MARINE ECOLOGY PROGRESS SERIES Vol. 70: 39-48, 1991 Published February 14 Mar. Ecol. Prog. Ser. ~ Spatial distribution and the effects of competition on some temperate Scleractinia and Corallimorpharia Nanette E. Chadwick* Department of Zoology. University of California, Berkeley. California 94720. USA ABSTRACT: The impact of interference competition on coral community structure is poorly understood. On subtidal rocks in the northeastern Pacific, members of 3 scleractinian coral species (Astrangia lajollaensis, Balanophyllia elegans, Paracyathus stearnsii) and 1 corallimorpharian (Corynactls califor- nica) were examined to determine whether competition exerts substantial influence over the~rabund- ance and distributional patterns. These anthozoans occupy > 50 % cover on hard substrata, and exhibit characteristic patterns of spatial distribution, with vertical zonation and segregation among some species. They interact in an interspecific dominance hierarchy that lacks reversals, and is linear and consistent under laboratory and field conditions. Experiments demonstrated that a competitive domin- ant, C. californica, influences the abundance and population structure of a subordinate. B. elegans, by (1)reducing sexual reproductive output, (2) increasing larval mortality, (3)altering recruitment patterns. Field cross-transplants revealed that the dominant also affects vertical zonation of a competitive intermediate, A. lajollaensis, by lulling polyps that occur near the tops of subtidal rocks. It is concluded that between-species competition, mediated in part by larval-adult ~nteraction,strongly influences the structure of this temperate anthozoan assemblage. INTRODUCTION the processes that underlie distributional patterns, long-term manipulation of natural populations is Scleractinian corals and other anthozoans exhibit necessary to reveal competitive and other interactions. diverse aggressive behaviors, which have been pro- However, field experiments on competition among posed to mediate interference competition on tropical tropical anthozoans have extended only 11 nlo reefs (references in Sebens 1976, Chadwick 1988a, (Romano 1990), and long-term manipulations have not review by Lang & Chornesky 1991). However, con- been conducted on temperate anthozoans. troversy exists over the impact of competitive behavior In temperate marine habitats, anthozoans such as on coral community structure. In some areas, disturb- actinian sea anemones, corallimorpharians, and aher- ances such as wave action and predation may limit matypic scleractinian corals may dominate benthic coral abundance to levels below those at which com- communities on hard substrata (Pequegnat 1964, petition is important (Connell 1978, Bradbury & Young Turner et al. 1969, Castric-Fey et al. 1978, Schmieder 1982, Dollar 1982). In the tropics, coral competitive 1985, Lissner & Dorsey 1986), and the extent to which interactions mediated by aggression appear to exert their competitive interactions influence community community effects mainly on reef slopes where physi- structure is of interest. An assemblage of 4 interacting cal disturbance is low and coral density high (Grigg & anthozoans on subtidal rocks on the California coast is Maragos 1974, Maragos 1974, Benayahu & Loya 1981, particularly amenable to experimental analysis, in con- Sheppard 1982). trast with tropical reef assemblages which contain up to Because field observations alone cannot elucidate 54 species of interacting corals (Sheppard 1979). Directed interspecific aggression, resulting in substan- Present address: Hopkins Marine Station of Stanford Uni- tial damage to opponents, has been documented in 1 of versity, Pacific Grove. California 93950, USA the Californian species, Corynactis californica Carl- O Inter-Research/Printed in Germany 40 Mar. Ecol. Prog. Ser. 70: 39-48, 1991 gren, 1936 (Chadwick 1987). It is possible also to The quadrat size was chosen as large enough to examine competitive interactions between larvae and enclose many polyps (up to 160), yet small enough for adults in this system, because members of another identification of each polyp. species, Balanophyllia elegans Vemll, 1864, release To assess abundances, photographic transparencies large, benthic larvae (Gerrodette 1981, Fadlallah & were projected onto a screen and the number of polyps Pearse 1982). of each anthozoan species determined per 12 x 18 cm Through field surveys, and field and laboratory area (= #/0.0216 m2) and then converted to the experiments, I demonstrate that: (1) some Californian number per 0.01 m2. Percent cover was estimated via a anthozoans exhibit subtidal vertical zonation, (2) they uniform array of 100 points superimposed on the pro- interact in a linear dominance hierarchy, (3) the abun- jected image. dance, distribution, and/or population structure of sub- Patterns of interaction. To investigate the possibility ordinate and intermediate species is influenced by the of competitive interactions within and between the competitive dominant, (4) larval-adult interactions species, I observed the outcomes of contacts between mediate some of these processes. randomly selected polyps at 8 to 15 m depth. Polyps Field work was conducted at the Hophns Marine were considered to interact if close enough for their Life Refuge (HMLR), Monterey County, California, extended tentacles or mesenterial filaments to touch, USA (36" 37.4' N, 121" 54' W). The site consists of within ca 5 mm inter-skeletal or inter-column distance. granite outcroppings interspersed with sand channels For each interaction, the outcome was noted as: dam- that slope down to a sand plain at 12 to 15 m water age to one individual, damage to both, or damage to depth. A dense forest of giant kelp Macrocystispyrifera neither. Injuries were visible as an area of dead or (Linnaeus, 1771) grows on these rocks during most of damaged tissue, or exposed calcareous skeleton along the year. Sessile invertebrates are especially dense on the region of contact. rocks in the deeper parts of the kelp forest where few Laboratory experiments were conducted to deter- understory algae grow (Pearse & Lowry 1974). mine the time course of interactions, and to compare At least 13 species of anthozoans occur in the kelp the outcomes of experimental and natural contacts. I forest at HMLR (Pearse & Lowry 1974). Three aher- collected the corals at HMLR, transported them to the matypic scleractinian corals and 1 corallimorpharian Joseph M. Long Marine Laboratory of the University of occupy exposed rock surfaces at 5 to 15 m depth in the California at Santa Cruz (USA), and cemented them outer parts of the kelp bed. The corallimorpharian underwater onto glass microscope slides (7.5 X 5 cm) Corynactis californica forms large aggregations of with Sea Goin' Poxy Putty (Permalite Plastics Corpora- polyps each 10 to 25 mm in diameter (Morris et al. tion, Newport Beach, CA 92663, USA). Adjacent to 1980, Chadwick 1987). The coral Astrangia lajollaensis each coral I attached a polyp of another coral species, Durham 1947 forms encrusting colonies of 5 mm at <5 mm inter-skeletal distance, but not in direct diameter polyps; each colony may live for 40 yr or more skeletal contact. Individuals of Corynactis californica (Fadlallah 1982). Balanophyllia elegans is a solitary also were collected from HMLR, and a piece of barna- coral with 10 mm diameter polyps (Morris et al. 1980), cle shell bearing a single corallimorpharian polyp was that each have a lifespan of 5 to 10 yr (Fadlallah 1983). cemented adjacent to each coral. I set up at least 20 Paracyathus stearnsii Verrill, 1869 produces solitary pairs of polyps in each of the 6 possible combinations of coral polyps with oral disks up to 20 mm in diameter the 4 species. To monitor effects of within-species con- (Pearse & Lowry 1974). tact, 20 individuals of each species were cemented into contact with conspecifics, and these slides interspersed with those bearing heterospecific pairs. Only full-sized, MATERIALS AND METHODS undamaged adults were used (sizes given above). The anthozoans were maintained in plastic trays supplied Patterns of spatial distribution. I quantified patterns with running seawater at ambient sea temperature (11 of vertical distribution and abundance of the above to 17 'C), and fed adult brine shrimp Artemia salina anthozoans on 3 large rocks that extended 2.5 to 4.5 m each week to saturation; the shrimp were readily con- in height above a sand plain at 11 to 12 m water depth. sumed by all polyps. Percent tissue damage to each The abundance of anthozoans was determined from polyp was determined monthly for 6 mo. photographs taken by height above the sandy bottom Field removal experiment. Based on results from the (the anthozoans appeared to be distributed with refer- above experiments, I investigated long-term effects of ence to height rather than depth, Pequegnat 1964). Six competition with Corynactis californica on Balanophyl- 12 X 18 cm framed photographs were taken of ran- lia elegans. The reverse experiment was not per- domly-selected areas at an interval of 0.5 m up the formed, because previous work (Chadwick 1987) indi- vertical sides and along the horizontal top of each rock. cated that C. californica unilaterally damages B. ele- Chadwick. Temperate coral distribution and competition 4 1 gans. I selected 3 adjacent rocks, each 8 to 10 m in Field transplant experiment. A short-term transplant height above sandy substratum at 15 m depth, with flat experiment was conducted on a subtidal rock that horizontal tops and