Please note that this is the English translation (with original title) to: Bertsch, Hans. 2008. Opistobranquios. In: Danemann, Gustavo D., y Exequiel Ezcurra (eds.), Bahía de los Ángeles: Recursos Naturales y Comunidad. Línea Base 2007. SEMARNAT, Pronatura Noroeste, SDNHM & Instituto Nacional de Ecología, México. Pp. 319-338. Figures and tables in original publication are accessible at: http://www.ine.gob.mx/publicaciones/consultaPublicacion.html?id_pub=546 Ten-Year Baseline Study of Annual Variation in the Opisthobranch (Mollusca: Gastropoda) Populations at Bahía de los Ángeles, Baja California, México Hans Bertsch Research Associate, California Academy of Sciences, 192 Imperial Beach Blvd., # A, Imperial Beach, CA 91932, USA, [email protected] With gratitude, affection and respect, I dedicate this work to Rosa del Carmen Campay–Villalobos, Tom Smith, and the people of Bahía de los Ángeles. IN MEMORIAM: Benjamin Casillas López (1976–2007) “The shores of the Gulf, so rich for the collector, must still be fairly untouched....We had not the time for the long careful collecting which is necessary before the true picture of the background of life can be established.” (John Steinbeck, 1941, The Log from the Sea of Cortez) Abstract From January 1992–December 2001 research dives were conducted at two shoreline sites north of the town of Bahía de los Ángeles (BLA), Baja California, using the total density per unit of time. This yielded 6095 specimens of opisthobranch molluscs, and approximately 81 species. Nine species were each encountered over 100 times, totaling 4999 specimens (82 % of the total number). Regression analyses on these species showed that over all the density of species per year significantly increased (P< = 0.01), but the number of specimens significantly decreased (P <= 0.007). Comparisons of 8 species of Chromodorididae nudibranchs between sites at BLA and Punta Arenas/Isla Cerralvo, Baja California Sur, indicate higher densities and larger total body lengths for 2 species. Introduction Baseline studies of the marine invertebrate fauna of Bahía de los Ángeles (BLA) are especially important to provide information that can be used to support the establishment of a natural park in this region, to establish guidelines for biodiversity conservation, and to monitor changes after the establishment of this reserve. In fact, as Hendriks et al. (2006) write, “We must improve our understanding of how the global ocean ecosystem works in order to design networks of protected areas that effectively preserve biodiversity.” Habitat descriptions throughout the Sea of Cortez Descriptions of the general biology, physiography and physical environment, and the lower intertidal and subtidal rocky reef zones at BLA and throughout the Sea of Cortez can be found in references such as Roden & Groves (1959), Barnard & Grady (1968), Robinson (1973), Brusca (1980), Bertsch (1991) and others. Schwartzlose et al. (1992) provide a comprehensive bibliography for the Sea of Cortez. Bertsch, OpPopulationsBLA: 3 Molluscan studies at Bahía de los Ángeles For many years, our knowledge of the molluscan fauna of BLA has been based primarily on sporadic collections or taxonomic lists of species collected over a wide range of localities throughout the bay, but without continuous monitoring of specific locations (e.g., see Steinbeck & Ricketts, 1941; McLean, 1961; Coan, 1968; Poorman & Poorman,1978; and Hertz, 1978 a, b). The resource sustainability of Modiolus capax (Conrad, 1837) is briefly discussed by Farmer (1990), and García–Aguirre & Buckle–Ramirez (1989). Two references to the fishery of the scallop Pecten vogdesi Arnold, 1906, give quite contradictory statements. Coan (1973) states “the fishery [is] apparently quite successful,” whereas Wolfson (1970) writes “a study of the disappearance of edible clams from this area is carried out by the author.” It is important to emphasize that this fishery has not existed in the BLA region for over 25 years (pers. obser.) because of over harvesting of the resource. Other molluscan bivalve studies include Serrano–Guzman & Avalos–Borja, 1991 (clam larval stages), and Islas–Olivares, 1982 (regarding the cultivation of the Japanese oyster, Crassostrea gigas). Short-term taxonomic studies have provided most of our knowledge about the opisthobranch fauna. The type localities of 11 named species are in this region: Okenia angelensis Lance, 1966 (named from the “lowest intertidal zone, Bahía de los Ángeles”); Cerberilla pugnoarena Collier & Farmer, 1964, and Eubranchus cucullus Behrens, 1985 (both named from Puerto Refugio, Isla Ángel de la Guarda); Nembrotha hubbsi Lance, 1968 (Isla la Ventana), now a junior synonym = Tambja eliora (Marcus & Marcus, 1967); Cuthona longi Behrens, 1985 (Isla Raza); and 5 species from Punta la Gringa: Bajaeolis bertschi Behrens & Gosliner, 1988; Polycerella glandulosa Behrens & Gosliner, 1988; Trapania goslineri Millen & Bertsch, 2000; Peltodoris lancei Millen, in Millen & Bertsch, 2000; Okenia angelica Gosliner & Bertsch, 2004; and Dendrodoris stohleri Millen & Bertsch, 2005. Long-term subtidal opisthobranch studies in this region have been reported by Bertsch, 1995, 2002 (natural history of Doriopsilla gemela); Bertsch, et al., 1998 (differences in community structures between Punta La Gringa/Cuevitas and Island/Puerto Don Juan sites); and Gosliner & Bertsch, 2004 (annual and monthly density variations, 1992–2001, of Okenia Bertsch, OpPopulationsBLA: 4 angelica). This present work is a 10-year baseline study of the opisthobranch populations at two shore sites approximately 10 km north of the town of BLA, to enable future comparisons to be made regarding the health of or changes in these populations. This is the first long-term data set of the annual occurrences and variations among species and specimens for a group of marine invertebrates at BLA. Importance and Selected Examples of Longitudinal Studies Long-term longitudinal studies are increasingly becoming recognized as significant in determining population extinctions, habitat degradation, and resource management. Among various long-term studies in other regions and habitats. Thomas et al. (2004) and Foster & Aber (2004) describe repetitive community surveys of terrestrial plants, birds and butterflies. Diller (2004) and Milius (2004) discuss population variations in the mollusc-eating penguin Spheniscus magellanicus (Forster, 1781) and killer whales (Orca). For decades, amateur aficionados have been providing significant density and diversity data for birds (Audubon Bird Count: www.audubon.org/bird/cbcl ) and marine fish (Great Annual Fish Count: www.fishcount.org ). Historical overfishing and the recent collapse of coastal ecosystems are described in Jackson et al. (2001) and Lotze et al. (2006). Numerous government and academic investigators are conducting long-term studies on members of coral reef communities. Topics have included coral recovery after major predation events (Walbran et al., 1989), the decline of coral reef ecosystems (Pandolfi et al., 2003), the effects of coral decline on fish biodiversity (Jones et al., 2004), and the future of coral reefs due to marine protected areas (Mora et al., 2006) and major limitations of reproductive failure, habitat degradation and fragmentation, and the role of sea temperature changes in coral bleaching (Knowlton, 2001). Published studies on opisthobranchs mainly involve temperate water intertidal locations. Todd (1981) and Trowbridge (2002) provide summaries and bibliographic sources. Trowbridge (1993) studied the population structure of two species of sacoglossan during a 4-month span. Nybakken (1974 and 1978) studied the abundance, diversity and temporal variability in central Californian intertidal nudibranchs over 27-month and 40-month periods. Bertsch, OpPopulationsBLA: 5 Materials and Methods In this survey within the synecdochic BLA, two sites accessible from the shore were studied: Punta la Gringa (29E 02.57' N’; 113E 32.3' W) and Cuevitas (29E 03.39' N; 113E 32.37' W). These sites are less than one nautical mile apart and are approximately 10 km north of the small fishing and tourist town of BLA. Both study sites have rocky basalt reefs and sandy/mud/silt and fine-grained rock and shell substrates (Bertsch, 1991). In the 10 year period from January 1992–December 2001, the author made 211 research dives, totalling 229.3 hours of search time. During each research dive, every opisthobranch specimen was identified, counted and measured. Camacho–García et al. (2005) and Behrens & Hermosillo (2005) provide descriptions and color photographs of all species. Data collected by safety diving partners were not used in this analysis to ensure rigorous and similar control over quality of data collection. Actual collection of specimens was the barest minimal for taxonomic purposes, so as not to skew the data by withdrawal of the populations’ members. The method of density study used related the number of specimens and species per unit of time searched (Nybakken, 1974, 1978). This method is statistically and realistically better in observing/finding opisthobranchs (rather than the traditional transect and quadrant system) because of the highly random distributional patterns of these organisms. Results During the 10-year period from January 1992–December 2001, 6095 specimens of opisthobranch molluscs were counted and identified at these 2 sites, with a total count of at least 81 species (some were not identifiable, and some may actually include more than 1 species, because