A Stepping-Stone to Marine Conservation

by Eric Crandall

Originally published in American Conchologist - December 2005

 

The Importance of Island-Hopping

 

Sometimes it helps to hop like a snail.  This odd image pops into my mind as the twin turboprop aircraft descends out of the blue South Pacific sky and alights on the narrow strip of white coral between the ocean and Rangiroa's lagoon.  Although I've come in search of snails, my trip through French Polynesia has been anything but snail-like.  In less than a month I've island-hopped through three archipelagos: the lush Society Islands to the southwest, the rugged Marquesas to the northeast, and now I've arrived in the Tuamotus - the vast scattering of ancient atolls that lies between them.  This array of low coral islands, like garden stepping-stones between the two high island archipelagos, provides exactly the right setting to study how snails hop between them.

"Hop" is probably not the first verb that comes to mind when you think of snails. It's difficult to imagine a snail traveling hundreds of kilometers between islands, yet this is what marine snails frequently do.  Some species, like pteropod sea butterflies, swim in the ocean as adults.  Most, however, undertake this journey in their youth, as tiny larvae called veligers.  In fact, the majority of marine organisms have similar tiny larvae that live the nomadic life of the plankton.  It is these larvae that achieve most long-distance movement in the ocean, while the adults of most species stay put in their favored habitat.  Cast adrift on ocean currents for days, weeks, or months, the larvae of many marine fish or invertebrates may often wind up quite far from where they were conceived.

At least, that is what most marine biologists thought until about a decade ago and with good reason.  They had found the larvae of intertidal animals in samples of plankton netted hundreds of kilometers from land. They had calculated the distance that a larva could travel in a given current over a given time, and had come up with estimates in the hundreds or thousands of kilometers.  It seemed clear that ocean currents were veritable highways for larvae; the California Current must be to larvae as the Pacific Coast Highway is to SUVs.

As is often the case in science, a closer look revealed that things were more complex than that.  Recent studies have utilized unique chemical and genetic signatures to tag larvae similarly to the way the DMV uses vehicle identification numbers to tag cars^{1, 2}. These studies have found, for the most part, that larvae do not move nearly as far as they potentially could. Although we thought that larvae regularly traveled the distance from San Francisco to Los Angeles, most only make it to Santa Cruz.  It seems that many larvae, especially larval fish, may be swimming vertically to escape major currents and get into nearby eddies or minor currents moving in the opposite direction. Essentially, some larvae are lollygagging in the rest areas, and some are exiting early, and sneaking home on the side streets.  However, even with this new knowledge, a basic understanding of how millions of larvae the size of this dot [.] move through the vastness of the open ocean remains a primary goal for marine biologists.

                                   

How to Think Like A Snail

In bad phrasebook French, I try to explain to the host at my pension: "Je etudie les escargots pour le universite."  I get the same smile and puzzled look that I've gotten throughout French Polynesia.  I wish my French were better, but it's hard to explain my snail obsession even in English.  I've come to French Polynesia in search of snails from the family Neritidae.  I've specifically targeted three intertidal species from the genus Nerita: N. plicata, N. albicilla, and N. polita. I'm also searching for their freshwater family members: Neritina canalis, Clithon spinosus, and Septaria taitans but I know I won't find them on this arid atoll.

All I have to do is step outside my bungalow to find one species: white globular shells with the characteristic spiral ribs grazing on some intertidal riprap Nerita plicata. This species is weedy they're happy on almost any surface in the high intertidal.  As I grab each snail, I stick a toothpick into the aperture to stop it from closing, then I drop the snail into ethanol to preserve it for genetic analysis.

Finding other species of Nerita requires more patience.  By night I use a flashlight to search the conglomerate platform for the swirled oval shape of Nerita polita and the splotchy N. albicilla.  I have to collect at night because by day these species bury themselves in the sand, or hide under rocks.  They also each seem to prefer a specific arrangement of sand and rock, so that I'll go for an hour without seeing any, and then suddenly spot three in a row.  After several hours of this evening easter-egg hunt, I tally only eleven N. polita and zero N. albicilla.  

                                                                                 

What I find most useful about Rangiroa is what is not here.  There are no freshwater streams in the Tuamotus.  On the volcanic high islands in the Societies and Marquesas, these streams, besides plunging over waterfalls into verdant pools for tourist brochures, also provide a home for the freshwater Neritids.  These species possess a trait that is rare in freshwater animals. Their larvae continue to return to the ocean to be dispersed by currents just like their marine cousins (such a life cycle is called amphidromous). 

So what can I learn from these snails about larval island-hopping?  Sometimes it helps to think like a snail.  To Nerita plicata, Rangiroa is Bali Hai - a giant ring of perfect intertidal habitat.  A larva from this species that is lucky enough to drift across it can settle down and have kids.  For a larva from a freshwater species, however, Rangiroa offers no amenities.  They must push on until they find an island with streams. Over time, I predict that Nerita plicata will find it easier to move between the Society Islands and the Marquesas because they can do it in a series of generational hops, using the Tuamotus as stepping-stones.  Freshwater neritids like Neritina canalis have to do it in a single giant leap of 1400 kilometers, which as I mentioned above, does not seem to happen very often. 

I plan to test this prediction using genetic methods to build a family tree for each species. If Nerita plicata is able to make it through the Tuamotus in only a couple of generations, then I would expect the ones in the Society Islands and the ones in the Marquesas to be close kin.  On the other hand, Neritina canalis samples from those archipelagos should be much more distant relatives.  If this proves to be true then these snails will have helped us to gain a slightly better understanding of how larvae move through the ocean - lots of short hops, one per generation.

On this map of French Polynesia, islands with significant year-round freshwater
streams that are accessible to freshwater snails are colored in green. 
The Tuamotus only offer habitat to marine organisms.

 

Stepping-Stones to Conservation

Using islands as stepping-stones to move through the ocean seems obvious. When the Lapita culture of coastal New Guinea rapidly colonized the islands of the South Pacific, they did so in stepping-stone fashion.  These ancestors of the modern Polynesians navigated single and double canoes using only stars and wave patterns as their guides.  They traveled farther and more accurately than any humans before them, but they didnÕt travel from New Guinea to Hawaii in a single voyage.  Instead they moved through the Solomons, Fiji, Samoa, the Societies and the Marquesas³. During World War II, General Douglas Macarthur used the Solomon islands as stepping-stones in the bloody struggle to approach within bombing distance of the Japanese islands. 

In 1967, another famous Macarthur , the ecologist Robert Macarthur, published a book together with E.O. Wilson called The Theory of Island Biogeography⁴.  This book became well-known for its prediction that the number of species on an island is proportional to its size. This concept has been important for designing networks of national parks and reserves (which can be viewed as islands) for the protection of terrestrial biodiversity.  In another chapter of the same book, Macarthur and Wilson also predict the importance of intermediate stepping-stone islands to increasing the movement of species between islands.  This has also gotten some attention in a terrestrial context.  Neither of these concepts has ever gotten much consideration in the realm of marine conservation, most likely because it was thought that even if the adults were limited to a single habitat, their larvae could travel almost anywhere.

Now, as fisheries around the world go belly up⁵, it is important for marine conservation science to catch up with terrestrial conservation science.  Recently, 161 marine scientists signed a consensus statement that calls for networks of marine reserves for the long-term conservation of marine biodiversity and fisheries⁶.  The reserves in these networks, whether they are along a coastline, or among islands, will serve as stepping-stones that catch incoming larvae, protect the adults, and allow them to reproduce safely.  A better understanding of the stepping-stone dynamics of these larvae will lead to better reserve design and a better conservation effort.

                                                                               

The next day I hook up with a tourist excursion to the next islet over in the circular chains of islets that make up Rangiroa'a atoll: Le Lagon Vert - The Green Lagoon.  While the rest of the group swims and sunbathes in this beautiful setting, I splash around the intertidal looking for snails (and stomatopod shrimp for my advisor), but to no avail.  I find plenty of N. plicata, a few shrimp, and lots of other cool shells, but not my target species.  This is a bit disheartening, but later I learn that N. albicilla and N. polita most likely do not occur as far east as Rangiroa.  As I prepare for my flight back to Tahiti and then home, I'm quite satisfied with what I've collected from French Polynesia. If everything goes well in the laboratory (and so far it has) Neritid snails may help move us one small step closer to understanding a big problem in marine biology.

 

Eric Crandall is a PhD Candidate at Boston University.  His trip to French Polynesia was funded by the Walter Sage Memorial Fund awarded by Conchologists of America, and the Lerner-Grey Fund, awarded by the American Museum of Natural History.

 

 

References

 

1.         Barber, P.H., et al., Sharp genetic breaks among populations of Haptosquilla pulchella (Stomatopoda) indicate limits to larval transport: patterns, causes, and consequences. Molecular Ecology, 2002. 11(4): p. 659-674.

2.         Swearer, S.E., et al., Larval retention and recruitment in an island population of a coral-reef fish. Nature, 1999. 402(6763): p. 799-802.

3.         Diamond, J., Guns, Germs and Steel. 1997, New York: W.W. Norton and Company, Ltd. 480.

4.         MacArthur, R.H. and E.O. Wilson, The Theory of Island Biogeography. 1967, Princeton: Princeton University Press. 203.

5.         Pauly, D., et al., Fishing down marine food webs. Science, 1998. 279(5352): p. 860-863.

6.         NCEAS, Scientific Consensus Statement on Marine Reserves and Marine Protected Areas. 2001.

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