Periodic inundation by sand is a very common feature of rocky coasts throughout the world. Even so, there have been few direct observations or experiments to investigate the role of sediments on intertidal rocky shores. We designed a field experiment in Mazatl´an Bay, Mexico, to test the initial impact and subsequent recovery of intertidal macrobenthic assemblages exposed to sand burial at two sites of varying wave exposure. Both sites supported different natural assemblages. Treatment plots for the addition of sediment and control plots (50×50 cm), separated by at least 1.5 m, were randomly placed across the mid-water tidal level. The initial response of the resident macrobenthos and the subsequent recolonization was monitored over a period of 95 days. The main effect of sediment deposition at both sites was mortality and removal of biota due to smothering. The recovery process was rapid and may in part have been the result of the mechanism by which the small, disturbed patches were recolonized. Most of the invertebrates colonized the patches as adults; several seaweeds exhibited vegetative growth as the major mechanism of colonization (e.g., Ulva lactuca Linnaeus, 1753, Amphiroa valonioides Yendo, 1902 and Chaetomorpha antennina (Borgensen) Kutzing, 1849). The rate of recovery varied between the sites, however. Recovery of species numbers proceeded quickly at the sheltered site (day 7), but took 95 days at the exposed site. In contrast, biomass reached control levels by day 45 at the sheltered site, but already by day 15 at the exposed site. By day 95, the assemblages recovered to 83.5% and 81% similarity with the controls at the sheltered and exposed sites respectively. Although differences in wave exposure could be very important in determining the different patterns of recovery at both sites, other biological processes may also play an important part.
The adults of Fortuynia atlantica sp. nov. are described in detail, including intraspecific variation of several morphological characters. Sensillus short, smooth, clavate; one pair of lamellar furrows; 14 pairs of notogastral setae vestigial c3; five pairs of genital setae, variations 6+5, 5+6, 5+4, 4+5; two pairs of anal setae, variations 3+2, 2+3. Leg setation (chaetome, solenidia): Leg I 1-4-2-3-18, 1-2-2; leg II 1-4-2-3-15, 1-1-1; leg III 2-3-1-3-15,1-1-0; leg IV1-2-2-3-13,0-1-0. The new species shows a remarkable secondary sexual dimorphism; males are characterized by a pair of rounded lateral notogastral protuberances, four large porose areas on notogaster, and very long, distally lanceolate notogastral setae la and lm, as well as by a less broad genital opening. The species colonizes the intertidal zone of rocky shores of the Bermuda Islands, where it lives mainly in crevices but also under covering mats of algae; it is absent from sandy beaches and mangrove trees. The species is well adapted to overflow with sea water. Feeding biology and other details in behaviour are discussed. It is the first described member of the family Fortuyniidae known from the Atlantic Ocean. This paper is listed as “Contribution #146, Bermuda Biodiversity Project, Bermuda Aquarium Museum & Zoo”.
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