EVOLUTIONARY PUZZLE

 

Click here for a printable version of this page.

                             Classification

          The living coelacanths, Latimeria chalumnae,and Latimeria menadoensis are possibly the sole remaining representatives of a once widespread family of Sarcopterygian (fleshy-finned) coelacanth fishes (more than 120 species are known from fossils)all but one of which disappeared at the end of the Cretaceous, 65 million years ago. The classification of coelacanths is a murky business with more than one vairation in the class category, but we'll give it a shot. Kingdom: Animalia, Phylum: Chordata, Class: Pices (fishes), Sub class: Gnathostomata- jawed fishes, Sub class: Teleostei- bony fishes (though cartilaginous, coelacanths are usually classed with the teleosts), Sub class: Sarcopterygii (lobed-finned fishes), Order: Crossopterygii, Family: Actinistia (coelacanths), Gennus: Latimeria, Species: chalumnae and menadoensis.

           The coelacanth appears to be a cousin of Eusthenopteron, the fish once credited with growing legs and coming ashore-360 million years ago. Today, scientists prefer to cite the tongue-twisting fossil candidates: icthyostega, panderichthys, acanthostega, and the newly discovered Tiktaalik roseae (2004), as the ancestor(s) of all tetrapods-amphibians, reptiles, and mammals, including ourselves.


fins to legs

Three candidates in the transition from fins to legs.

(Adapted from the BostonGlobe 1/14/2014 -courtesy Wendy Winslow)

           But for some scientists, debate still rages as to whether the coelacanths, presumed to be close relatives of the Rhipidistia fishes from which tetrapod amphibians supposedly arose, are our closest tetrapod ancestors, or if lung fishes, another very ancient line, are more closely related to tetrapods than the Rhipidistia and thus claim the oldest closest living relative title. (There are three living genera of lung fishes.) Good genetic and morphological evidence points in both directions. Another line of thinking, based on physiological and anatomical analysis, identifies coelacanths with sharks and other cartilaginous fishes, but this view seems to have fallen from favor.

           Fossils of ancient coelacanths have been found on every continent except Antarctica. They were first identified from an English fossil by naturalist Louis Agassiz in 1836. (Ironically, Agassiz became a firm opponent of Charles Darwin's theory of evolution!) 250 million years ago there were as many as 30 species living at the same time, about a third of them in fresh water. With a couple of exceptions ancient coelacanths were small, seldom exceeding 55 cm. In a recent issue of The Journal of Vertebate Paleontology, Andrew Wendruff and Mark Wilson, described an aggressive fast swimming species they call Rebellatrix. It had a muscular, forked tail which allowed it to chase prey in the open seas. Rebellatrux lived from about 250 mya, until it lost out to the better adapted sharks at an undetermined time in the deep past.

                                      SIZE

            Today's coelacanths can reach almost six feet (2 meters) in length and weigh up to 150 or more lbs,(the giant Mozambique female shown on this site was 180 centimeters long and 95kg) but they are usually somewhat smaller, particularly the males, which average under 165cm.

                                   FEEDING

         Coelacanths are oportunistic feeders, scarfing up prey probably on or near the bottom. Stomach contents have included lantern fishes, stout beard fishes, cardinal fishes, cuttle fishes, deep water snappers, squids, deepsea witch eels, snipe eels, swell sharks, and other fishes normally found in their deep reef and volcanic slope habitats- and now even garbage (!) (There are as yet no reports on the specific prey of the South African canyon dwelling coelacanths.)

                             COLORATION

         Coloration is vey dark blue, almost dark gray, with distinctive white flecks that can even be used by researchers to designate individuals. (Indonesian coelacanths were first thought to be more brown than blue, but this was probably a post mortem condition). The white flecks afford camouflage against a backdrop of dark lava walls encrusted with lywhite oyster shells.

                               LIFESPAN

          Scientists believed individual coelacanths may live as long as 60 years, but there is still confusion as to how many scale growth rings are laid down each year: one or two- possibly resulting from multiple nutrient cycles in the shifting currents. Reseacher, Hans Fricke, however, writing in Marine Biology, takes a different tact. He says it is virtually impossible to detect age or growth changes in coelacanths observed by submersible for 20 plus years at the Comoros. On top of that, the rate of replacement in observed colonies indicated only two or three deaths per year. Comparing coelacanths, to a well known species of grouper, Dr Fricke estimated individuals may live to the age 103!

                        REPRODUCTION

          Coelacanths are ovoviparous, giving birth to as many as 26 live pups which develop from eggs in the oviduct, feeding off a large yolk sac until birth. Nothing is known about mating behavior or even juvenile habitat.

     GENERAL DESCRIPTION AND SIGNIFICANCE

            The coelacanths date back 410 million years to the beginning of the Devonian epoch. One of the incredible aspects of the living coelacanth, Latimeria, is that it offers a genetic and anatomical snapshot of life in those times. The backbone of this fish is composed of a fluid-filled cartilaginous tube, which provides a firm yet flexible support for muscles. Hollow fin spines, identified in fossils, are what got the fish its name- "coelacanth" which literally means 'hollow spine'from the Greek. The sucking maws of jawless predecessors have transformed, through a modification of one of the gill arches, into hinged, rigid structures with teeth on the bottom ridge and upper palate- true jaws. The tiny brain, is encased in a hardened skull, which hinges in the middle to increase the gape of the mouth while feeding (a feature also found in frogs!) The eyes are well developed, with reflecting cells called tapita to enhance night vision. A chambered heart pumps blood in prototype to our own. Three indentations on either side of the snout lead to a peculiar cavity, a jelly-filled rostral organ, which very likely functions as an electro-receptor to help in the location of prey. Along the sides a pressure sensitive lateral line is well developed to sense the proximity of other fishes and surrounding structures- no doubt useful in the submarine caves where coelacanths pass their days. Two back, or dorsal, fins and one protruding beneath the nape of the tail are complimented by paired lobed pectoral and pelvic fins. These contain in their trunks bones mimicking those of Eusthenopteron which later might have developed into arms and legs. While coelacanths have not been observed to "walk" on the bottom, their pectoral and pelvic fins can be seen as "pre-adaptations" to land locomotion. Used under water their action maintains stability and balance. But in their cousin Eusthenopteron, the same action might have become four-legged land walking. Coelacanth scales are thick, and lined with serrated rows of hardened toothpick-pointed denticles. Perhaps most distinctive of all is the trilobated tail with its extra trunk and fin protruding from the middle. It was this feature that made fossil coelacanths so easily recognizable and helped clinch the case for the identification of the first living specimen. While the living coelacanths retain many ancient features they have also, contrary to their public image, done some evolving along the way. Live bearing, for example, would seem to be a modern feature.

 


Coming in from Search Engine - No Navbar Go to Coelacanth Home Page