The sharks, rays, and chimaeras are all grouped into the class Chondrichthys, commonly called the cartilaginous fishes (although as noted above the hagfishes and lampreys are also cartilaginous fishes). They are divided into two main evolutionary lines, the subclass Holo-cephali (chimaeras) and subclass Elasmobranchii (sharks and rays), with the former being the most primitive.
The chimaeras are grouped into 1-3 families, the divisions usually based on the shape of the snout (plownose. shortnose, and longnose chimaeras). There are usually two dorsal fins, the first being preceded by a spine that in some cases has a poison gland associated with it.
The males have paired claspers used for internal fertilization. They also may have an accessory clasper-like structure in front of the eyes, the function of which is still not well known. The eggs are encased in brown horny capsules.
Most chimaeras are found in cool to cold deep marine waters. They grow to fairly large sizes (normally to 1 m but up to 1.5 m) and do not take to handling very well. All this causes difficulties to those wishing to keep any in captivity. They normally do not last more than a few months in public aquariums although one has been kept alive for more than two years by University of Washington aquarists. These rather poor swimmers eat small invertebrates and fishes and are sometimes caught on hook and line, perishing quickly once removed from the water.
Sharks are a fairly large (approximately 250 species) and highly diversified (about 13 families in the superorder Selachimorpha) group. Most people are familiar with some sharks, particularly since the popular motion picture Jaws and its sequels were released. Almost every public aquarium or oceanarium keeps their share of sharks in large tanks that are preferably circular or doughnut-shaped so that the larger free-swimming species can be in constant motion, a requirement for such species. Even with such facilities many sharks fare quite poorly in captivity, ceasing to feed and often almost blindly ramming into the sides of the tank. Certain sharks do well in captivity, and exhibitions of their feeding draw considerable crowds. Home aquarists are more limited in their selection and normally must choose small, bottom-dwelling sharks for their aquaria.
Almost all sharks are marine. There are a few species that are able to ascend rivers into pure fresh water; the almost land-locked shark of Lake Nicaragua is quite well known. They are found around the world, mostly in tropical and subtropical waters with fewer numbers in the temperate regions and practically none in the cold polar waters. They vary considerably in size from those barely 46 cm long to giants 15-20 m long.
Shark reproduction involves internal fertilization. Males are provided with claspers on the inner edges of the pelvic fins. Development in most cases is ovoviviparous, while in others it may be oviparous or viviparous. In those that deposit eggs, the eggs are encased in horny capsules, sometimes highly ornamented, with long tendrils at the ends that become entangled (and thus anchored) with objects on the ocean floor. Some of the egg capsules have been successfully hatched in captivity.
Sharks are, without exception, carnivorous. Many are provided with very sharp, often serrated teeth that can do considerable damage to objects a lot harder than human flesh. Most sharks feed primarily on fishes, while others are scavengers feeding on whatever becomes available. Still others have teeth modified to feed on hard-shelled molluscs or crustaceans. The huge whale shark and basking shark, on the other hand, feed on small planktonic forms, primarily crustaceans. As for attacks on man. these are usually few and far between. Even then, in many cases the shark was provoked or it was a case of mistaken identity, the human victim appearing to the shark as its more natural prey (ex. seals).
Aquarists should be warned that even the small sharks that are kept in home aquaria have the potential of inflicting a severe bite on anyone foolish enough to become careless when handling them. These sharks are surprisingly strong and often are able to twist enough to bite at anything close enough, usually an arm. They are also quite adept at getting tangled in nets and slashing at the fingers that are freeing it from the mesh.
The two families of the order Hexanchiformes (frill sharks. Chlamydoselachidae, and cow sharks, Hexanchidae) have a single, spineless dorsal fin and six or seven gill slits. The Frill Shark (Chlamydoselachus anguineus) is snake-like in appearance and lives in deep water where it feeds mainly on squids. Of the cow sharks, the one most likely to be seen is the Sev-engill Shark (Notorifnchus maculatus), which is common in California bays as well as in the western Pacific. Sharks of both families are ovoviviparous.
The single family in the order Heterodontiformes, Heterodontidae, contains species that find their way into marine aquarists' fanks. Heterodontus portusjacksoni, the Port Jackson Shark, is one of these. It is also called the oyster-crusher because of its feeding habits. Aquarists should be wary of the sharp spines preceding each of its two dorsal fins. Horny egg cases with spiral flanges are laid by members of this family.
The order Lamniformes (with about seven families) contains most of the sharks. There are two spineless dorsal fins, an anal fin. and five gill slits. Some of the sharks in this order are small and very colorful or have interesting patterns and so are of interest to aquarists. Some of the carpet or nurse sharks, for example, are generally available to aquarists. First among these is probably the Atlantic Nurse Shark (Ginglymostoma cirratum), although it soon outgrows the tank in which it is housed. It is not very colorful but is quite docile, comparatively speaking. The carpet sharks are very colorful and in addition have fringes or lappets that add to their appeal. Most of these sharks also outgrow their tanks quite rapidly.
The family Scyliorhinidae (cat sharks) is by far the most important family of sharks to aquarists. It contains the smaller, more colorful species that do well in captivity. Almost all are bottom-dwellers feeding on the invertebrates and small fishes that are found there. Aquarists should be aware that some species grow large (over a yard) and many are from temperate regions, requiring cooler tank water. Most of the species kept in aquaria are from the Indo-West Pacific. Included in this family are the swell sharks that can fill their stomachs with air if they are removed from the water, much like a pufferfish. The species are ovoviviparous. No special care is needed for these small sharks other than a suitably large tank, plenty of good food, and perhaps an open area of sand.
Other members of the Lamniformes include the whale shark (family Rhincodontidae), the sand tigers (family Odontaspididae), the thresher, basking, and mackerel sharks (family I.am-nidae), the hammerhead sharks (family Sphyrnidae), and the smooth dogfishes and requiem sharks (family Carcharhinidae). Almost all of these are too large for private tanks, and only a few are regularly seen in public aquariums. Among the better known sharks in this group are the Whale Shark, a monster that is said to reach as much as 15 m in length, the Gray Nurse Shark (perhaps responsible for many of the shark attacks in Australian waters), the thresher sharks with their unusually elongate upper tail lobes, the Great White Shark ("Jaws"), the Mako, and of course the Tiger Shark with its tiger-like stripes and reputation to match. Stomachs of Tiger Sharks have contained a wide variety of items from indigestible, inanimate garbage items to human body parts. With such a reputation many public aquariums have tried (mostly unsuccessfully) to keep them alive. Among the more unusual sharks can be included the hammerheads, so-called because of the bizarre development of their heads, with the eyes on the end of lateral extensions.
The order Squaliformes contains sharks with two dorsal fins (with or without spines), no anal fin, and five gill slits. Three families are currently included, the dogfish sharks (family
Squalidac), the saw sharks (family Pristiophoridae). and the angel sharks (family Squatini-dae). Only some of the smaller squalids are kept in home aquaria, such as species of Squall and CephaloscyIlium, and it is Squalus acanthias that is the shark commonly used in compara tive anatomy labs. The smallest shark, Squulioius laticaudus, which grows to about 25 cm. belongs to the Squalidae. The saw sharks should not be confused with the sawfishes (family Pristidae), which are rays, although the saw sharks have many ray like characters and have a great superficial similarity to those fishes. The saw sharks have laterally positioned gill slits while those of the sawfishes are ventrally located. The angel sharks also are somewhat ray-like but they also have the gill slits laterally positioned. The angel sharks are also called monkfish. As their flattened body might lead one to guess, they live on or close to the bottom, often partially hidden in the sand.
The superorder Batidoidirnorpha. Order Rajiformes, contains the rays (in the broad sense including the sawfishes, guitarfishes, skates, etc.). Only a few colorful species are kept by marine aquarists. All of these fishes are characterized in part by having the gill slits opening on the underside of the body,
Small members of the sawfishes (family Pristidae) are sometimes kept in home aquaria and commonly in public aquariums. They should be handled with care as the elongate snout provided with rows of sharp teeth (actually modified scales) along each side can cause considerable damage. Many an unlucky person who thought it was safe to hold one by the tail has scars to prove the fallacy of this idea. Normally the saw is used to slash through schools of fishes (mullets, etc.). impaling some on the saw teeth. Sawfishes arc found around the world in shallow tropical waters. They may stray into brackish or even fresh water, and, like a shark cousin, one species has become almost land-locked in Lake Nicaragua. The guitar fishes (family Rhinobatidae) are similar in shape to the sawfishes but arc without the saw. They are too big for home aquarists but may find their way into public aquariums. More fascinating to aquarists arc the electric rays (family Torpedinidae). These ovoviviparous fishes can generate electrical charges, some quite intense, and should be handled with great care-Most encounters are by swimmers or waders who inadvertently step on one on a sandy bottom. These rays are poor swimmers and rely mostly on their shocking ability rather than (light for protection. Some of the species are quite colorful and, regardless of the risk, are kept in home tanks. Their food is mainly bottom invertebrates or fishes. The eagle rays (family Myliobatidae) and manta rays (family Mobulidae) are much too large for home aquaria, but some smaller ones are kept in oceanariums. They always attract attention with their graceful "flying" motions. Both eagle and manta rays are noted for their ability to leap from the water and come down with a resounding crash. The two most speciose families are the skates (family Rajidae) and stingrays (family Dasyaticlae), and it is from these families, particularly the latter, that aquarium species are selected Most skates and rays live inshore on the bottom, feeding on shellfish and/or crustaceans. Skates prefer more temperate waters and in many areas are fished for commercially. Males have long claspers for use in internal fertilization. They lay eggs encased in horny capsules that have tendrils or projections that help anchor them to objects on the bottom. Many are washed up on shore and have been given the names sailor's purse and mermaid's purse. Stingrays are more tropical fishes, spending much of their time almost completely buried in the sand or soft bottom material. In addition to the crustaceans and molluscs, they will readily feed on worms when available. Stingrays have the base of their tail armed with one or more spines provided with venom-producing tissue. Perhaps the most commonly kept stingray is Taeniura lymma, a colorful species with blue spots scattered over its upper surface.
The remaining fishes all belong to the bony fishes (class Osteichthys), which have skeletons composed, at least in part, of true bone.
The Coelacanth (Plate 17)
The Comoro Islands Coelacanth (order Coelacanthiformes, family Latimeriidae. Ixitirneria chalumruie) is the only kruiwn survivor of the crossopterygians, fishes usually considered to be ancestors to the entire lineage of present-day vertebrates. All the Coelacanths have been collected in relatively deep water (to 360 m) where there is a rocky bottom. Their large scales are mostly a deep metallic blue. They feed exclusively on other fishes. What makes them very unique: is their lobed fins. The second dorsal, pectoral, and pelvic fins arc supported by fleshy stalk like structures earning them the common name of lobefins. Most public aquariums and oceanariums would love to have a living Coelacanth on display, but they have not been very successful in their endeavors. Obviously a rare 1.2-1.5-m fish reaching a weight of 72 kilos would not bo available to private aquarists. These ovoviviparous fishes were found to contain eggs up to 9.1 cm in diameter and young in the oviduct with a length of up to 33.5 cm total length.
Tarpons, Ladyfish, and Bonefish (Plate 17)
The tarpons (Megalopidae), ladyfishes (Elopidae). and bonefishes (Albulidae) are all included in the order Elopiformes where the pelvic fins are abdominal, the tail is deeply forked, and all have a fork-tailed leptocephalus-like larva. None are of great interest to aquarists, although all are sought after avidly by fishermen. Often called the silver king, the Atlantic Tarpon (Megalops atlanticus) grows the largest, attaining up to 2.4 111 in length and a weight of 135 kilos. Once hooked, the tarpon puts up a tremendous battle (especially on relatively light tackle) with spectacular leaps, giving the fisherman the time of his life. The bony mouth makes it hard to set the hook and many fish arc lost. Even when landed, however, this fish is often released, partly in respect for the pleasure it has just given. Bonefish (Albula vulpes) are smaller and instead of leaping make powerful runs stripping line from the reel at a fast clip. Like the others just mentioned, the Atlantic Ladyfish (ELops saurus) is a game fish. It resembles the tarpon more in appearance and fighting characteristics but averages closer to 2.25 kilos. It also lacks the filamentous last ray characterizing the tarpon. Most of these species are found in shallow inshore waters where they feed on small fishes and invertebrates. There is a deep-water genus of bonefish (Ptcrothrissus) with a long dorsal fin.
Eels (Plates 18-29)
All the true eels, comprising about 20 families, are included in the order Anguilliformes. The body is elongate, and the dorsal and anal fins are usually connected to the caudal fin. while the pelvic fins (occasionally also the pectoral fins) are absent. There is a leptocepha-lus larva (elongate, very flattened, and almost transparent) but. contrary to that of the Elopiformes, it has a tail tapering to a point rather than a forked one. The elongate form of the eels has been suggested as being an adaptation for moving through small openings and in some cases there are further adaptations for burrowing. Although most eels art* bottom-dwellers, there are at least some adapted to a bathypelagic mode of life.
Of the many eel families, only a few contribute species to the marine aquarium on a regu lar basis. Others may be kept as curiosities as the occasion allows. Freshwater eels (which usually spend much of their life in fully marine waters) of the family Anguillidae are collected occasionally and may find their way to home aquaria of marine enthusiasts but are then usually released again when the novelty wears off. The freshwater eels generally spawn far out in the open ocean where as many as 20 million eggs are released by each female. After spawning the adults die. The leptocephali then travel back to the continental shores (this trip may be up to several thousand kilometers) where they ultimately metamorphose into young eels called elvers that move toward fresh water The snake eels (family Ophichthi-dae) are borrowers, the tails of some genera being modified into a hard, fleshy point without fins. Burrowing is done tail-first. Several species from this family, especially the colorful members of the genus Myrichthys. are regularly kept in marine aquaria. The worm or spaghetti eels (family Moringuidac) are extremely thread-like, without or with only feeble pecto ral fins, and with eyes that are small and covered with skin.
The conger eels (family Congridae) arc distinguishable from the moray eels by possessing pectoral fins. The larger species are usually passed over by aquarists, but several members of the garden eels (subfamily Heterocongrinae) make interesting aquarium inhabitants and are seen from time to time. Garden eels are found in nature usually in rather large colonies (gardens) where there are many burrows. The garden eels will be seen hovering just above the burrows feeding on small organisms that are carried to them in the current. At the approach of some threat the eels will retreat tail-first into their burrows only to appear again slowly when the danger is past.
The moray eels (family Muraenidae) lack pectoral fins, have small, restricted lateral gill openings, and most have long, fang like teeth. Most live on tropical and subtropical reefs in relatively shallow water and are quite colorful. Most are large, generally up to a meter in length, with others reaching a length of over two meters. Divers frequently encounter morays that are in holes in the reef with only their heads sticking out. As they respire the mouth opens and closes exhibiting the formidable teeth. Although this deters any diver from messing with them, unfortunate divers have been bitten when they stuck a hand into a hole after a fish or shell and were unaware that it was occupied by a moray eel. Normal food of morays includes small fishes and some invertebrates. Apparently squid are a delicacy for most morays, and aquarists able to supply such food should do so. Morays of the genus blchidyia. generally have teeth modified for crushing the shells and carapaces of crustaceans and molluscs. Some morays even feed on sea urchins. Even with such a bad reputation (or possibly because of it) aquarists keep many species of morays (mostly from the genus Gymnothorax) in their home tanks. Public aquariums almost always have several species on exhibit as well.
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