Outline:
Essay:
Surprisingly simple strategies can exploit shear or oscillatory fluid flows to enable successful recruitment of island intertidal species, and even benthic organisms inhabiting near-surface seamount summits.
On an isolated island or seamount, an extreme reproductive risk is advection of gametes or larvae beyond the habitable coast by inherently unpredictable currents. Thankfully, even in the open ocean there are oscillations in the current with tidal or inertial frequency; in the shallows or confined coastlines of islands, these motions will be accentuated. Pending local variations like storms, the rotating current field usually averages out to a unidirectional mean velocity relative to a reference point like an island. Consequently, species inhabiting island intertidal zones can expect a range of scales of physical motion: wave action (dependent on local exposure), long-shore transport, tidal flows, and rotating open-ocean advection. Successful recruitment will depend on one or a combination of these phenomena to first disperse eggs, sperm, or larvae, and sometime later return viable young individuals -- foremost to the coast, and secondarily to an acceptable depth range and environment.
Let us consider a species of barnacle which avoids the tenuous encounter rates associated with external fertilization by dispersing nauplii (larvae, rather than eggs or sperm) directly from the benthic adult. Long shore transport, a consequence of waves breaking at an angle to a coastline, is a likely dispersal mechanism for such intertidal recruits. After surging flows and turbulent diffusion remove nauplii from their original habitat, long shore movement will commence. The key, especially on an island for organisms with limited swimming ability (Jumars, p. 203), is to get back to shore (and the right depth) before long shore currents sweep you off a point, or larger-scale circulation moves you into the open ocean.
Superimposed on the long-shore advection is a vertical shear (akin to that in estuaries) -- also a consequence of waves. Pouring milk (whole's best) into the clear water off of a sloping beach reveals that fluid moves slowly beachward in the surface layers, while the flow is reversed near the bottom. The suggested, simple strategy for nauplii is to start swimming upwards (or be buoyant) upon release. The vector sum of long- and on-shore velocities will be slightly different for each nauplius due to different release times and subsequent positions in the on-/off-shore shear. The resultant advantages are wide dispersal along the downstream coastline, a range of exposure to predation, and minimal losses to the open sea. Littoral cell circulation (Duxbury and Duxbury, 1991) may provide additional recruitment opportunities for nauplii that continue to act in accordance with the "swim up" strategy. If the substrate or depth is undesirable when a nauplius first is returned to the shore, breaking wave turbulence and the pursuant near-shore circulation (surge channels and rip tides) will again efficiently transport the individual seaward, beneath the surface (shoreward) velocity fields. Swimming upward a short distance during this seaward movement will return the larvae to the shoreward fluid flow, and ultimately to a new potential intertidal habitat.
This hypothetical strategy may underlie larval barnacle behavior I have observed in my aquarium. A stream of larvae is ejected (within a jet that penetrates a few cm) from the parent and is advected away. Simultaneously, the nauplii begin to propel themselves (at about 0.5 cm/s) upward, invariably congregating in the upper corner of the aquarium, apparently aiming for the sun!
Observation of bottom fauna (scallops, sea urchins, hermit crabs, polycheates, tube-dwelling annelids, anemones, and brittle stars) on the summit of Cobb Seamount (Budinger, 1957) suggests that tidal flows may be amenable to similar recruitment strategies. Selection may be strong for hopping behaviors in which larvae depart from the seafloor with a frequency equal to the dominant (elliptical) current frequency, but re-settle shortly thereafter; riding less than one forth of the tidal cycle will result in roughly linear cumulative advection, the direction depending strongly on the phase of the oscillation at the time of departure. Larvae utilizing such a hopping strategy could disperse in all directions if they were released over a full tidal cycle. Indeed, a small fraction of recruits could even disperse upstream (relative to the cumulative advection), a highly advantageous trait for larvae from an adult positioned on the downstream edge of a seamount, or an isolated island. Reliance on a tidal oscillation entails risk, however; dispersal during a storm when tidal oscillations are dominated by unidirectional wind-driven circulation could be disastrous, and such unpredictable variability could result in missing year classes.
A promising alternative to moving with the currents is to produce a larval stage that is temporarily parasitic on intertidal nekton. Especially in environments in which there is a persistent unidirectional advection, such a strategy could enable dispersal upstream that would otherwise be impossible, possibly precluding local extinction of a population by literally (littorally?) being advected off an island. Adult intertidal fish, especially-mobile predators, could prove a highly reliable transport mechanism, delivering larvae directly to a new and likely habitable environments.
References:
Budinger, T.F. 1957. Geology, biology, and hydrography of Cobb Seamount. University of Washington Masters thesis, pp. 47-48.