You’ve heard of wolves placental and marsupial, mantises and mantispids — but crayfish and Palæmon?
A few years back, I stumbled across an 1879 work by Thomas H. Huxley (“Darwin’s bulldog”) entitled The Crayfish: An Introduction to the Study of Zoology. This fascinating piece is available in full through the University of Alberta as well as Google Books. In Chapter VI, Huxley introduces us to one of the least-known — and most striking — cases of evolutionary convergence I’m aware of.
Several species of prawns (Palæmon) abound in our own seas. Other marine prawns are found on the coasts of North America, in the Mediterranean, in the South Atlantic and Indian Oceans, and in the Pacific as far south as New Zealand. But species of the same genus (Palæmon) are met with, living altogether in fresh water, in Lake Erie, in the rivers of Florida, in the Ohio, in the rivers of the Gulf of Mexico, of the West India Islands and of eastern South America, as far as southern Brazil, if not further; in those of Chili and those of Costa Rica in western South America; in the Upper Nile, in West Africa, in Natal, in the Islands of Johanna, Mauritius, and Bourbon, in the Ganges, in the Molucca and Philippine Islands, and probably elsewhere.
Many of these fluviatile prawns differ from the marine species not only in their great size (some attaining a foot or more in length), but still more remarkably in the vast development of the fifth pair of thoracic appendages. These are always larger than the slender fourth pair (which answer to the forceps of the crayfishes) ; and, in the males especially, they are very long and strong, and are terminated by great chelæ, not unlike those of the crayfishes. Hence these fluviatile prawns (known in many places by the name of “Cammarons”) are not unfrequently confounded with true crayfishes; though the fact that there are only three pairs of ordinary legs behind the largest, forceps-like pair, is sufficient at once to distinguish them from any of the Astacidæ.
Take a look at today’s species of Palaemon Weber, 1795. None of them bear much resemblance to Huxley’s river-dwelling monsters. What happened?
Well, the genus once served as a catch-all for hundreds of marine, estuarine, and freshwater prawns, but subsequent paring-down has reduced it to 34 species, most of them inhabitants of temperate and tropical coastlines in salt or brackish water (Knowlton et al., 2004). To top things off, some species now included in Palaemon were originally described in the genus Leander E. Desmarest, 1849 (ibid).
So where did Huxley’s “cammarons” go? Most (including the huge species remarked on in the second paragraph) were placed in Macrobrachium Bate, 1868. Another genus, Palaemonetes Heller, 1869, was separated from Palaemon primarily on the basis of morphology — “the absence of a mandibular palp” (ibid).
Most species of Palaemon tend not to venture very far up rivers, and Lake Erie is stomping ground only for Palaemonetes. Though there is overlap with these other two genera, most of the locales Huxley rattles off refer to the distribution of Macrobrachium (river prawns, long-armed prawns, or — as the Aussies would have it — cherabin). Not all of them are passable crawdad-analogues (these prawns range from tiny burrow-diggers to lanky, stilt-limbed creatures with no real equivalents anywhere else in the animal kingdom) but some do a remarkably good job.
As far as the big chelae go, Huxley is right on the money — count the number of walking legs (pereiopods) following the largest claws. What other differences can you spot?
O, Macrobrachium. Over 200 species strong, inhabiting every continent but Europe and Antarctica, it’s about time that you long-armed beasts make your blogosphere debut.
Huxley, Thomas Henry. The Crayfish: An Introduction to the Study of Zoology. 1879. Vol. XXVIII. International Scientific Series. New York: D. Appleton and Company, 1880.
Knowlton, R.E. & Vargo, C.K. (2004). The larval morphology of Palaemon floridanus Chace, 1942 (Decapoda, Palaemonidae) compared with other species of Palaemon and Palaemonetes. Crustaceana, Volume 77, Number 6, pp. 683-715(33)