Larvae of the Holarctic marine chironomid, Halocladius variabilis (Staeger), have strong fidelity to the tuft-forming brown alga, Elachista fucicola (Velley) Areschoug, an abundant epiphyte on intertidal fucoids of the North Atlantic. We show that larvae are sufficiently motile to select an algal host in a Petri dish within 3–4 cm, and that larvae show differential behaviour with respect to host selection in the presence or absence of a predator. In the absence of predators 53% of larvae found an algal host within 1 hour; however, after 24 hours, there was no significant difference in host selection. When an isopod predator (Idotea sp.) was present, more larvae found a host within 1 hour (81%) and Elachista was chosen over three of the four other hosts. Furthermore, when larvae were present in Elachista, predator (Carcinus maenus) success was significantly reduced relative to two other algal hosts. The adaptive significance of Elachista as a refuge from predation was confirmed by experiments demonstrating that larval growth with other algal hosts was greater than with Elachista. These experiments suggest that microhabitat selection by larvae of H. variabilis reveals important tradeoffs for growth and predator avoidance.

The commensal (and possibly mutualistic) symbiosis between the marine chironomid, Halocladius variabilis, and brown algal epiphytes of Fucus vesiculosus and Ascophyllum nodosum is described for the first time from Europe, based on field studies from the White Sea, Russia. While the primary host, Elachista fucicola, and the secondary host, A. nodosum, are the same as in eastern Canada where the symbiosis was first described, White Sea populations have a wider range of primary hosts that include Pylaiella littoralis and Dictyosiphon foeniculaceus. About 64% of E. fucicola thalli on A. nodosum in the low intertidal zone were colonized. Significantly lower frequencies were found on E. fucicola when the latter was epiphytic on F. vesiculosus at the same tidal height or on either secondary host when they were in the subtidal zone. For a given tidal height, or secondary host, frequency of H. variabilis was reduced on P. littoralis and further reduced on D. foeniculaceus. We suggest that subtidal populations are colonized by dispersal of larvae from the intertidal zone on to suitable substrata, and that there is limited potential for these individuals to reach the intertidal zone as adults for reproduction.