Heterobranch Sea Slugs from San Luis Obispo Bay Across Forty Years, and the Historical Absence of the Doridid Aegires albopunctatus at Avila Beach

The rocky shore at Cave Landing (35.1752°, –120.7223°), next to Avila Beach in San Luis Obispo Bay (SLO Bay) was sampled by R.A. Roller and colleagues (including author GM) for benthic heterobranch sea slugs from 1967–1973 and by author JG from 2001–2023. This enabled comparison of species composition across decades in a small embayment with an upwelling shadow, including consideration of hypotheses for the unexpected early absence of Aegires albopunctatus. Twenty-one species were observed in one or the other time period, and the period of occurrence of these was significantly associated with geographic range group, with most of the northern species observed only during 1967–1973, and all of the southern species observed only in 2001–2023. Out of the total 49 species recorded in this study, 79% of southern species increased in frequency of occurrence from the early to late sampling period, while 67% of northern species decreased, a result consistent with recent poleward range shifts by marine species globally. Within each time period the prevalence of southern species relative to northern species tracked the El Niño Southern Oscillation. Aegires albopunctatus, a widely distributed species with a unique larval development, stood out among species found in only one time period by its high (92%) frequency of occurrence in 2001–2023. Although absent 1967–1973, it was present during that period at sites just outside SLO Bay, suggesting a bay-specific factor had caused its early absence at Cave Landing. Of six hypotheses considered, we conclude that water-borne hydrocarbons from major petroleum infrastructure located until 1999 at Avila Beach most likely caused this absence by disrupting early development of A. albopunctatus or by negatively impacting its calcareous sponge prey.