Tumbling under the surf: wave-modulated settlement of intertidal mussels and the continuous settlement-relocation model

Abstract

For many mussel species, the model of planktonic development followed by metamorphosis and settlement in the benthic habitat is complicated by the existence of planktonic post-metamorphic stages and/or pediveliger benthic stages that can relocate after initial settlement. This has led to the long-standing hypothesis of 'primary' settlement from the plankton onto intertidal algal substrate followed by 'secondary' relocation to mussel beds. Here, we investigate settlement of the intertidal mussels Perumytilus purpuratus and Semimytilus algosus in central Chile to test this hypothesis and explore physical drivers. Our results indicate that: (1) these species do not have planktonic post-metamorphic stages, (2) larvae typically arrive to the intertidal zone at a size 80-150 µm larger than the largest planktonic larva, which based on growth rates, corresponds to a 3-20 d delay, (3) there are no differences in pediveliger sizes between different algal substrates, mussel beds, or artificial collectors, and (4) there is no evidence that larvae metamorphose in the intertidal and grow in alternative habitat before relocation to mussel beds. In 2 summers, daily settlement of both species was tightly and positively associated with wave height, despite large inter-annual variability in wind conditions. Our results reject the primary-secondary settlement hypothesis and support a new settlement model in which, after metamorphosis beyond the surf zone, the negatively buoyant settlers become semi-benthic and readily sink to the bottom. There, they can be transported onshore through the surf-zone by wave-driven near-bed transport. The process of tumbling under the surf may take from a few hours to several days, with larvae arriving at the shoreline in a wide range of sizes at any given time. For some larvae, relocation continues in the intertidal zone for months.

Transport of larvae and detritus across the surf zone of a steep reflective pocket beach

“…Larvae that sink or swim down may enter near-bed streaming and thus be transported shoreward. Our data, along with that of Navarrete et al (2015) and Morgan et al (2009) suggest that competent larvae can enter the surf zone via near-bed streaming, and given the universality of the associated physics, that this transport mechanism could be exploited by larvae on all shores. …”

“…This hypothesized mechanism of surf zone entrainment of detritus works when waves are small enough that detritus and zooplankters can settle to the seabed and remain there long enough for streaming to transport them into the surf zone. Navarrete et al (2015) presented data suggesting that 2 species of intertidal mussels in Chile use streaming to cross the inner shelf and surf zone to settle in the intertidal. Larvae of these species settle and metamorphose offshore, but likely close to shore.…”

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

“…Benthic streaming may increase entry of zooplankton into the surf zone at the RSZ, where only demersal taxa, including harpacticoid copepods, juvenile parasitic isopods seeking host shrimp and cyprids were more abundant inside than outside the surf zone. Streaming is suppressed by breaking waves at the seaward edge of the surf zone, but zooplankton, detritus and sediments near the bottom enter the surf zone by entrainment into breaking waves [25][26][27]. Although some species of cyprids have been reported to be more abundant near the sea surface [28][29][30], we previously demonstrated that cyprids of all species recruited almost entirely to the bottom of moorings just outside the surf zone over 5 years on this coast [31].…”

Surf zones regulate larval supply and zooplankton subsidies to nearshore communities

“…Plankton near the surface may be transported into the surf zone via the sea breeze during the daytime (Jacinto and Cruz ) or via Stokes drift in large shoaling waves that are refracted toward shore (Morgan et al a ; McPhee‐Shaw et al ). Plankton near the bottom may be transported together with sediment through the surf zone by benthic streaming (Lentz et al ; Navarrete et al ; Shanks et al ). Benthic streaming may deliver heavy zooplankters that passively sink to the bottom following mixing by large waves (Navarrete et al ) or swim to the bottom in response to turbulence (Fuchs and DiBacco ; Roy et al ; Fuchs et al ).…”