Bad news for the Great Red Island?
Marbled crayfish, or Marmorkrebs, are parthenogenic members of Procambarus (Decapoda: Cambaridae), an important genus of North crayfish; I covered the Nilotic shenanigans of P. clarkii, infamous Lousiana expat, here and here.
I say “members” because they have no specific name — asexual, all-female mudbugs first discovered in German fish tanks in the ’90s tend to be iffy as far as taxonomists are concerned. This reproductive quirk, otherwise unheard of in decapod crustacea, has caught the attention of geneticists and developmental and evolutionary biologists on the lookout for model organisms (see here).
Their recent discovery in Madagascar (Jones et al., 2008) will do little for the blood pressure of the Eighth Continent’s already-beleagured wildlife advocates. The perfect invader: a parthenogenic crayfish poses a new threat to Madagascar’s freshwater biodiversity, published August 2008 in Biological Invasions, has the score. In 2005, University of Antananarivo biologists noticed a strange new decapod being sold at markets near the capitol city. After a tip-off from Japanese researchers who’d pronounced them marmorkrebs in early 2007, a crack team of local and foreign investigators was soon on the ground. What they found was not pretty.
Marmorkrebs (sequencing of their 16S mtDNA genes confirmed the ID) were found at 37 sites around Antananarivo … “at very high density.” Their habitats were predominantly artifical or anthropogenically disturbed (e.g., drainage ditches and rice fields) — as is often the case for invasive species (Byers, 2002). They were probably introduced some time in 2003, perhaps from a road-building project done by foreign contractors, but then again perhaps not. What was certain was this: like a metastasizing tumor, they were reproducing (many captured were bearing eggs or young) — and spreading (brought live to market via major roads).
So what could this mean? The fecundity of marmorkrebs would be troubling in and of itself, but it’s hard to listen to its jet-setting relatives’ ecological charms without getting a sense of impending doom:
1) They may carry a fungal disease (Aphanomyces astaci) known as crayfish plague. North American species are immune but Astacoides [Madagascar's endemic crayfish] are likely to suffer extremely high mortality. Where plague has been introduced to non-immune populations, local extinctions have followed.
2) Procambarus are voracious predators of tadpoles and aquatic invertebrates and may cause serious changes to aquatic ecosystems in general.
3) Where Procambarus has been introduced in rice growing areas (e.g. Spain and Portugal) they have become an important pest of rice fields, damaging banks and disrupting irrigation through burrowing and eating seedlings.
Point 1 really cannot be overstated. Crayfish plague, near-ubiquitous in the North American crayfish that are its resistant characters, has proven devastating for European crayfish species. Responsible for the extinction of countless local populations, the spread of this fungal disease has paved the way for replacement of locals by Nearctic ne’er-do-wells like the signal crayfish, Pacifastacus leniusculus (Decapoda: Astacidae). Though Astacoides have not yet been experimentally exposed to this pathogen (Jones et al., 2007), the reactions of their Australian Parastacoid kin are not promising. Simply put: “Infected susceptible crayfish do not survive – 100% mortality is normal” (Bower, 2006).
Assays for A. astaci infection in Madagascan marmorkrebs came back mercifully negative — though, the authors note, fixative solution could have affected their results (Jones et al., 2008) …
What’s more, marmorkrebs were not found in “natural forest areas” — the stronghold of slower-reproducing native crayfish (six times slower (!) : Jones et al., 2008). That this will remain the case is far from certain. Jones and his colleagues were unable to make firm pronouncements on implications for either biodiversity or rice farming, but concluded that complete eradication would be “impossible or prohibitively expensive.” Marmorkrebs, they suggested, would continue to spread through human transportation (overturned trucks on the way to market, cash-strapped farmers “seeding” ponds, and endless other scenarios) in lieu of legal prohibition.
So, if worst comes to worst, what do we stand to lose?
Let’s set aside Madagascar’s amazingly unique freshwater fish (Benstead et al., 2003) and crab (Cumberlidge & Von Sternberg, 2002) fauna for a moment, and look just to its crawdads.
Madagascar’s lone genus of crayfish, Astacoides (Decapoda: Parastacidae), is found nowhere else on Earth. “The only uniquely tropical crayfish on this planet” (Jones et al., 2007), they man the parapets of a Gondwanan fortress separated from its counterparts in Australia and South America by thousands of miles of open ocean and the tireless grind of geologic time. By the time marmorkrebs arrived, the siege was already well underway — habitat loss and overharvesting have hit A. crosnieri and A. petiti hard enough to earn them endangered status on the IUCN Red List.
The day we bring our erasers to a distribution map drawn before the birth of the Atlantic is the day there is no denying that we live, forevermore, in a Planet of Weeds. May it never come.
Benstead, J., De Rham, P., Gattolliat, J., Gibon, F., Loiselle, P., Sartori, M., et al. (2003). Conserving Madagascar’s Freshwater Biodiversity. BioScience, 53(11), 1101-1111 . doi: 10.1641/0006-3568(2003)053[1101:CMFB]2.0.CO;2.
Byers, J. E. (2002). Impact of non-indigenous species on natives enhanced by anthropogenic alteration of selection regimes. Oikos, 97(3), 449-458. doi: 10.1034/j.1600-0706.2002.970316.x.
(Crustacea: Decapoda: Potamoidea). Zoosystema 24(1), 41–79.
Jones, J., Rasamy, J., Harvey, A., Toon, A., Oidtmann, B., Randrianarison, M., et al. The perfect invader: a parthenogenic crayfish poses a new threat to Madagascar’s freshwater biodiversity. Biological Invasions. doi: 10.1007/s10530-008-9334-y.
Jones, J., Andriahajaina, F., Hockley, N., Crandall, K., Ravoahangimalala, O. (2007) The ecology and conservation status of Madagascar’s endemic freshwater crayfish (Parastacidae; Astacoides). Freshw Biol 52:1820–1833. doi:10.1111/j.1365-2427.2007.01766.x.