Sea Squirts, SLOSS and of Course Porn

Castilla et al. (2007) are reporting in a recent PNAS article an interesting property of sea squirt pornography and local oceanography. I know, I'm a frequent purveyor of tunicate smut, but this utter filth may have consequences in the debates surrounding marine reserve design. These authors studied the spawning behavior of intertidal tunicates (Pyura praeputialis, an invasive) from chilean coasts. What they found will make all decent folk turn their insides out. These filthy denizens of the seas let it all out together in a mass orgy. Oh the indecency. Are you sure the children are asleep and not watching over your shoulder? You may want to tuck them in before continuing on.

Sea squirts have mobile "tadpole"-like larvae whose role is typically dispersal since they do not feed. The also don't stay in the water column for too long, preferring to stick their heads to a rock somewhere (figure below shows larval metamorphosis).

Diagram from Cornell University's introductory biology autotutorial on chordates.

So what happens when tunicate sperm and egg meet seawater? An explosion of bio-foam! This foam is created by surfactants in Pyura's gametes (see figure below). This reduces the surface tension of the seawater. So basically, sea squirts spawn en masse, the high amount of gametes in tidal channels releases a high amount of surfactants which react with the seawater and its associated protein debris creating foam, the surface tension is reduced so larvae are not carried out to sea, thus larvae are retained in the tidal channel! As simple as that. Castilla et al. (2007) document the effects this foam might have on larval dispersal ability by monitoring ping pong balls in the channels. When there is no foam present, they all head out the channels to sea. When foam is present, 50-60% of the ping pong balls were retained in the tidal channel (depending on ebb or flood tidal conditions).

A) Mass splooging of P. praeputialis. B) Sperm (SS) and egg (ES) suspensions left dripping in the intertidal. C) 2 meters of foamy splooge. D) Chilean researcher knee-high in sea "squirt" if you know what I mean... Fig. 1 from Castilla et al. (2007).

So what is the end result of this filthy, spineless mass bukkake? (That ought to bring in the hits!) Larvae are retained in the tidal channels resulting in massive colonies over several generations. They are not trapped here though since theoretically, as shown with the ping pong ball experiment, nearly half of the larvae escape out to sea where they can start new colonies in other tidal channels perhaps. But this suggests that other larval forms are also retained during foamy times. Hence this bio-foam acts as resistor to the current of larvae out of a particular channel.

This has some interesting connotations for the ever debatable Single Large Or Several Small (SLOSS) dilemma facing marine reserve designers. If rare or endangered species occur in such tidal channels, for instance, you would want to grant protection to that channel. It is acting as quasi-reservoir for recruitments. Populations would appear to build up in these channels (excluding all other ecological forces that determine population size like competition, resource availability, etc.) and proportionally send out more larvae with each generation. This research suggests that protecting individually foamy channels (those with gametes secreting surfactants, or channels with high protein debris content) would more sufficiently protect the potential source of larvae for a particular species of interest. You just need to find the right areas, the foamy ones.

Of course this opens up a lot of questions. As someone interested in communities and diversity. I would like to understand how surface tension reduction affects the structure and composition of communities in foamy channels and compare that to non-foamy channels. Or, how planktonic communities respond to this potential stress? Does the composition of the seston track the cyclic patterns of the foam production and tides? Is diversity higher in non-foamy channels because foamy ones tend to have a higher dominance (and lower evenness) of a few species like Pyura praeputialis. Does the foam and reduced surface tension, act a barrier to immigration for new species and recruits? Its a pretty interesting system and though I don't know the foam literature well, I get a sense that this isn't well-studied. I will be looking for a postdoc very soon.... (hint hint).
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Castilla JC, Manriquez PH, Delgado AP, Gargallo L, Leiva A, Radic D (2007) Bio-foam enhances larval retention in a free-spawning marine tunicate. Proceedings of the National Academy of Sciences 104:18120-18122. doi:10.1073/pnas.0708233104