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Patterns of intraand interspecific competition on artificial substrata were studled in Patagonia, in an epifaunal community composed of 19 species of bryozoans, 4 spirorbid polychaetes and 1 colonial tunicate. The latter was found to be dominant over bryozoans and both groups were able to overgrow spirorbids. Apparent cessation of growth occurred in intraspecific encounters among bryozoan colonies. Several cases of homosyndrome were recorded in the dominant species Umbonula alvareziana. Competitive ability was correlated with zoarial height in cheilostome but not in cyclostome bryozoans. Lack of correlation between abundance and competitive ability could be related to the relatively early successional stage of this assemblage, where a high proportion of the surface is bare. Analysis of the interaction matrix indicates a clearly hierarchical con~petitive organization of the community. Colony size was found to be an important factor influencing the outcome only in encounters between species with similar competitive abilities. Relationships among number of ovicells, colony shape and overgrowth ability were analyzed in 2 closely related species of Celleporella: the runner-like C. patagonica compensates its poor conlpetitive performance by reaching sexual maturity earlier and having a significantly higher rate of ovicell production than the sheet-like C. yagana. INTRODUCTION competitive ability, ovicell production and morphological characteristics in some of the most frequent species. Studies on competitive overgrowth interactions Although the present study focused mainly on intraamong encrusting invertebrates in ep~faunal comand interspecific competition among bryozoan colmunities have Increased considerably In recent years. onies, some observations on overgrowth abilities of Since substratum availability is often the limiting factor spirorbids and a colonial tunicate are also included. in sessile benthic assemblages (Connell 1961, Dayton 1971, Paine 1974, Jackson 1977a, Osman 1977), mechanisms for acquiring and holding space are funSTUDY AREA AND METHODS damental for both solitary and colonial organisms. Data on competitive interference abilities have been The study was carried out at Puerto Deseado, Santa obtained in different temperate and tropical habitats, Cruz, Argentina (47'45'S, 65"55'W), which is located such as coral undersurfaces (Jackson & Buss 1975, Buss on the northern shore of an estuary about 40 km in & Jackson 1979, Jackson 1979a), intertidal and subtidal length and oriented in an E-W direction in its outer hard bottoms (Gordon 1972, Osman 1977, Buss 1980a, zone. Water temperatures are typically subantarctic b , 1981, Quinn 1982, Rubin 1982, Paine 1984), algae and influenced by the Malvinas Current, varying from (Stebbing 1973a, b, O'Connor et al. 1980), hermit crab about 4.5"C in winter to 13.5"C in summer. Extreme shells (Karlson & Shenk 1983) and artificial substrata tide amplitude reaches almost 6 m , producing strong (Keen & Neill 1980, Russ 1982). Competition for space tidal currents. Salinity varies around 33 %O Kiihnemann has also been studied in fossil epifaunas by Taylor (1970) has described the Puerto Deseado area in more (1979, 1984) and Liddell & Brett (1982). detail. This paper analyzes patterns of overgrowth competiTen sand-blasted acrylic panels (24 X 14 X 0.2cm) tion in a Patagonian bryozoan community growing on were provided as surfaces for settlement and growth of artificial panels, as well as the relationships among sessile organisms (Fig. 1). They were screwed to a 0 Inter-Research/Printed in F. R . Germany 122 Mar Ecol. Prog. Ser. 51. 121-130, 1989 rectangular iron frame in 2 superposed series of 5 adjoining panels. The whole set was maintained in a vertical position ca 2 m beneath low tide level for 9 mo (1 April to 28 December 1977). Panels were then fixed in 5 % formalin, rinsed in water, air-dried and examined under a stereomicroscope. Percentage cover of each species was estimated by point sampling (Jackson 1977b, 1979a, Sutherland & Karlson 1977, Russ 1980). One hundred random points were marked on a transparent sheet which was laid on the panels and organisms underlying these points were recorded. Percentages were then calculated from data pooled over all panels. Following Stebblng (1973a), overgrowth in bryozoans was defined as the elevation of the growing edge of a colony over another so as to cover the apertures of zooids. All cases of competitive interaction between bryozoan species were counted, with the exception of intraspecific competition in Umbonula alvareziana, in which a maximum of 100 encounters was examined. The methodology proposed by Russ (1982) for statistical treatment of interspecific overgrowth, as well as the arrangement of data in matrices of competitive interaction, was adopted. Species involved in 5 or less encounters were not included in the matrix. Chi-square l-sample tests were performed on interactions with 5 or more cases, so as to test the null hypothesis that no differences in competitive ability exist between both competitors (1 degree of freedom, a = 0.05). In interactions with ties (i.e. apparent cessation of growth on the line of contact between colonies), a score was calculated for each species, made up of the sum of 2 points for each win plus l point for each tie. The chi-square test was then performed on these scores (Russ 1982). In order to test the influence of zoarial size on the outcomes between species with similar competitive ability, an interaction matrix was calculated among 7 of the 12 studied bryozoans, taking into account the relative size of both colonies. Species with very poor overgrowth abilities, which generally showed low proportions of ties and reversals, were not included in this matrix. As a measure of competitive ability the wins/losses ratio (W/L) of each bryozoan species was calculated (Jackson 1979a). Zoarial type, number of spines in marginal zooids and marginal thickness of the colony (including spines) were also recorded for the 12 most frequent bryozoan species, in order to relate overgrowth ability with morphologic characteristics of zooids and zoaria. Values for these 12 species (Table 1) were ranked for percentage cover, W/L ratio and colony thickness. Paired correlations were calculated using Spearman's rank order coefficient (Siege1 1956). An index proposed by Rubin (1982) was used to measure the degree of intransitivity of the assemblage: where P,, [W] = probability that species i will overgrow species j ; P, [L] = probabhty that species i is overgrown by species j ; n = number of species. This index yields a value of 1 in a perfectly intransitive association. However, a value of is obtained in a perfectly transitive relationship, which would be inadequate as a definition for the upper limit of the index. To overcome this problem, Rubin (1982) calculated values of nearperfect transitivity of 3.37, 4.09, 4.61, 5.01, 5.32 and 5.57 for associations of 3, 4 , 5, 6, 7 and 8 species respectively, giving each species a Pij [L] = 0.1 in its cncounters with the 2 immediately below in the hierarchy. In order to compare the rates of ovicell production of Celleporella yagana and C. patagonica, 30 ovicelled colonies of each species were randomly chosen among those having unobstructed growing margins. Obstructed colonies were discarded because growth detention is likely to produce some effect on ovicell production. Numbers of ovicells and zooids were counted on camera lucida drawings of the colonies. Indirect computation of zooid number by measurement of colony area (only applicable in this case to C. yagana) was avoided, for it would have introduced a bias in the data. As a previous step in the analysis, a third-order polynomial regression of ovicell number over zooid number was fitted in both species by means of the POLY program of the statistical package BIOM (Rohlf 1982). Results of this analysis indicate that the functional relation between both variables can be adequately accounted for by a linear model in C. yagana. On the other hand, C, patagonica seems to show a slight trend to curvilinearity. However, while the linear term is highly significant (P P>0.01). As a consequence, a linear model was also fitted in this species, and considered valld until more data can be gathered about ~ t s reproductive pattern. The functional relation between ovicell and zooid number are described by the slope of the major axis of the confidence ellipses in a Model I1 regression (Sokal & Rohlf 1981).

Overgrowth competition in an assemblage of encrusting bryozoans settled on artificial substrata