Posted by: Rob Viens | September 4, 2012

Darwin’s Shrimp

The beginning of September saw Darwin getting tired of the constant battle with the sea, as the Beagle was tossed around by wind and waves.  He writes:

“We have remained all day at our anchorage: the weather has been cloudy for some days past & it is almost necessary to obtain observations of the sun to ascertain our situation.— I am throughily tired of this work, or rather no work; this rolling & pitching about with no end gained.— Oh for Baia Blanca; it will be a white day for me, when we gain it.” (Sept 4)

The last line, of course, is some Darwinian humor as “Bahia Blanca” is “White Bay”.  Even in his dismay, Darwin tries to lighten up his diary.

As I have mentioned earlier though, these past few days have resulted in pages and pages of descriptions in Darwin’s Zoological Notebook.  These notes include detailed descriptions of bryozoans, comb jellies, fish, isopods, and shrimp (to name just a few).  Today, a couple of notes on the “shrimp” (dated between September 2 and 4 in the notebook)…

Shrimp are crustaceans from the Phylum Anthropoda (not surprisingly the same family as the insects and spiders). Along with their crustacean cousins the lobsters and crabs, they belong to the Class Malacostraca.

Darwin’s first shrimp encounter is with a species of mantis shrimp (Order Stomatopoda). mantis shrimp are ocean predators (as is evident by their large and powerful claws), and they can grow to over a foot in length. (There are even reports of some species breaking through aquarium glass!)

Here are a few comments from Darwin, who, alas, seems to have only encountered mantis shrimp in their larval stage located at 15 fathoms and “4 miles from shore”. He starts with the feet and claws, ordered by pairs:

“1st pair of “pieds machoire” long, cylindrical, terminated by ciliæ: 2nd strong with “griffe”, penultimate joint broad, receiving griffe in a grove protected on each side by recurved spines: 3rd & 4th pairs, with claw, & penultimate joint enlarged, globular; vesicles at base: 5th rudimentary without claw.— True feet 6 in number, mere stumps: 5 pair of circular ciliated caudal swimmers, when at rest they are applied indifferently either towards head or tail.”

After some additional details he describes the general look of the body and how it swims:

“Body transparent, colourless, excepting the eyes which are dark green; all that was to be seen, when animal was in the water, were two black spots, the eyes.— In its motions not active; swims in oblique direction; & frequently rolls from side to side:— Has the power of withdrawing large part of body from beneath shell.”

Mantis shrimp (from Wikipedia Commons):

mantis shrimp

The second “shrimp”, labeled as belonging to the taxon Mysidacea, appears to have been very abundant as, “the sea contained vast numbers of this species”.  Although that name is no longer used, it would appear from Darwin’s description below (and another reference in the footnotes to “opossum shrimp”), that this is different type of shrimp-like crustacean belonging to the Order Mysida. These “opossum shrimp” are not technically shrimp, but do have several shrimp-like characteristics.  (Unless you are an expert, f you saw the picture below you’d probably call them a shrimp.)  But Darwin notes a major distinction between these critters and true shrimp when he described in some detail a female with young shrimp in a “pouch”:

“Body coloured slightly red: especially 2nd pair of “pieds machoires”, inner part of: Females had attached near to base of last pair of legs, a curved circular ciliated membrane, when folded in, forming prominent pouches; in each of these were two young animals, length about 1/15 of inch; differed from old Specimens by the greater proportional largeness of eyes; also by the less distinct separation of thorax & tail.— [note (b)] In the membrane were dark coloured vessels, much branched.— & I suppose by these pouches convey nutrition to the young animal.— [note ends] They possessed but very little irritability.— The females with young were larger & darker coloured than the others.”

Mysida species (from University of Tasmania Marine Zooplankton index)

mysida species

Mysida, and other members of the Superorder Peracarida to which they belong, are defined by the presence of a “brood pouch” called a marsupium (which has the same meaning when we refer to the marsupial mammals such as kangaroos). How cool is that – marsupial “shrimp”.  Darwin must have been very excited.

I wonder if Darwin tried to eat these guys?  Past experience suggests that it was not out of the question but no record remains J (RJV)

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Responses

  1. […] Darwin's Shrimp « The Beagle Project […]

  2. […] Darwin’s Zoological Notebook suggests that much of what he was finding were smaller swimming Crustacea such as amphipods or the larva of larger orders such as the Mantis Shrimp (for more on these fellas see Darwin’s Shrimp). […]


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