The amazing Life History Strategy of Pacific Island Inverts: Tahiti, Samoa and Fiji oh my!

Just when you thought you couldn’t learn anything more incredible about invertebrates…

Amphidromy is a specialized form of anadromy, where a species lives for its juvenile and adult life in a tropical stream habitat but releases its larvae to the open ocean for development.  Yes, you read that right:  several species of molluscs and crustaceans (snails and shrimp) turf their tiniest members out to the ocean to live among marine plankton before they metamorphose and recruit back to freshwater streams.  Amphidromous species dominate the fauna of the streams on tropical islands, and it is generally assumend that this life history strategy evolved as a mechanism to cope with these unpredictable habitats.  Streams on small islands are rare (only islands that are tall enough to generate their own adiabatic rainfall can have year-round stream flow) and in many cases ephemeral (likely to dry up), so a strategy by which to find a new stream when an old one is no longer hospitable makes a lot of sense.

A recent study asked questions surrounding the ability of snail veilger larvae to colonize new areas.  How far do larvae generally travel before settling back into a freshwater home?  Two (somewhat opposing) evolutionary pressures could be at work here:  First, if freshwater habitats are rare (and they are), it would make sense not to stray too far from your native habitat since finding a new one could be difficult. However, the unpredictability of tropical stream habitats should favor the ability to travel greater distances in order to find a new habitat if the old one becomes unsuitable.

To travel or not to travel?  That is the question…

Researchers predicted a middle ground:  it was hypothesized that mitochondrial DNA signatures of snail larvae (two Neritid species) would show genetic structure from separate island archipelagos.  In other words, snail larvae were expected to be able to disperse within an island network, but not across large distances.  This prediction makes a lot of sense to me, a certain level of recruitment ability should evolve for organisms to find new habitats within the same general area if theirs becomes unsuitable.

This is NOT what was found!

Genetic data showed that the Neritid snails across the Western Pacific (an area spanning nearly a thousand kilometers) are essentially one panmictic population!  This confirms that these veligers 1) have the ability to travel great distances during this dispersal phase and 2) that there is a significantly long larval lifespan (researchers calculated that 8 weeks would be an absolute minimum for Pacific currents to carry a tiny veliger the approximate distance between Fiji and Vanuatu or Samoa).  This is much longer than the approximate larval lifespan of several marine invertebrate species (around 3-4 weeks).

So there you have it.  Charismatic megafauna like whale sharks and sea turtles aren’t the only animals making a trek across the Pacific.  Joining them (with a great deal of pride I hope!) are the larval stages of several freshwater invertebrates.  Yeah!!!

Crandall, E., Taffel, J., & Barber, P. (2009). High gene flow due to pelagic larval dispersal among South Pacific archipelagos in two amphidromous gastropods (Neritomorpha: Neritidae) Heredity, 104 (6), 563-572 DOI: 10.1038/hdy.2009.138