Epibiontic macrophytes on the Mole crab Emerita analoga in a bay in the central coast of Peru

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

Lima, Perú

Conferencia; Internacional Conference on the Humboldt Current System; 2006

Instituto del Mar del Perú; Institut de Recherche pour le Développement; Food and Agriculture Organization

Introduction- The mole crab Emerita analoga is one of the most abundant decapod of the Peruvian sandy beach communities. They play an important role in secondary production as prey for many crustaceans, fishes, birds, and even humans (Alvitres et al. 1998). Its geographical distribution in the Pacific Ocean extends from Alaska (58° N) to the Beagle Channel (55° S). In Ancón Bay (11º46’ S, 77º11’ W), a small bay close to Lima in the Peruvian central coast, the mole crab inhabits all sandy beaches, and they have been found with epibiontic macrophyte. Until now, such interaction had never been reported. Epibiosis is one of the closest possible associations between different species. Through direct and indirect interactions, this association has major effects on the species involved and on community dynamics (Wahl & Mark, 1999). In this study we described the epibiosis, assessed its occurrence in the population and tested its effects in E. analoga burrowing abilities. Methods- Based on rostrum individual percent cover by algae, a 3 levels qualitative scale was designed: level 0 (no epibiontic algae observed), level 1 (algae covering 0 to 50% rostrum surface), and level 2 (algae covering 50 to 100% rostrum surface). Classification was tested by weighting dry biomass of epibiontic algae from fouled individuals collected in San Francisco Grande (SFG), a beach of Ancón Bay, in October 2005. To analyze E. analoga population characteristics and epibiosis occurrence, we performed, in February 2006, samplings in two beaches of Ancón Bay: SFG and Las Conchitas (LCB), and one south of the bay, in Huaquillo beach (HB). Samplings were performed in three transects parallel to the shore, corresponding to saturation, resurgence and retention zones. To assess direct effects of epibiosis, in March 2006 burrowing ability experiments were performed at HB, where burrowing time and depth were measured. We collected variously sized E. analoga with and without epiphitic macrophytes. For statistical analysis only ovigerous females were considered, as they were the only group with enough fouled and non-fouled individuals to make measurements comparable. The burrowing times of individual crabs were timed from the initiation of burrowing to the disappearance of the crab under the sediment surface. Burrowing speed was calculated as the rate CL / burrowing time. Maximum burrowing depths, were estimated by measuring the length of a nylon string glued to the carapace, from the sediment surface to the tip of the rostrum. Results- The epibiontic algae found on E. analoga was identified as Enteromorpha spp., and was settled mainly on the rostrum, but was also found on primary and secondary antennae and maxillipeds. In some cases, Polisiphonia spp. has been also found under Enteromorpha canopy. Algae dry weight ranged from 0,0004 to 0,0091g for epibiosis 1 and from 0,0062 to 0,1899 g for epibiosis 2. These values of algae dry weight corresponding to categories 1 and 2 of our visual-based scale were significantly different (T-test, P<0,001). In SFG the density was 433 ind/m2, the male:female ratio was 6,75:1, and 0,8% of individuals had epibiosis 1. In HB the density was 1091 ind/m2, the male:female ratio was 4,63:1; 0,32% of individuals had epibiosis 1 and 1,76% epibiosis 2. In LCB the density was 284 ind/m2, the male:female ratio was 1,81:1, and there weren’t fouled individuals. Ovigerous females with epibiosis 2 spent more time in burrowing than ovigerous females with epibiosis 0 and 1 (ANCOVA, P<0,001 Tukey post hoc test, P<0,001 for both comparisons). Burrowing speed was significantly lower for ovigerous females with epibiosis 2 than for ovigerous females with epibiosis 0 and 1 (ANCOVA, P<0,001; Tukey post hoc test, P<0,05 for both comparisons). The degree of epibiosis had no effect on the maximum burrowing depth of ovigerous females (ANOVA, P>0,05). Conclusions- The designed scale resulted to be effective in assessing the degree of fouling. Densities and sex rates resulted consistent with other studies in the Peruvian coast. Our results show that epiphitic algae modify E. analoga burrowing ability, slowing their escape activity. This may play an important ecological role in their population dynamics by modifying their availability to predators, mainly seabirds and fishes; although further studies are necessary to assess how epibionts affects E. analoga in recognition and handling by predators. Enteromorpha has been characterized as opportunist algae, proliferating in eutrophicated shores, therefore we conclude that the interaction reported in this work should occur in other impacted areas of the large distribution of E. analoga.