Reproductive periods and larval abundance patterns of the crabs Panopeus africanus and Uca tangeri in a shallow inlet (SW Spain)

Abstract

Adult females of Panopeus africanus A. Milne Edwards, 1867 and Uca tangeri (Eydoux, 1835) were sampled fortnightly from April 1989 to November 1991 in the Rio San Pedro inlet (SW Spain) to estimate the seasonal timing of their reproductive periods. Planktonic larvae were collected monthly from July 1991 to June 1992 by pumping during nocturnal flood and ebb 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, tidal and lunar phases, and vertical position in the water column). Both species showed a short, late-spring/summer reproductive period: mature and ovigerous females peaked from May to July. Seasonal occurrences of first zoea larvae also indicated short reproductive periods for both species. First 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, suggesting that larvae are released in the inlet and exported to the bay. The abundance of the first zoea of P. africanus was higher during high and ebb tides, at night and at the surface of the water column. For this species, there was a significant interaction between lunar phase and diel phase: zoea I were most abundant during nocturnal neap tides. Zoea I of U. tangeri showed greater density at ebb tides; they were more abundant at the surface of the water column during ebb than at high tide, suggesting a vertical migration between these tidal phases. Considerable short-term (consecutive or alternate days) and long-term (interannual) variation in the abundance of larvae was observed for both species. The comparison of our results with the scarce information available on the reproductive periods and larval patterns of both species suggests that, at the northern limit of their latitudinal ranges, water temperature may control the seasonal timing of reproductive periods, but populations under different hydrodynamic regimes (different water conditions) might show peaks of larval release during different diel/tidal/lunar phases; this association of larval release with rhythmic cues may serve as a mechanism that may aid escape of larvae from unsuitable environmental conditions.