Reproduction and recruitment of the barnacle Chthamalus montagui at Aberystwyth (mid-Wales)

Data on the recruitment of Chthamalus montagui were collected over the 7 yr period 1979-85 at a site in Wales close to the species northern geographic limit. When compared with data from more southerly sites they show that the breeding period, the period of larval settlement, and the density of recruits are all reduced in the northern part of the species range. Intensive studies of settlement carried out during 1985 confirmed that poor recruitment was a result of low numbers of cyprids reaching the shore. Such poor larval supply occurs annualy; it is therefore proposed that competition with Semibalanus balanoides, which Connell (1961) suggested sets the northern limit od C. montaqui, can only act to intensify the effects of a restricted breeding period and the consequential low level of larval recruitment. The range of the barnacle Chthamalus montagui (Southward 1976) extends from the north of Scotland to the Mediterranean Sea (Crisp et al. 1981). On the shores of the Firth of Clyde, Scotland, towards the species' northern limit of distribution, competition with the boreal Semibalanus balanoides (L.) limits C. montagui to a relatively narrow band at the top of the shore from which the former is excluded by its intolerance to desiccation (Connell 1961, Foster 1971). It has been suggested that as summer conditions become less severe further to the north, the boundary between the 2 species should move upshore and eventually reach a level where C. montagui is totally displaced. On the east coast of the USA, Wethey (1983) has demonstrated a similar relationship between S. balanoides and Chthamalus fragilis and suggested that the northern limit of the latter is set by competition. In view of both the unusually high levels of Semibalanus balanoides settlement in Connell's study area (Kendall et al. 1982), and the latitudinal variation in the breeding cycle of other barnacles (Hines 1978), it would be unwise to restrict discussion of the factors setting the geographic Limits of Chthamalus montagui to competition alone. Hines (1978) discussed the way in which a number of barnacles with wide distributions have a restricted breeding period in the northern part of their range. For example, in central and southern California Pollicipes polymerus reproduces throughout the year but in more northerly populations breeding is restricted to summer (Hines 1978). No comparable field data for C. montagui exist although Pate1 & Crisp (1960) have shown in the laboratory that 'C. stellatus' (probably C. montagul] will breed only at temperatures above 15°C. This would suggest that the extent of its breeding season should vary latitudinally, becoming progressively shorter with distance north. In an intensive study of Semibalanus balanoides (Kendall et al. 1985) it has been shown that annual variability in the density of recruitment is the result of differences in the cyprid supply to the shore. In such cases, factors acting after larval settlement could only serve to modify the final total of spat. The possibility that the low annual recruitment of Chthamalus montagui to sites close to its northern limit could, similarly, be the result of a poor supply of larvae was therefore investigated during the summer of 1985. The population of Chtharnalus rnontagui studied (at Aberystwyth) appears to be typical of many others close to the species' northern limit. Although annual reruitment is low (see below), as a result of the species' increased longevity in the north (Lewis 1964) the rock surface within the zone it occupies is generally totally covered. Methods. The study was carried out on a small jetty of hard igneous rock situated about 200 m to the north of the promenade at Aberystwyth. Dyfed, mid-Wales (52"25'N, 4"05'W). In June of each year from 1979 to 1985, 3 areas of 250 cm2 on each side of the jetty were cleared of all flora and fauna; the uppermost of these sites was positioned close to the highest surviving Chtharnalus rnontagui and the lowest at the overlap between that species and Semibalanus balanoides, a O Inter-Research/Printed in F. R. Germany 0171-8630/87/0038/0305/$ 03.00 306 Mar Ecol. Prog. Ser. 38: 305-308, 1987