MARINE ECOLOGY - PROGRESS SERIES Vol. 30: 261-273, 1986 Published May 22 1 Mar. Ecol. Prog. Ser. - -- l Effects of octopus predation on motile invertebrates in a rocky subtidal community Richard F. Ambrose* Department of Biology, University of California. Los Angeles, California 90024, USA ABSTRACT: Octopus birnaculatus, a common member of intertidal and subtidal communities in southern California, consumes many motile benthic invertebrate species, including snails, chitons, hpets, bivalves, and crustaceans. An analysis of octopus ddholes in empty snail shells indicates that 0. bimaculatus is a major snd predator in the community. The addition of marked snails to a natural reef with unmanipulated octopus densities demonstrated that octopuses can quickly reduce snd abundances, consuming 24 % of the available snails within 24 d. During 5 yr at 1 site, octopus abundance declined from 40 to 8 octopuses, while total prey abundance increased from 16 to 80 ind m-'. A second site experienced a similar drop in octopus density and increase in prey density over 3 yr. At both sites, snails and hermit crabs showed the greatest increases. Sedentary grazers, bivalves, and crabs and shrimps occurred at low densities throughout, possibly because the abun- dances of these preferred prey were depressed by octopus predation even during years of low octopus density. In splte of the strong negative association at the 2 sltes between octopus and prey over time, for a single time penod at 15 different s~tesprey densities were not negatively correlated wth the densities of their predators. However, octopus densities were much higher when they appeared to reduce the abundances of their prey than when the 15 different sites were sampled. I conclude that octopus predation can dramatically reduce prey densities when octopuses are unusually common, but that in most years the abundance of octopuses 1s too low to disrupt the basic patterns of prey abundance that result from other processes. The hghest prey species richness and diversity at the 2 study sites occurred during years of lowest octopus densities. Species richness, chversity and evenness of the prey assemblage were not correlated with octopus, lobster or predatory gastropod abundances at the 15 different locations; when the effects of habitat structure were held constant by partial correlation, seastar abundance was negatively correlated with species nchness and diversity. The relation between predation and species chversity suggests that, when predation is effective in this assemblage, it decreases diversity. INTRODUCTION dator-prey interaction, since new defenses against pre- dation are available through movement. Running Predators often influence the structure of com- responses are very common in motile benthic inverte- munities. The effect of predation on the number of co- brates (Feder 1963, Ansell 1969, Schmitt 1981, McKil- occurring species in a community (Paine 1966, Harper lup 1982). Motility also allows prey to use temporary 1969) has perhaps attracted the most attention. Under- refuges, thereby reducing exposure to predation (Hall lying the effects of predators on diversity are their et al. 1970, Nelson & Vance 1979, Wells 1980, Bern- effects on the abundances and/or distributions of their stein et al. 1981, Menge & Lubchenco 1981, Hines & prey. In marine communities, most studies of predation Pearse 1982). These responses may have important have involved sessile or sedentary organisms (Lub- consequences for prey distributions and the organiza- chenco 1978, Tribble 1981, Connell 1970, Russ 1980, tion of the prey assemblage (Vance 1979, Markowitz and many others). Remarkably few studies have evalu- 1980, Menge & Lubchenco 1981, Schmitt 1982, Fawcett ated how predation affects assemblages of motile 1984). benthic species. In spite of motility-related defenses, predation on Motdity itself may influence the nature of the pre- motile species has been reported to be important in p -- some communities. In temperate subtidal habitats, sea ' Present address: Department of Biological Sciences, Uni- otters, fish, and crabs may drastically reduce the versity of California, Santa Barbara, Cahfornia 93106, USA abundance of urchins (Muntz et al. 1965, Duggins O Inter-Research/Printed in F. R. Germany 262 Mar Ecol. Prog. Ser. 30: 261-273, 1986 1980, Bernstein et al. 1981) and abalone (Hines & Octopus bimaculatus is a generalist predator that Pearse 1982). Slower-moving predators such as sea- consumes a wide variety of motile benthic inverte- stars may affect gastropod (Paine 1969, Schmitt 1982) brates; at Santa Catalina Island, snails make up 75 O/O and urchin (Bernstein et al. 1981) distnbutions and of its diet, with chitons, bivalves, crabs, and hermit abundances. crabs comprising most of the remaining portion Octopuses have been reported to be major predators (Ambrose 1984). Invertebrate predators that cooccur of motile species in several marine communities (Onuf with 0. bimaculatus and consume the same prey, plus 1972, Fotheringham 1974, Wells 1980, Schmitt 1982, some sessile species, include the seastar Pisaster Arnbrose & Nelson 1983, Fawcett 1984). From 1978 to giganteus, the lobster Panuhrus interruptus, and the 1982, I examined the interactions between Octopus snail Kelletia kelletii. This assemblage of predators bimaculatus Verrill, a common member of intertidal and prey species has previously been studied by and subtidal communities in southern California, and Schmitt (1982), who described species abundance pat- its prey, assorted motile benthic invertebrates on sub- terns over a variety of rocky substrates, including cob- tidal rocky substrates. My intent was to determine the ble areas. impact of octopus predation on prey species abund- ances and species richness. In ths paper, I examine first the importance of octopus predation, relative to METHODS other predators, using field observations and experi- ments. To evaluate the impact of octopus predation on l examined 15 locations on the lee side of Santa the abundances of its prey, 1 monitored octopus Catalina Idand, off the coast of California, USA abundance and prey densities at a single location for (33" 27' N, 118" 29' W; Fig. 1).The 15 locations include 5 yr. For 3 of the 5 yr I also m.ade observations on many substrate and vegetation types, but all are rela- octopus and prey densities at a second location. tively protected horizontal areas in less than 10 m of Finally, 1 test the expectation that;if predators are a water. The most common substrates are small rocks, significant mortality source for their prey, prey abun- boulders and bedrock, somet~mesinterspersed with dances will be negatively correlated to predator sand and shelly debris; because few predators occur in abundances by relating the abundances of octopuses cobble areas (Schmitt 1982, pers. obs.), I have not and other predators to prey abundances and the com- included this habitat. Vegetation on these rocky sub- postition of the prey assemblage at different locations strates characteristically consists of 'bushy' algae and at Santa Catalina Island. erect and encrusting coralline algae. Most rocky reefs support forests of giant kelp Macrocystis pyrifera; Eisenia arborea is locally abundant at shallow (3 to Natural history of Octopus bimaculatus 5 m) depths. At some locations an introduced species, Sargassum muticum, achieves extremely high The two-spotted octopus Octopus bimaculatus densities in winter (Ambrose & Nelson 1982). The sites occurs from Santa Barbara, California and the Channel are described in more detail in Ambrose (1982b). Islands to the southern tip of Baja California and in the A permanent study site was established at the east Gulf of California (Hochberg & Fields 1980). 0. blma- end of Bird Rock ('East Bird Rock'), an islet 0.5 km off culatus is found in holes and crevices in a wide range Santa Catalina Island (Fig. 1, Site #14). Since 1976, of hard-substrate habitats. Although shelters may be giant kelp has been nearly absent and Sargassum scarce in some areas, it appears that shelters generally muticum has been seasonally very abundant. The do not limit the size of 0. bimaculatus populations study site depth varied from 4 m (at the lower edge of (Ambrose 1982a). Many octopuses inhabit the same an Eisenia arborea bed) to a maximum of 10 m. East shelter for an extended period of time (at least 1 mo; Bird Rock was chosen because it supported a large Ambrose 1982a). Mating occurs year-round, but peaks octopus population. Lobsters were also active on the in winter. Females brood their eggs for 1 to 3 mo in late site, although they concentrated their foraging spring through early summer and die soon after the activities in a nearby intertidal mussel bed. Seastars eggs hatch. The young are planktonic for one to sev- and Kelletia kelletii, though present, were never eral months before settling to the bottom. In spite of numerous. nearly synchronous hatching at Santa Catalina Island, Shell censuses. Octopuses typically kill their shelled young 0.bimaculatus settle throughout the year. After prey by drilling a small hole in the shell to inject settlement, juveniles live in small shelters and kelp poison (Pilson & Taylor 1961). Octopus drill holes have holdfasts, and apparently feed primarily on small cms- a characteristic shape and can easily be distinguished taceans. The lifespan of 0, bimaculatus is probably from the drill holes of predatory gastropods. It IS pos- 12 to 18 mo, dependmg on the time of settlement. sible to obtain a rough estimate of octopus