Amphidrome

For those of you following the Gregorian calendar, please accept my belated welcome to 2009!

As someone interested in the evolutionary history of river prawns of the genus Macrobrachium, a crustacean taxon poorly reported from the fossil record despite a global distribution and reasonable antiquity (probably late Oligocene to early Miocene: Murphy & Austin, 2005), I know how frustrating an bad paleontological record can be.  Given the large size, wide range, and heavily calcified claws of the American M. carcinus (pictured in the title bar of this blog), I’ve often wondered why it’s never been documented — so far as I could tell — in fossil or subfossil deposits.  Middens might be a good place to search for more recent material (Losey et al., 2004), which could perhaps lend insight into historic prawn abundance and shifting patterns of exploitation.

It was with great interest, then, that I read “New Cretaceous and Cenozoic Decapoda (Crustacea: Thalassinidea, Brachyura) from Puerto Rico, United States Territory” (Schweitzer et al., 2008), a paper from the Bulletin of the Mizunami Fossil Museum.  Although the article is also notable for having the first records of [edit:] Cretaceous decapods from the island, I was most intrigued by its treatment of Pleistocene material — another novelty for Puerto Rico:

A crushed cheliped recovered from a cave was matched to the still-extant crab Cardisoma guanhumi (Brachyura: Grapsoidea: Gecarcinidae).  Fossil fragments identified with this species had heretofore only been known from the Pliocene of Panama, the Pliocene of Costa Rica, and the Pleistocene of Jamaica.  “The freshwater crab families,” the authors elaborate, “have a poor fossil record; thus, the occurrence is noteworthy and may document dispersal of the crab by humans.”

The male C. guanhumi shown above leaves no doubt as to the basis of its species’ common name, “blue land crab”.  In males, pronounced asymmetry of the claws is typical. (Image Source)

In females, by contrast, “even-handedness” seems to be the norm. (Image Source)

Now, when we think of anthropogenic transport of live animals — especially prior to the rise of the global seafood and pet trades — crabs like C. guanhumi don’t figure very large.  Do any aspects of C. guanhumi‘s distribution or biology suggest otherwise?  Schweitzer and colleagues continue:

Extant members of Cardisoma guanhumi are known from throughout the Caribbean region and along the coast of Brazil as far south as São Paulo (Rathbun, 1918b). Other extant species of the genus Cardisoma have been reported from the Caribbean, the west Atlantic Ocean, Brazil, and West Africa (Rathbun, 1935b; Türkay, 1973; Manning and Holthuis, 1981). Türkay (1974) and Davie (2002) document a circumtropical distribution for the genus based upon the occurrence of species in Australia and many Pacific islands. Türkay and Sakai (1976) documented the presence of Cardisoma carnifex in Japan.

Well, what does this imply?

The extremely broad distribution of a terrestrial brachyuran species in remote, isolated locations introduces the possibility of dispersal by humans. Specimens of land crabs could have served as a food resource on oceanic voyages, providing a venue for introduction of the animals into otherwise inaccessible locales.

Cardisoma are still prized as edibles; no Bahamian gourmet worth his salt is anything less than familiar with “crab and rice”.  As shipboard companions-cum-rations go, their formidable claws and swift scuttling might have posed less of a problem than one might expect; Cardisoma seem to keep well even in trussed-up-bundles hawked on roadsides, and the occasional dip over the side would suffice to moisten their gills.

Ucides occidentalis wrapped in twine at Ecuador’s Latacunga market (Image Source)

But let’s not get too far into the realm of speculation — is  the claim of human introduction adequately warranted to start with?

In my view, no.  A likelier alternative seems to present itself in the very next paragraph (emphasis mine):

Rathbun (1918b) reported that the crabs live in a variety of terrestrial habitats ranging from swamps to forests, that they produce deep burrows, forage at night, and return to the sea to reproduce. The occurrence of the fossil specimen from Puerto Rico in a cave deposit and in association with fossils of terrestrial organisms is consistent with this interpretation. Their habit of returning to marine environments to reproduce is supported by the observation of Collins and Donovan (1997) that Cardisoma guanhumi has been collected from a marine unit, the Port Morant Formation in Jamaica.

It is a mistake to consider Cardisoma purely “freshwater” or “terrestrial” organisms when the entirety of their larval lives are spent amongst the plankton of warm coastal seas — floating … drifting … wafting from shore to shore with the currents.

Costlow & Bookhout (1968 b) raised C. guanhumi from newly hatched eggs to the first crab stage in 35 ppt saltwater, reporting there to be five zoeal stages and one megalopal stage.  The necessity of salt was affirmed in a series of epxeriments conducted by Kabler & Costlow (1968), whereby C. guanhumi were found to hyperregulate in 10 ppt seawater and hyporegulate at 40 ppt for the first third of their larvae life, with progressively less saline isosmotic points thereafter).   “From the time of hatching,” they conclude, “the osmoregulatory pattern of developing C. guanhumi fits them for deep penetration of estuaries and for crossing steep saline gradients.”

What implications could this life cycle have for genetic variation between far-flung populations of C. guanhumi?  De Oliveira-Neto et al. (2008), sampling across 5 Brazilian states, reported that “Populations of the blue land crab are characterized by a high level of genetic variability that is homogeneously distributed throughout the entire studied region along the Brazilian coast.  High genetic diversity is thought to result from a combination of high mutation rate in the study fragment and the large population size of the species.”  Do we  have to invoke ocean-going travellers with living larders to account for this?  Not at all.  “The dispersal capacity of their larvae is probably amplified through coastal currents, such that the genetic compositions of populations in different estuaries are homogenized” (ibid.).

Cardisoma armatum (Image Source)

In larval physiology — and thus dispersal potential — C. guanhumi seems to be rather typical for its genus, if geographic distributions are any indication.  Costlow & Bookhout (1968 a) report C. guanhumi larvae to survive to the first crab stage (i.e. complete larval development) “in salinities of 15–45 p.p.t., 25° and 30°C.”  Precise salinity, they suggest, is less crucial than temperature.  C. armatum of West Africa offers few surprises.   The first four zoeal stages tolerate salinities of 15–45‰, and 15–35‰ during later development, though 15‰ salinity “tended to cause higher mortality and a significantly delayed development in most stages, while 25‰ allowed for maximum survival through metamorphosis” (Cuesta & Anger, 2005).   For both species, larval euryhalinity makes both estuary-hopping and trans-oceanic dispersal theoretical possibilities.

Even beyond Cardisoma, we find indications aplenty that larval transport through ocean circulation can be a perfectly adequate explanation for wide distributions.  The mangrove crab Ucides cordatus cordatus (Ocypodidae) has a distribution nearly identical to that of C. guanhumi — “along the subtropical and tropical Atlantic coast of America from Florida to Uruguay, and on the Caribbean Islands” (Burggren and McMahon 1988).  As one might expect, its salinity tolerance is also quite similar: “larvae only survived to megalopa at salinities ≥15, with highest numbers at salinity 30 (72%) [the highest salinity treatment] and lowest at 15 (16%)” (Diele & Simith, 2006).

Ucides cordatus (Image Source)

Not only have genetic studies of U. cordatus revealed a “very modest degree of differentiation over a wide geographical area”, suggesting “preponderance of the larval export strategy” throughout the species’ evolutionary history (Oliveira-Neto et al., 2007), researchers in the Furo Grande Region of Brazil’s northern State of Pará have actually observed “synchronized massive releases of larvae that were exported up to 200 km off the coast” (ibid.; Diele, 2000).

The parallels with the land crabs of Christmas Island (previously blogged about here) are obvious.  If the planktonic young of crabs on a tiny Indian Ocean island are able to satiate the largest fish in the sea, I can’t help but be astonished by the trophic implications of “massive release” along huge swaths of continental coast.

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