SEAHORSES, ICEFISH,

ELECTRIC FISH

 

EVOLUTIONARY MODEL

If evolution is true, then dramatic adaptations such as the body shape of seahorses, the unique adaptations of icefish, or the electrical conduction of electric fish should be modified versions of strucutres found in other fish. Other groups of fish might develop similar adaptations.

CREATIONISM MODEL

If the creationism model is correct, dramatic adaptations could represent unique structures, unrelated to those of any other fish which show no signs of acquisition through gradual change. It is not expected that other groups of fish develop similar adaptations.

INTELLIGENT DESIGN

If intelligent design is correct, then dramatic adaptations could not represent modified versions of ancestral structures nor could they develop through transitional states. In order to have any function for an organism, they must be supernaturally created in a complex state.

SEAHORSES

pipefishpipefish
The family Syngnathidae includes seahorses, pipehorses, seadragons, and pipefishes (pipefish are depicted above). Genetic evidence and a number of shared anatomical features (such as a prehensile tail, a brood pouch, and fin structure) support the monophyly of this group and a nested hierarchy of relationships within the group. Other related families also include species with elongated bodies (Teske, 2004; Kawahara, 2007).

pipefish seahorses

In the family Syngathidae (seahorses and pipefish) males may possess a brood pouch for the care of the young (seahorses), a partial brood pouch which opens in the middle (some pipefish), or may lack a brood pouch (other pipefish in which the female's eggs simply adhere to the male's underside). Seahorses have binocular vision, unlike most fish (Moyle, p.282-3).


ICEFISH
The development of polar environments in the southern ocean is a relatively new phenomenon, dropping from about 20oC to -2oC in the past 55 million years. About 274 fish have adapted to Antarctic waters. These fish are members of 49 separate fish families including hagfish, lampreys, skates, sharks, and teleosts (Eastman, 1993, p. 29). While most fish went extinct, teleosts of the group Notothenioidei adapted to this environment, reducing the amount of hemoglobin in their blood to compensate for the thickening of fluids which occurs at lower temperatures. Of the 129 species classified in the sub-order Notothenioidei, 101 in five families have adapted to Antarctic waters (Di Prisco, 2007). Anatomical and genetic studies indicate a nested hierarchy of relationships between the diverse lineages of notothenioids. All cichlids form a monophyletic clade and within notothenioids, there are a number of smaller clades which have been identified (Di Prisco, 2007; Di Prisco, 2002).

Some notothenioids have reduced the number of different hemoglobins produced (fish in general typically produce multiple hemoglobins, perhaps as a way of adapting to different environments). Some notothenioids produce 5 different hemoglobins while others produce two or one. One family, Channichthyidae, completely lacks hemoglobin. The blood of white-blooded notothenioids can only carry 10% of the oxygen of the red blood of other notothenioids. The icefishes (Channichthyidae) suffered a deletion of hemoglobin at the base of their clade after which the genes were inactivated. Traces of the globin genes, but not the globin genes, still remain. This family has scaleless skin, allowing them to perform cutaneous gas exchange. Oxygen concentration in Antarctic waters is high and oxygen demands of the fish are low (Hays, 1996; Bargelloni, 1998; Di Prisco, 2002).

One study found that myoglobin was absent from the oxidative skeletal muscle and auricle of 8 species of icefishes (Channichthyidae). Of these 8 species, five possessed myoglobin in their heart's ventricles (and one produced the mRNA but not the protein). All of the species possessed a myoglobin gene and analysis suggests that the loss of myoglobin expression occurred multiple times within this family (Sidell, 1997).


Some larval eels and deep sea fish also lack hemoglobin.

Antifreeze glycoproteins are present in the plasma in only some members of the subfamilies Eleginospinae and Nototheniinae. (Eastman, 1993, p. 173)
In related Antarctic icefish, the liver weight/total body weight can very from .9 to 3.2% (Eastman, 1993, p. 161).


ELECTRIC

fish

In Mormyridae, the electric fishes, electricity is generated for electrogenesis and electroception using an electric organ. The electric organ is a modified muscle which is no longer contractile. This muscle produces an action potential large enough to produce an electric field around the fish (Katz, 2006; Lavoue, 2000).
In Mormyridae, the cerebellum is so large that its size compared to that of the body approaches the ratio found in humans. These fish typically engage in behavior which has been described as play, something typically attributed to more advanced vertebrates (Moyle, p. 232; Van Der Bank, 1995).

The families Mormyridae and Gymnarchidae form a clade which can be broken into subgroups. The single species of Gymnarchidae lacks caudal and ventral fins and the electric organ is different than that of the mormyrids. An electric organ with stalkless electrocytes had evolved by the common ancestor of Mormyridae and Gymnarchidae, and subsequently further advances occurred in the 188 species Mormyridae which are endemic to Africa (Lavoue, 2000; Van Der Bank, 1995). Within mormyrids, electric organs began as stalkless structures. They later became stalked and in some species the stalked electric organs were modified further(Hopkins, 1995).


Mormyrids are not the only electric fish. The African mormyrids and the South American gymnotids have independently evolved electric organs and electrosensory systems through the duplication and subsequent modification of sodium channel genes (Katz, 2006). Thus, two groups of electric fishes (the mormyriforms of Africa and the gymnotiforms of South America) have independently modified a sodium channel gene (Nav1.4a) and changed its area of expression from muscle to the electric organ (Zakon, 2006). Other fish capable of generating electric pulses are electric skates and species in the genera Malapterurus, Clarius, and Synodontis (Hopkins, 1995).