Introduction to Chondrostei

The surviving 25 species of chondrostean fishes are divided between the bichirs and reedfishes (18 extant species of the genera Polypterus and Erpetoichthys) in the Brachiopterygii, and the acipenseriform sturgeons and paddlefishes (Acipenser, Polyodon, and Psephurus). Both groups share primitive characters such as spiracles and a spiral valve, but Polypterus with its closely-set rhomboid scales covered with shiny ganoin is much closer to its Devonian palaeoniscid ancestors. Indeed, the only speculation that E. S. Goodrich, the most distinguished of all comparative anatomists since Cuvier, ever permitted himself was to entitle a paper “Polypterus, a paleoniscid?” (Goodrich, 1928).

Geoffroi St. Hilaire, who traveled with Napoleon to Egypt, said: “If I had discovered only this species in Egypt it would compensate me for the pains usually involved in a long journey.” He refused to give his collections to the British after the surrender of Alexandria, finally managing to get them to the museum in Paris where he described the fish. Cuvier himself remarked that the discovery of Polypterus alone justified Napoleon’s Egyptian expedition, (Champollion might have felt that the Rosetta stone took precedence, although this did fall into the hands of the British).

The paired lung-like septated swimbladder, connected to the esophagus via a ventral glottis, probably shows the ancestral condition. Bichirs and reedfish are air-breathers and die if denied access to the surface, where they gulp air in, in a curious way. The swimbladder is emptied by intrinsic muscles and refilled by the elastic recoil of the scales. Polypterus uses its swimbladder to produce moans and thumping sounds.

Sturgeons have a mainly cartilaginous skeleton, with an enormous unconstricted notochord, but some dermal skull elements are ossified as are a line of lateral body scutes. The swimbladder is not respiratory, but is of some economic importance as it is used in fining wine and beer, and, as aged European readers will recall, in preserving eggs. Of course, the most valuable product from sturgeons is caviar, and a beginning has been made in their aquaculture. Most sturgeons live in oceans or in large inland seas, ascending rivers to breed, where they lay their sticky eggs on the bottom, to be fertilized externally. The spiny larvae that hatch spend a year or so in the river after metamorphosis before descending to the sea. Some sturgeons are large (5 m, 1000 kg) and, while rooting around for their food on the bottom tasting with four large barbels, take fish as well as invertebrates in their diet. In the UK, sturgeons are rarely captured (as by-catch in trawling) and are the property of the reigning monarch. Our research boat at Plymouth (RV Sarsia) captured one, so we offered it to the Queen, who served it to de Gaulle at a banquet (but not before we had abstracted two barbels to silver stain receptors).

The Mississipi paddle fish (Polyodon) filters plankton from the water with a gill raker sieve, but the Chinese Psephurus feeds on small fish and crabs, which it catches with its protrusible jaws. Both, like sturgeons, are equipped with many electroreceptors on their snouts, probably used to detect muscle action potentials from their prey.