Epibiont and basibiont relationships can have positive and negative effects on both organisms involved, ranging in intensity from minor to major effects. Limpets of species Lottia pelta are commonly found with two algal species growing on their backs, Ulva lactuca and Acrosiphonia spp. Previous research has shown that basibionts (substrate organism) and epibionts (organism growing on the surface) have complex interactions that can be positive, negative, or neutral. A force transducer and flume were used to measure the drag forces experienced by a limpet at various water velocities. Presence of either epiphyte significantly increased limpet drag. Acrosiphonia produced a greater drag effect than U. lactuca, increasing the force substantially. When dropped in a tank, limpets with algal growth landed foot-down significantly more often than limpets without algal growth. Acrosiphonia spp. had a greater effect than Ulva lactuca. Lastly, limpets in a wind tunnel with algal growth (especially Acrosiphonia) had cooler body temperatures than limpets without algal growth. In conclusion, the effects on the basibiont of this relationship were found to be both positive and negative. 1. Introduction In the rocky intertidal environment, primary space is often a limiting resource. Because of this, many organisms have developed a life-history strategy of growing on the surface of another organism. The basibiont (substrate organism) and the epibiont (organism growing on the surface) often have complex direct and indirect interspecific associations [1]. The effects of epibionts on the basibiont may show positive, neutral, or negative consequences [1–4]. The mechanical effects of epizoic growth are of particular concern for the basibiont in a marine system. Hydrodynamic effects of epibionts have been well studied in a number of epibiont-basibiont systems. Epibionts may increase drag and lift in environments with high water flow, such as wave-swept intertidal zones [5]. Mytilus byssus receives a 2- to 6.7-fold increase in stress induced by drag forces from epibiotic kelp [6]. Epibiotic algae on Mytilus mussels increased dislodgement during storms, regardless of the size of the algae [4]. Indeed, mussel dislodgment due to algae may be more important than predation in certain systems [6, 7]. Hydrodynamic effects on the basibiont are not the only possible negative impacts of epizoic organisms. In Littorina littorea, for instance, the presence of barnacles growing on the shell causes an increase in the snail’s volume and weight and a decrease in the locomotionspeed and
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