Shark Page 18
Flat sharks
The superorder Batoidea comprises guitarfishes (Rhinobatiformes); wedgefishes (Rhiniformes); sawfishes (Pristiformes); skates (Rajiformes); electric rays (Torpediniformes); and stingrays (Myliobatiformes). They are collectively known as the batoids. Researchers use the terms ‘Batoidea’ and ‘Rajomorphii’ interchangeably, but the common term ‘rays’ is frequently used to refer to all members of the superorder. The exact nature of the relationship between the batoids and their ‘true’ shark cousins the distinctly flattish angel sharks, as well as to some bottom-dwelling dogfish sharks, remains uncertain. The fossil record seems to indicate that the rays are shark derivatives, a linking lineage known as the Hypnosqualea hypothesis, but some researchers argue a case of ‘molecular phylogenetic evidence refuting the hypothesis of Batoidea (rays and skates) as derived sharks’.80
The batoids are at the heart of what has been described as a cladistic (classification) elasmobranch revolution. As a distinct group they are—even by the standards of the sharks already discussed in this book—under-researched and poorly known, so much so that it was not until 2004 that the batoids were formally assessed for the International Union for the Conservation of Nature’s (IUCN’s) Red List of Threatened Species. One reason for this is that most scientists and researchers tend to focus on the ‘true’ sharks.81
In 2004, the 631 identified species of batoids represented just over half the total number of species of the planet’s cartilaginous fishes. Although more speciose than the sharks, the batoids do not have such bewilderingly diverse physical characteristics. Most, although all rules have exceptions, have a body that is flattened dorsoventrally, that is, from top to bottom, and a tail that is either narrow with very reduced fins (skates), or whiplike with no fins (rays). The batoids’ characteristic disc-like shape is a result of the pectoral fins being greatly spread out and fused to the sides of the head. The eyes are on the top of the head, the mouth and gills underneath. Behind the eyes are large spiracles to take in water that is then passed over the gills, which as often as not are barely above or on the seabed, or even submerged under the sand. The batoids’ basic body plan—directly complementary to that of the ‘true’ sharks—perfects the distribution of the cartilaginous fishes into every conceivable aqueous niche: fresh, saline, estuarine, shallow, midwater, deep, warm, temperate, cold, clear, murky, dark or lightless.
The batoids’ jaws have evolved to take every possible advantage of their life as bottom feeders. Hard-shelled prey is a major component of their diet, but this means they have to separate out shell and other indigestible or useless matter. Unlike bottom-dwelling suction-feeding sharks they do not repeatedly take in and spit out prey, a feeding mechanism which eliminates unwanted matter before swallowing but which risks exposing valuable food to scavengers. Instead, the batoids’ elaborate lower jaw apparatus allows them to separate out wanted and unwanted material internally, with the latter then ejected. This is described as a ‘largely hydrodynamic form of manipulation, permitting exploitation of benthic invertebrates with exoskeletons’,82 through a number of cranial muscles—which sharks lack—that manipulate the lower jaw. This is seen as the benthic equivalent of large predatory sharks being able to protrude their upper jaws when biting prey. (Elasmobranch jaw manoeuvrability is the evolutionary equivalent of teleosts using their pharyngeal jaws and tetra-pods their limbs to manipulate and process food.)
Guitarfishes (Plate 15)
Classification
• Four genera in three families
• At least 47 species
Biology
• Elongated snout, tapering sharklike body with two dorsal fins
• Ovoviviparous
Habitat
• Temperate and tropical inshore waters globally
As an order they are the Rhinobatiformes, but as a suborder of the rays they are the Rhinobatoidei. Furthermore, the single-family order Rhiniformes—the sharkfin guitarfish, also known as the wedgefish or shovelnose ray—simply confounds what is already a confusing taxonomy. The Rhinobatiformes include thornbacks, fanrays, panrays, banjo rays and fiddler rays (pesce violino in Italian). The common names are the key here, for these fishes combine the disc and the fusiform body plans. The rhiniform rays, with their pectoral fins fused to their heads, are described as having ‘sharklike dorsal fins’ with a ‘trunk thick and sharklike’.83 Physiologically, if not zoologically, they appear to be an intermediate form, and there is no reason why there should not be such forms. Shovelnose rays can also be distinguished from guitarfishes by the relative placement and size of their pelvic fins and lower caudal lobes, while fiddler rays differ from guitarfishes in their snout shapes.84
Most guitarfishes are small or medium-sized inhabitants of shallow inshore warm and tropical waters, including estuarine environments. A few species are found in temperate waters as far south as Australia’s Bass Strait. The paired, prominent dorsal fins are set well back along the thick, tapering tail, which provides vigorous sharklike locomotion, while the spread pectoral fins are effective in both acceleration and manoeuvrability. Guitarfishes swim at a positive angle: the head is slightly higher than the tail. During the day, guitarfishes tend to bury themselves in sand or other substrate, leaving just their eyes and spiracles above the surface. They are ambush predators, grabbing unwary crustaceans and bivalves, often with the help of the rostrum which, like a limb, pins the prey down and prevents it from escaping. Guitarfish have small, numerous and blunt teeth, designed for crushing shells. By night, guitarfishes are more active, actively hunting small fish. Guitarfishes are viviparous, bearing up to twelve live young.
The difficulties in classifying the batoids can be demonstrated by the case of two species of shovelnose rays inhabiting the continental shelf off the coast of north-western Australia. Theirs is an intriguing story. In the 1970s, commercial Taiwanese trawl fishing in the area led Australian authorities to set up a research program to identify exactly what was being caught in the trawlers’ nets. This CSIRO program ran for ten years, and was the first thorough study of the local fish fauna. Many new species were discovered, including two small shovelnose rays which did not quite conform to known shovelnose species. As with many taxonomical orphans, for some years the literature referred to them as ‘undescribed species’.85
The two rays, both just over 50 centimetres long, were finally described in 2004 by senior CSIRO scientist Peter Last.86 They are the goldeneye shovelnose ray (Rhinobatos sainsburyi) and the spotted shovelnose ray (Aptychotrema timorensis), the latter described from just a single specimen taken at a depth of over 100 metres in the Timor Sea. The spotted shovelnose appears to be the northern Australian equivalent of the continent’s western shovelnose ray (Aptychotrema vincentiana) and the eastern shovelnose ray (Aptychotrema rostrat), but can be distinguished physiologically by having definite white spots rather than blotches or no patterning at all on its skin; a slightly different snout apex; more angular dorsal-fin apices; a lack of dark marking at the snout’s ventral apex; and fewer vertebral centra, resulting in a smaller caudal fin. Easy!
The morphometric table for the holotypes of these two new species painstakingly measures, to parts of millimetres, no fewer than 62 external parts of their little bodies which, upon reflection, are surely as worthy of admiration as those of their breaching great white cousins. As has been suggested of the shovelnose guitarfish in faraway California: ‘This ancient ray has been playing it flat for over 100 million years’.87
Wedgefishes
Classification
• Two genera in one family
• Five species
Biology
• Intermediate body form between shark and ray
• Small blunt teeth
• Two large dorsal fins
• Well-developed caudal fin
• Ovoviviparous
Habitat
• Shallow coastal tropical and subtropical waters
The bowmouth guitarfish (Rhina anclyostoma) is relati
vely common in warm reef waters from northern Australia through the Indonesian archipelago to eastern China. It is described as:
an unmistakable guitarfish with a broad, rounded snout, large, high pectoral fins, and heavy ridges of spiky thorns over the eyes and on the back and shoulders; jaws with heavily ridged, crushing teeth in undulating rows . . . Grey or brownish above, white below; numerous white spots dorsally on fins, body and tail; black spots on head and shoulders but no eyespots or ocelli . . . Inhabits coastal areas and on coral reefs, close inshore . . . Found on sand and mud bottoms . . . Sometimes found in the water column . . . Feeds mainly on bottom crustaceans and mollusks . . . A row of large spines present above the eye, on the center of the nape, and on the shoulder have a defensive function (can be used for butting). Caught commonly by demersal tangle net, and occasionally trawl and longline fisheries . . . Difficult to handle and can damage the catch when caught in trawls . . . Utilized fresh and dried-salted; the pectoral fins are the only part which is eaten.88
Sawfishes (Plate 16)
Classification
• Two genera in one family
• Seven species
Biology
• Elongated snout is a toothed rostrum without barbels
• Sharklike body with two dorsal fins
• Ovoviviparous
Habitat
• Coastal marine waters, one freshwater species
The sawfish is a ray. Its pectoral fins are distinctly detached from its head, it has large spiracles behind its eyes to take in water and its mouth, nostrils and gills are ventral. The trunk is flattened dorsoventrally but, despite this, its body form is decidedly unraylike, having a thick shark lower body and tail supported by large dorsal and pelvic fins. Its snout extends to a toothed rostrum which in some species is long enough to class the sawfish as ‘gigantic’, possibly exceeding both the great white and the tiger shark in total length. The scientific literature invariably describes the sawfish as a highly modified ray and it is not to be confused with the sawshark. Sawfishes are inshore dwellers in warm waters, both salt and fresh. They are among the most endangered of all marine life.
Sawfishes are classified in the order Pristiformes, comprising just two genera of approximately seven species:
However, due to considerable taxonomic confusion this number may in fact vary between four and ten. Among the reasons for this taxonomic disarray is that many of the original species descriptions were extremely abbreviated, and in some cases not even based on specimens, or based only on isolated anatomical parts; only two of the six type specimens are available for examination today; poor representation of specimens in collections, which mostly consist of dried rostra or very young specimens; and scarcity of these animals in their natural habitat due to overfishing.89
(Bizarrely, the caudal fin of a sawfish that Dutch ichthyologist Dr P. Bleekers had used to describe the holotype of a specimen in 1852 was later found stashed in the body cavity of another specimen in his collection.)
The species described here are:
• Dwarf sawfish (Pristis clavata)
• Freshwater sawfish (Pristis microdon)
• Smalltooth sawfish (Pristis pectinata)
• Largetooth sawfish (Pristis perotteti)
• Green sawfish (Pristis zijsron)
The species that prefer freshwater environments have broad, tapering rostrums; those with a preference for saltwater have non-tapering rostrums. The evenly spaced rostral teeth are modified denticles and number from about fourteen to 23 on each side. There is not always the same number of teeth on each side. The seventh species has a distinctly narrower saw with flatter teeth. The teeth of the rostrum are self-sharpening, through constant abrasion against sand and stone, and a broken tooth will grow again as long as the base is not damaged.
No species is found exclusively in freshwater environments. The dwarf sawfish, which inhabits the shallow estuarine and tidal mangrove waters of tropical Australia, moves into freshwater rivers during the wet season, possibly in order to breed, as does the much larger and more widespread freshwater sawfish which grows to about six or seven metres. Juveniles spend their first three or four years in rivers before moving to shallow coastal waters. However, adults are also known to travel as far as 100 kilometres upriver. In the Top End of Australia, this migration may be associated with wet season flooding.
Sawfishes are recorded in river systems throughout southeast Asia, India and eastern Africa, many of which are also home to the bull shark. Given their very different feeding strategies, competition between them is likely to be minimal. In the western Atlantic the smalltooth sawfish and the rare largetooth sawfish exhibit similar habitat diversity:
Florida’s sawfish are most often found within a mile of land such as in estuaries, river mouths, bays, or inlets. They occur in a wide range of habitat types including grass flats, mud bottoms, along oyster bars, sand bottoms, artificial reefs, under or adjacent to mangrove shorelines, associated with docks, bridges or piers. They can also be found miles up rivers in low salinity conditions. Large sawfish can occasionally be found living in close association with artificial reefs or wrecks, hard bottoms, or mud bottoms.90
The sawfish’s rostrum is clearly its principal evolutionary feature. How and why did such a long, inflexible toothed extension to the snout come about? Until more is known about the sawfish’s biology and behaviour this question will be difficult to answer, but a chance capture of a sawfish off the coast of Queensland may provide a clue. A 3.6-metre green sawfish became entangled in a commercial gillnet and was set free unharmed, after scientists had fitted it with an acoustic tag—the first sawfish to be monitored this way in Australian waters. The tag tracked its movements over a 27-hour period, during which time it moved north along the Gulf of Carpentaria from the Port Musgrave estuary. It began feeding within an hour of its release and spent most of the tag period in water less than two metres deep. This suggests a specific design of the saw to be most effective in very shallow water for feeding, despite the fish itself growing to a hefty five metres and more.
Studies indicate that males have longer saws than females and more teeth, which could mean that the saw has a role in courtship competition and mate selection. Juvenile and small adult sawfishes use their saws to dig for and scuff up bottom prey, while larger sawfishes use them to disable prey, particularly shoaling fish such as mullet and herring. These small fishes invariably inhabit shallow waters, and shallow-water substrate provides a rich concentration of molluscs and bivalves. In other words, the prey bases of both young and adult sawfishes are located in similar waters so there is no real need for them to penetrate deeper ocean waters. Perhaps this explains why sawfishes, as coastal dwellers, have adapted to weakly saline and then freshwater systems.
Skates
Classification
• 26 genera in three families
• More than 260 species
Biology
• Pointed snout, broad pectoral disc, small narrow tail
• Paired, single or no dorsal fins
• Large dermal bucklers
• Oviparous
Habitat
• Mostly deep offshore
Skates belong to the large order Rajiformes, comprising three families divided into some 25 genera, totalling approximately 250 species, many of which are physiologically very similar and almost none of which, despite being plentiful in certain parts of the world, are well understood. Skates are the least-studied elasmobranchs. They are unobtrusive bottom dwellers, usually found in deep waters and, with the exception of some species, are not of great commercial value. Skates do not have the appeal or drama of some of their fusiform cousins but this makes them no less important in the marine biota. In fact, as the most speciose benthic elasmobranchs, they are a critical link in the food chain.
A skate is readily identifiable by its flat, pan shape and pointy snout (which has sensory capabilities), appendage-like pelvic fins, large paired claspers on males and short
, straight, slender tail with miniature dorsal fins near the tip. Most species are small, some no bigger than a sideplate. Skates’ dermal denticles are unique in that they are not uniform, but grow in a wide variety of shapes and areas on the body. The longer denticles are known as thorns: those along the centre of the skate’s back and along the top of its tail are called bucklers; those around the eyes malar spines; and those at the disc edges alar spines. The thorns have a protective function, and it is thought that the alar spines may also help the male to grip the female during copulation. Scientific analysis of very similar dermal covering in different skate species has led to questions about the validity of some species classifications, and ongoing research is being carried out as Project Odontobase at the Museum of Natural History in Paris.91
Skate disc shapes vary and have various formal descriptors: circular, rhomboidal, oval, heart-shaped, quadrangular, sub-circular, wedge-shaped. Disc shape, thorns and snout shape have traditionally been the three principal species identifiers. A skate swims by undulating its pectoral fins, using its tail as a kind of rudder. The pelvic fins move the animal forward with punting-like motions.