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.
Fig. 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.
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.
Fig. 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.
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.)