The aim of this study was to examine the impact of bioturbation by the Manila clam, Ruditapes philippinarum, on sediment stability. A laboratory benthic annular flume system (AFS) was deployed to evaluate the relationship between sediment stability of a subtidal mudflat and density of the infaunal clam under the influence of different current velocities. There was a significant correlation between mean erosion rate and current velocities in all treatments with clams (p < 0.001). There was also a significant correlation between mean erosion rate and R. philippinarum density (p < 0.001), reflecting bioturbation-enhanced sediment erosion. The effects of clam density on sediment erodability were more marked at the lower current velocities. In the control, the critical erosion velocity ( " U crit ) was about 32 cm s )1 . With increasing R. philippinarum density, " U crit decreased down to the minimum value of about 20 cm s )1 at a density of 206 clams m )2 . This study demonstrated that the burrowing activity of R. philippinarum reduces sediment stability, particularly at relatively low current velocities (25 cm s )1 ) and at densities below those found in the clam cultivation areas within the Sacca di Goro lagoon.
The reproductive cycle of the non-indigenous mytilid Musculista senhousia was studied over a one-year period in the Sacca di Goro, a lagoon in the Po River deltaic area, where the species was recently accidentally introduced. Four main stages were recognized: gonad spent, developing, ripe, and spawning. Oocytes in previtellogenesis appeared in March and coexisted with growing oocytes. Spermatogenesis preceded oogenesis, and sperm clusters were observed from May. Yolk-filled eggs were present from June, but, instead of being spawned, underwent summer degeneration. Conversely, spawning started in mid-September, coinciding with water temperature decrease, and lasted to November, after which the gonads were spent. The species is strictly gonochoric. Both the cohort-of-the-year and the survivors of the cohort-of-the-previous-year started maturation. Such an overlap of reproducing generations is probably an adaptation for survival in a variable environment. The reproductive output of M. senhousia was determined following stimulation trials using hydrogen peroxide. Emission of gametes was completed within 4 h, and a single female might release as many as 137·10 3 egg cells. Swimming D-shaped larvae appeared within 24 h of fertilization. Larval life had a duration of approximately 15-18 days.
A comparative study was carried out on the functional feeding behaviour of the infaunal Manila clam, Ruditapes philippinarum, and the epifaunal Asian date mussel, Musculista senhousia, inhabiting the Sacca di Goro lagoon, in response to changing temperature and variable quantity of food. The rates of clearance, ingestion, absorption, respiration, and energy budget (scope for growth, SFG) were measured and compared. The two non-indigenous species show a similar trend for all parameters measured in the two seasons considered (late winter and summer). SFG followed a clear seasonal trend, reaching higher values in the warmer period (July) for M. senhousia and in the cooler period (March) for R. philippinarum. Probably, this seasonal divergence was a consequence of the two species' different reproductive cycles.
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