Impacts associated with the recent range shift of the aeolid nudibranch Phidiana hiltoni (Mollusca, Opisthobranchia) in California

Abstract

In 1977, <i>Phidiana hiltoni</i> (O'Donoghue in J. Entomol Zool (Pomona College, Claremont, California) 19:77-119, 1927) began spreading northward from Monterey, California. By 1992, it had reached Duxbury Reef (37° 53' 23″ N, 122° 41' 59″ W), 100 km to the north, where other nudibranchs subsequently appeared to decline. The role of <i>P</i>. <i>hiltoni</i> in this decline was investigated through diet analysis, feeding trials, and comparison of historical and recent abundance data. In the wild, <i>P</i>. <i>hiltoni</i> preyed largely on hydroids, but also showed evidence of predation on nudibranchs. In the laboratory, <i>P</i>. <i>hiltoni</i> attacked most of the dendronotid and aeolid nudibranchs presented to it, ingesting small individuals whole. The pooled abundance of nudibranchs vulnerable to attack by <i>P</i>. <i>hiltoni</i> declined an average of two-thirds at Duxbury Reef since its arrival, compared to (1) no change in the non-vulnerable species and (2) no change in either group at two other sites where <i>P</i>. <i>hiltoni</i> was one to two orders of magnitude less abundant. <i>Phidiana</i><i>hiltoni</i> therefore appears to have caused this decline, likely through a combination of direct predation and competition for prey. A brief larval period, combined with cyclonic re-circulation in the lee of Point Reyes, may be driving self-recruitment of <i>P</i>. <i>hiltoni</i> at Duxbury Reef, as well as hindering further northward spread.