Testing the habitat harshness hypothesis: reproductive biology of the wedge clam Donax hanleyanus (Bivalvia: Donacidae) on three Argentinean sandy beaches with contrasting morphodynamics

摘要

In order to test the habitat harshness hypothesis (HHH) the reproductive biology of Donax hanleyanus was studied histologically, comparing populations from three beaches with contrasting morphodynamics (dissipative, intermediate and reflective) over 25 months. The reproductive phase of D. hanleyanus was extended at the reflective beach compared to the other two. Males and females from the dissipative and intermediate beaches were significantly smaller and had lower biomass at maturity than those at the reflective beach. Recruits were significantly more abundant and the recruitment period was extended significantly at the dissipative beach. Spawning events took place twice each year at the dissipative (early spring and spring-summer) and the intermediate beach (winter and summer), whereas continuous gamete releases were noted at the reflective beach. Size and biomass at first maturity were lower at the dissipative beach, whereas monthly mean abundance of D. hanleyanus was higher at the reflective beach. The gametogenic cycle correlated significantly with sea-surface temperature, relative spermatozoon abundance, condition index, ash-free shell-free dry mass, and mean size and abundance of oocytes, for all three populations. At the population level, many of these reproductive responses to physical variables were opposite to those predicted by the HHH, including: greater abundance, extended reproductive cycle, extended period with spawning individuals, and larger size and higher biomass at first maturity at the reflective beach. This suggested that the hypothesis of habitat safety (HHS), originally proposed for supralittoral species, may be extended to intertidal species; a combination of narrow swashes and steep slopes makes reflective beaches a safer and more stable environment for intertidal species such as D. hanleyanus.