rates on E. analoga with and without epibiosis; and 3) to estimate mortality rates due to predation of E. analoga with and without epibiosis. Results indicated that epibiosis prevalence in the intertidal population was low (~1%), and that only females were epibionted. Several bird species (Larus modestus, L. belcheri, L. pipixcan, Haematopus ater, H. palliatus and Numenius phaeopus) were observed foraging on E. analoga with variable intensity; among them, L. modestus was by far the most abundant. Despite predation rates on epibionted and non-epibionted E. analoga were variable, all of the bird species preferred epibionted over non-epibionted individuals, being consumed at a much higher proportion than their availability in the intertidal. Estimates of mortality rates indicated that 0.2% of the non-epibionted intertidal population and 76% of the epibionted intertidal population is consumed every day. This suggests that epibionted individuals must be replaced continuously with individuals from the subtidal to maintain predation and mortality rates in the calculated levels. Epibiosis by macroalgae had a negative effect on intertidal E. analoga by increasing mortality by predation on fouled individuals. However, other individual effects of epibiosis such as changes in moulting frequency and hence in growth rates, metabolism, changes in fecundity, etc. should be further studied to determine the costs and benefits of epibiosis and to evaluate the net effects on the entire E. analoga population.