Polypterus

“if I had discovered only this species in Egypt, it would compensate me for the pains usually involved in a long journey”. These are words of scientist Geoffry Saint Hilaire who was participating in a scientific expedition organised by Napoleon along the invasion of Egypt. Sitting next to the Nile river he discovered a 1 meter sized  green pre-historic fish, which had been known since the time of the Pharao’s. Local called the fish something like Bee-sheer, probably phonetically spelled by the French as Bichir. In 1802 this species was described as Polypterus bichir.

The current family Polypteridae are the sole survivors of the ancient group Polypteriformes, dating from the late Cretaceous period, 100-60 million year ago, referenced as the last phase of domination of dinosaurs.  Distribution of Polypteriformes  covered an area stretching current Africa and South-America.

Polypteridae (common name bichir’s) are one of the most current primitive ray-finned fish. Overall distribution covers freshwater systems of Africa, mostly 10 degrees north and 10 degrees south of the equator. Fossil records of Chad point out that the current species already lived during the late Miocene (5.3 milion years ago). The fossil is said to be very similar to the current P. Bichir and P. endlicheri, species that are still found in that area nowadays.

Evolution may seem to take place in slow-motion concerning this ancient fish. However, under influence of geographical distribution and different habitats different species and variants arose. The familiy Polypteridae consists of 2 genera:

1. Erpetochthys, constisting only one species: E. Calabaricus
2. Polypterus, consisting 13 species and several variants

The last twenty years the existing description of the species was subject to a lot of confusion among ichthyologists, struggling to fit in variants that were newly discovered. Amended insight led to structural changes in classification, distinguishing 3 groups of Polypterus:

• Senegalus group including Erpetoichthys. Species within these group  have a larger upperjaw or of similar length. As a result the mouth opens to the lower side, equipping these species for feeding on (dead) food from the bottom. This upperjaw group consist E. calabaricus, P. senegalus, P. palmas, P. delhezi, en P. polli.
• Weeksii group is another upperjaw group  with a larger upperjaw or of similar length. Member of this group are P. weeksii, P. ornatipinnis, P. retropinnis, P. teugelsi en P. mokelembembe.
• Bichir and Endlicheri group with a larger underjaw then the upperjaw, resulting in a mouth that opens to the upside. These species are more specialised in hunting living animals,  zoals amphibian, aquatic insects and fish. To this group belong P. bichir, P. ansorgii en P. endlicheri. Underjaw species tent to grow considerably larger then upperjaw species.

Description

Bichirs have a cylindric bodyshape, similar to that of a reptile. The body is covered by multilayered scales, rhombiod in shape, with an outer layer of ganoine a middle layer of dentine a layer of isopidine and an inner dermal, cosmine bony layer and are referred to as ganoid scales. They have 7 to 16 triangle formed fin dorsal din (Polypterus means many fins), which make a dragon-like appearance. Pectoral fin are places exactly where you would expect legs on a lizzard. The swim bladder is modified to a primitive lung, enabling the fish to take supplementary gulps of air. Combined with the gills a useful adaption to swamps with a low oxygen content. Even when the oxygen level is sufficient, Polypterus will gulp air from the surface on a regular base. Young specimens have external gills, that are clearly visible and are similar to salamander. While growing these external gills dissapear. The scales prevent Polypterus from drying out quickly when out of the water. In dense conditions Polypterus can survive out of the water up to 2 days, using their pectoral fins to crawl over land.

Eyesight of Polypterus is poor. Food is found by smell. Most of the time when they smell food they follow the general direction of the smell. Often they stumble upon the food, smell again and swallow. Sometimes they hoover past or over the foodsource to discover they just missed it, and simply hoover back. Even blind specimens are able to find their food. Polypterus have two bony plates in the underjaw which help to crush food. In the natural habitat a deathroll is observed when the fish stumbles on bait to large to swallow.  Like  crocodile the fish bites into the bait and pivots to tear of pieces of meat.