The successful settlement of the African pea crab Afropinnotheres monodi in the Bay of Cadiz was analysed to get new insights into the evolution of pea crab parasite life history traits. The pea crab lives symbiotically and with high prevalence in the bivalves Cerastoderma glaucum and Mytilus galloprovincialis, and with low prevalence in Scrobicularia plana. A remarkable monopolisation of host (one crab/bivalve) occurred irrespective of host species and crab demographic categories (males, hard females, soft females), probably as an optimisation of resources in smallsize hosts. However, there was a clear asymmetry in host use by the different crab 2 categories. A 1:1 sex ratio was found in crabs harboured by C. glaucum and S. plana, with most of the female crabs being small and hard specimens, whereas crabs inhabiting M. galloprovincialis were primarily large reproductive females. Ovigerous females were found throughout the year in M. galloprovincialis and there was a strong correlation between female size and fecundity, suggesting that the females harboured by this host were the major contributors to the reproductive effort of the studied population. Conversely, most of the new crabs recruiting to the population were harboured by the remaining two host species. We hypothesise that such a generalist but asymmetrical usage of bivalve hosts by pea crabs may have clear benefits for species in geographical expansion as A. monodi, facilitating its dispersal to new locations and its successful settlement in sheltered systems, as the Bay of Cadiz.

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The composition and seasonal variation of the ichthyoplankton community in a shallow tidal channel in Cadiz Bay was studied for 5 years (1984 1988), taking fortnightly samples with a conical tide-strained net (0.5 mm mesh). Thirty-nine species belonging to 21 families of teleosts were caught. Four species (Pomutoschistus microps Kreryer, Atherina hoyeri Risso, Liza rumuda Risso and Lizu uurafu Risso) totalled more than 90% of the annual ichthyoplankton abundance.Three species, assumed to be permanent residents, dominated the number of fish (837%), and were represented by all their developing stages; 26 species (66.7%) were temporary residents, contributing 17.2% to the total abundance and were represented only by early developing stages; the remaining 10 species were classified as rare. Monthly numbers of species and individuals showed seasonal variation with a main annual peak in spring and in late spring-early summer, respectively. The diversity (H') and evenness (S) indices of this community were low (0.13 to 1.83 and 0.03 to 0.71. respectively), with higher values in winter and late summer, and both were significantly correlated with the log-transformed density of temporary residents (r = 0.43 and 0.41, respectively; P

ABSTRACT. Adult females of Panopeus africanlis A. Milne Edwards, 1867 and Uca tangel-i (Eydoux, 1835) were sampled fortnightly from Apnl 1989 to November 1991 in the Rio San Pedro lnlet (SW Spaln) to estimate the seasonal timing of their reproductlve periods. Planktonic larvae were collected monthly from July 1991 to June 1992 by pumping during nocturnal flood and e b b spring tides to establish seasonal larval abundance patterns. Additional 24 h series of samples were collected in July 1991 and May/June 1992 to analyse larval abundance in relation to the main environmental cycles (diel, tldal and lunar phases, and vertlcal position In the water column). Both specles showed a short, latesprlng/summei-reproductive period: mature and ovigerous fenmles peaked from May to July Seasonal occurrences of first zoea larvae also indicated short reproductlve periods for both species. Flrst zoea was the most abundant stage, representing 99.5':,, ( P africanus) and 99.9% (U. tangeri) of all individuals collected. On most sampling occasions, there was a net output of zoea I of both species from the Inlet, suggestlny that larvae are released in the Inlet ancl exported to the bay. The abundance of the first zoea of P afrlcanus was higher durlng hlgh and e b b tides, at nlght and at the surface of the water column. For t h~s species, there was a sign~ficant Interaction between lunar phase and diel phase: zoea l were most abundant during nocturnal neap tides. Zoea 1 of U. tangeri showed greater d e n s~t y at e b b tides; they were more abundant at the surface of the water column during e b b than a t high tide, suggesting a vertical migration between these tidal phases. Considerable short-term (consecutive or alternate days) and long-term (Interannual) vanation in the abundance of larvae was observed for both species. The comparison of our results w~t h the scarce information available on the reproductive periods and larval patterns of both species suggests that, at the northern llmlt of their latitudinal ranges, water temperature may control the seasonal timing of reproductive penods, but populations under d~fferent hydrodynamic regimes (different water conditions) might show peaks of larval release during different diel/tidal/lunar phases; this associat~on of larval release with rhythmic cues may serve as a mechanism that may aid escape of larvae from unsuitable environmental conditions.

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