The below mentioned article provides a note on the evolution and classification of fishes.
Fishes are generally grouped into three broad categories: the placoderms, the cartilaginous fishes and the bony fishes. The placoderms include the primitive and extinct fishes, the cartilaginous fishes comprise the elasmobranchs and the holocephali, while the bony fishes include the Actinopterygians, Crossopterygians and Dipnoans.
Each of these groups is characterised by having a number of individual specializations. But the interrelationship between the different groups of fishes becomes extremely difficult to interpret. The most important obstruction stands on this particular aspect is the absence of good fossil link.
The placoderms were the earliest known gnathostomes and they are regarded by many as the progenitors of the modern fishes, both the bony and cartilaginous forms. The placoderms represent a polymorphic group and exhibit wide range of adaptive radiation.
Of the placoderms, the acanthodians were the most primitively constructed forms and possibly hold the ancestry of the other fishes. But the existence of the operculum in acanthodians becomes sometimes difficult to interpret, specially in the consideration of the phylogeny of the elasmobranchs.
The old idea that the elasmobranch fishes with cartilaginous skeleton represented the most primitive forms does not hold true, because recent workers on this line are unanimous on the primitiveness of the placoderms. This is attested by paleontological records.
The fragments of bony fishes were abundant in mid-Devonian deposits. From the existence of the fossil records, the emergence of these forms prior to mid-Devonian strata is obvious, possibly in the early Devonian, if not late Silurian times. Whether the two subclasses of the bony fishes, viz., the Crossopterygii and the Actinopterygii, have immediate common ancestry is not certain.
It is most likely that the Actinopterygians and the Crossopterygians had common ancestory in the upper Silurian or early Devonian period. After originating from the common progenitors these two groups underwent wide diversification.
The Crossopterygians, despite their abundance in the early part of evolutionary history, exist today with a few living representatives, whereas the Actinopterygians have reached the pinnacle of evolutionary success and survive today by a countless number of forms.
The emergence of the elasmobranchs is still a debated issue. The absence of bony elements led many earlier workers to hold the view that the bony fishes have evolved from the elasmobranchs. But such a concept is rejected on the plea that the bone is actually a primitive material in the geological history.
The earliest known fragmentary remains of the elasmobranchs (although rejected by many recent Ichthyologists) were discovered in the mid-Devonian deposits. But the records of the fossils of the bony fishes dated back to the early Devonian and late Silurian period. So the idea of the elasmobranch progenitor of the bony fishes is unacceptable.
The elasmobranchs represent a well-defined group and their evolution from the shark-like polymorphic placoderms is supported by remote and incomplete evidence. The elasmobranchs have cartilaginous skeleton with no trace of true bony elements.
So how can this group be evolved from the placoderms where the skeleton is bony is difficult to explain. Many recent authors incline to think that the elasmobranchs have evolved from bony ancestor (the placoderms) through neoteny.
Evolutionary plasticity of the youthful stages and their subsequent role in evolutionary dynamics have been greatly emphasized by Garstang and Willey. It is likely that the bony ancestral forms during their larval phase became sexually mature and reproduced in that condition. Such a neotenous form possibly holds the key of the emergence of the elasmobranchs. There are evidences in favour of this idea.
The evidences are:
(1) The cartilage is an embryonic tissue. So the presence of exclusively cartilaginous skeleton indicates the embryonic condition.
(2) The neotenous condition is observed in a few living teleosts as exemplified by Clariallabes.
An amphibian larval form Axolotl exhibits the same phenomenon. So the cumulative evidences force us to believe the emergence of the elasmobranchs from the bony fishes through the phenomenon of neoteny.
Cladistic classification:
Today more or less two major evolutionary phylogenetic systems are used in the classification of organisms.
The older one which is called evolutionary systematics and the newer one is phylogenetic systematics or cladistics, Cladistics is one method, related to the branching sequences of the phylogenesis and the phylogeny is reconstructed on the basis of shared derived characters that analyze the primitive and derivative characters.
This system of classification performs a good degree of objectivity in the selection of morphological characteristic features that are employed in reconstructing characteristic features that are employed in reconstructing phylogenetic relationships.
Clade is the phylogenetic lineage evolving from a common ancestor. Fig. 6.70 is a cladogram of fishes which represents the hypothesized relationship of taxa. In this cladogram, among gnathostomes have many common features including jaws, paired fins, scales, gills, vertebrae and semicircular canals. By employing these common shared derived features the relationship among major groups of fishes has established.
General Notes on Fishes:
Fishes have undergone extensive adaptive modifications and exhibit an endless variety of forms in different environments. The organisation of the fishes is so diverse that it is difficult to give a complete and comprehensive account of the group.
Size:
The fishes vary extensively in size ranging from 10 mm long tiny Pandaka (Philippine goby) to the giant tertiary elasmobranch, Carcharodon of about 24 m long. The largest living fish is Rhinodon (oviparous whale shark of tropical seas) which attains a length of 21 m. The dogfishes of the genus Squalus grows to an average length of about 1 m. Majority of the sharks do not exceed 2.5 m.
The rays are mostly 30-90 cm in length and the giant ray, Manta birostris (Devilfish) measures 5.1 m in length and 6.5 m across the pectoral fins. Bathytoshia (a Pacific sting ray) reaches 4.2 m in length and a width of about 2 m. The chimaeras are usually less than 90 cm in length.
The bony fishes are variable in size and no bony fish attains a length rivaling that of the largest known sharks. Most of them are under 1 m in length. Huso huso (a sturgeon) grows to a length of 7.2 m. Sphyraena (tropical and subtropical barracudas) reaches about 2.4 m and Serrasalmus (river-dwelling Piranha) rarely exceeds 60 cm in length.
Mistichthys luzonensis (a Philippine goby) measures 13 mm in length. Tetrapturus (spear-fish) grows more than 4.2 m in length, Istiophorus (sail-fish) is about 3.6 m and Arapaima gigus (South American Spear-fish) attains a length of about 4.5 m.
Skin:
The skin of fish is composed of epidermis and dermis. A thick basement membrane is present between the dermis and epidermis. The epidermis is multilayered, but the stratum corneum is absent. The dermis is made up of connective tissues, blood vessels, nerves and smooth muscle fibres. The scales remain embedded in the dermis.
The epidermis of fishes contains large mucous cells or Becker’s cells and chromatocytes. The chromatocytes are also present in the dermis. Besides the mucous cells, two types of sensory cells (granular sensory cells and club cells) are encountered particularly in Actinopterygian fishes. The dermis is composed of the stratum laxum, the stratum compactum and a subcutaneous layer.
The stratum laxum is a laminated fibrous layer into which the bases of the scales are held by Sharpey’s fibers while the stratum compactum is made up of a reticulum of delicate fibres. All these three layers are sharply demarcated from each other.
The skin glands are modified in many fishes. In Lepidosiren and Protopterus the skin glands produce abundant secretion to form ‘cocoon or capsule’ during aestivation. The poison glands and the photophores are modified skin glands.
Multicellular poison glands are present in many elasmobranchs and a few teleosts. These glands are derived from epidermis and are protective in function. Photophores or luminous organs are present in deep-sea elasmobranchs and teleosts inhabiting in total darkness.
The photophores are modified skin glands. The pterygopodial glands situated at the root of the claspers of some rays and skates are modified skin glands. These glands help in sexual activities.
Parental care:
Most fishes do not care for their eggs or youngs and leave the spawning grounds after laying the eggs. But there are many fishes where definite parental care has been evolved.
Various devices have been adopted to ensure proper development of the eggs into adults. One or both the sexes may participate in the process. The different ichthyologists like Ridley (1978), Baylis (1981), Potts and Wootton (1984) have discussed different modes of parental care.
Formation of nest:
The male of many fishes Etheostoma (dorters), sunfishes, cichlids prepare shallow basin-like nest for laying eggs by the females. The stones and pebbles are removed from such nest and the male keeps close watch over the eggs till hatching.
Many freshwater fishes prepare crude nest with aquatic vegetation where eggs are laid. Protopterus and Lepidosiren prepare deep hole into which the females lay eggs. Males protect the nests till development is complete.
Amia (Fig. 6.104) constructs crude circular nest made out of aquatic vegetation. Youngs are seen to leave the nest under the protective guidance of their father.
The nest prepared by three-splined stickleback, Gasterosteus aculeatus (Fig. 6.105) is very peculiar. The male collects aquatic weeds which are joined together by a sticky secretion produced from the kidneys of the male. When the nest assumes a considerable size, the male makes a small tunnel.
After the formation of tunnel, the male drags a mature female into the tunnel for laying eggs. After laying eggs, the- female swims away and the male keeps watch over the fertilized eggs till development is over. In addition, foamy nests prepared by blowing of bubbles of air and sticky mucus are also encountered in many fishes.
The bubbles of air and mucus adhere to form a floating mass of foam. The eggs are collected by the male in his mouth cavity and he throws them in such a way that the eggs can adhere to the lower surface of foamy nest. This type of caring for eggs is found in Betta, Macropodus and many other fishes.
Mouth cavity as shelter:
In many fishes, the fertilized eggs develop within the mouth cavity. In cichlids, the females carry the eggs in their mouth cavity. Even after hatching, the fry uses the mouth cavity as shelter at the time of danger (found in Tilapia) (Fig. 6.106). In a cat-fish, Arius the males carry the eggs as well as youngs fry in the mouth cavity.
Coiling round eggs:
In butter fish, Pholis, (Fig. 6.107) the eggs are rolled into a rounded ball and one of the parents, possibly male, guards the egg ball by coiling round it.
Attachment of eggs to the cephalic hook:
The male nursery fish, Kurtus (Fig. 6.108) bears a hook-like process of dorsal fin spine. It carries egg mass in two bunches.
Formation of integumentary cups:
In a cat-fish, Platystacus, the caring for eggs is of advanced type. The skin of the lower surface of body of the female becomes soft and spongy during spawning season.
Immediately after the fertilization of the eggs, the female presses her body against the eggs in such a way that each egg becomes lodged in a small integumentary depression. Each egg is attached inside the cup by an inconspicuous stalk. The eggs remain in this position till hatching.
Development of brood pouch:
In Syngnathus and Hippocampus (Fig. 6.50D) the eggs develop within the brood pouch of the male. The eggs are transferred into the brood pouch by the female and development takes place within the brood pouch.
Viviparity:
The highest degree of parental care is found in viviparous fishes where young develop within the oviduct of the female. The nutrition is drawn by forming yolk-sac placenta in most cases.
Examples:
A. Chondrichthyes (Rhinobatus, Trygonorhina, Spinax, Acanthias, Trygon, Torpedo);
B. Osteichthyes (Cambusia, Anableps, Sebastes norvegicus, Zoarces viviparus).