Sharks with head claspers (sort of)

September 30, 2009 • 9:00 am

by Greg Mayer

In my post on the genitalia of ratfish (which are shark relatives), I noted that although no extant sharks had similar structures, some fossil ones did, so here are what two species of these sharks looked like. Both are members of the family Stethacanthidae, known for its sexual dimorphism.Falcatus falcatus, by Smokeybjb, from WikipediaFig. 1. Female (above) and male Falcatus falcatus (Carboniferous of North America). Note the pelvic claspers on the male, and the roughened denticles atop the head, as well as on the head clasper

How exactly the male ratfish uses his head clasper during mating is obscure (at least to me), and the use of the head clasper (the spine of the first dorsal fin) in the shark Falcatus would also be obscure, except that a pair has been found fossilized in flagrante delicto, the female grasping the head clasper in her mouth, her body parallel to and above the male’s.  There would have to be more to their mating than this to bring the male’s pelvic claspers in to position, but it does provide at least a partial picture of mating and courtship in this fossil species. A nice photo of the fossil pair is at the fine website on Fossil Fishes of Bear Gulch maintained by Richard Lund and Eileen Grogan. They also have a photo of the rather similar Damocles serratus (presumably so named because its own sword [clasper] was always hanging over its head).

Equally bizarre is Stethacanthus, with a brush-like set of denticles atop the first dorsal fin, a large first dorsal fin spine, and roughened denticles atop the head. It’s not clear exactly how, or for what, this structure was used, but the fact that it occurs only in males, and that the related Falcatus (and almost certainly Damocles as well) used a similar structure in mating, suggests some sort of sexual behavior function.

Stethacanthus by Dmitry Bogdanov, from Wikipedia

Fig. 2. Male (to left) and female Stethacanthus altonensis (Carboniferous of North America).

Lund and Grogan provide further discussion and illustration at their website, and one of Lund’s papers is in the American Museum of Natural History’s digital library of its scientific publications. Matt Celeskey at the Hairy Museum of Natural History has reconstructions of both Falcatus and Stethacanthus.

Filling a demand that I didn’t know existed, in the mid 1990’s two excellent and well-illustrated popular accounts of the history of fishes were published, both emphasizing the fossil record. WEIT readers should enjoy both; Long has more on these odd sharks and ratfish.

Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore.

Maisey, J.G. 1996. Discovering Fossil Fishes. Henry Holt, New York.

That’s not ratfish genitalia. That’s ratfish genitalia.

September 28, 2009 • 8:58 am

by Greg Mayer

Over at Pharyngula, PZ has linked to a story at Deep Sea News about the description of a new species of ratfish with “forehead genitals”. While it’s a great concept, the tentaculum, or cephalic claspers, of ratfish are not genitals.

Male chimaeridFig. 1. That’s not ratfish genitalia. A male ratfish (family Chimaeridae) showing the tentaculum.

The genitalia of male ratfish are the pelvic claspers, modifications of the medial side of the pelvic fins used as intromittent organs for the introduction of sperm into the female’s reproductive tract. The ratfish’s pelvic claspers are bifid and especially spectacular.

Claspers of male chimaerid.Fig. 2. That’s ratfish genitalia. A male ratfish’s (family Chimaeridae) pelvic claspers; note the bifid structure, giving the appearance distally of four claspers. The anterior of the fish is to the bottom of this photo. The medial lump anterior to the pelvic fins is the rectum, prolapsed.

For comparison, here’s a female ratfish with unmodified pelvic fins.

Female chimaeridFig. 3. A female ratfish (family Chimaeridae), showing unmodified pelvic fins.

Ratfish comprise the Holocephala, one of the two major subdivisions of the cartilaginous fishes, the Chondrichthyes, the other major subdivision being the sharks and rays (elasmobranchs). Pelvic claspers are found throughout the modern cartilaginous fishes, which therefore have internal fertilization (most bony fish have external fertilization). Although living species of sharks do not have tentacula, some fossil ones (e.g. Falcatus) did, and others had other sorts of spine encrusted bits on their front ends which may have been involved in courtship and mating.

Whether or not tentacula are genitals is a matter of the definition of genitals, of course, but the term is, to my knowledge, reserved for structures involved in the transfer and reception of gametes. If parts of the body used in courtship are considered genitals, then the throat fans of anoles and the long fingernails of turtles would have to be considered genitals, too; indeed, so would the entire human body.  Many commenters at Pharyngula  have remarked about ratfish having penises on their heads (or something to that effect), which, of course, they don’t: their genitals are in the normal place (for cartilaginous fish), alongside their pelvic fins.Female (left) and male chimaerids.

Fig. 4. A ratfish couple.

(I tried to post a short comment to this effect at Pharyngula last night, but found I wasn’t registered to do so, and then I thought, “Why talk about it, when you can show pictures”, so I waited to take some photos this morning and posted here.)