The second dorsal fin is about two-thirds the size of the first and is located behind the pelvic fins. There are sharp dorsal fin spines at the anterior margins of the dorsal fins with the first about half as long and the second nearly as long as the anterior margins of their respective fins. The pectoral fins form nearly perfect equilateral triangles with rounded rear tips and slightly concave rear margins.
The pelvic fins are closer to the second dorsal fin than the first dorsal. There are low lateral keels located on the caudal peduncle. There is no notch on the upper caudal lobe and the lower caudal lobe is not well-developed. There is no anal fin on the spiny dogfish. Coloration The dorsal surface of the spiny dogfish is slate-colored and may have a brownish cast.
There is a lateral row of small white spots along each side from above the pectoral fins to above the pelvic fins. These spots of conspicuous on immature fish, fading with growth until they disappear entirely from some individuals. The edges of the first and second dorsal fins and the caudal fin appear dusky at birth but quickly fade.
The ventral surface of the spiny dogfish ranges from pale gray to pure white. Dentition The upper and lower teeth are small and similar in shape with oblique points bent toward the outer corners of the mouth.
The cusps are deeply notched outward with a single sharp point. These form a nearly continuous cutting edge from one corner of the mouth to the other. There are 28 upper teeth and lower teeth in the jaws of the spiny dogfish.
Denticles Dermal denticles of the spiny dogfish are small and low with three cusps. The central ridge is prominent and the lateral extensions are wing-like in appearance. Size, Age, and Growth The average size of the spiny dogfish is inches cm with adult males ranging from inches cm and adult females from inches cm in length.
The maximum length of males is 39 inches cm and females 49 inches cm. Mature females reach weights of 7. The all-tackle game fish record is Females reach maturity at 12 years of age and from They live up to years of age. Food Habits Dogfish have earned a bad reputation among fishermen for their voracious appetites. They are known to drive off commercially caught fish including mackerel and herring, while consuming large numbers of them.
Spiny dogfish have been observed biting through nets to get at fishes, releasing many of them in the process. Schooling pelagic fishes make up the majority of the diet of the spiny dogfish. These include herring, menhaden, capelin, sand lance, and mackerel. Other consumed species include wolffish and flatfishes, as well as squid, jellyfish, shrimps, crabs, octopus, and sea cucumbers. It is believed that spiny dogfish rarely feed during the winter months when they stay in deeper waters based upon their very thin appearance in early spring in coastal waters.
Reproduction Mating typically occurs in offshore waters with fertilization occurring internally. This is followed by ovoviviparous development. If these factors are integrated - late sexual maturity with small litter sizes and a long pregnancy time - it is by no means surprising that excessive fishing has a destructive influence on spiny dogfish populations.
Their "slow" reproduction rate does not allow easy restoral of its unstable populations. Their numbers are negatively influenced both by fishing and their relatively small size, which makes them easy prey for other sharks, sea lions and swordfish. Spiny dogfish often live in swarms consisting of thousands of animals. These swarms are formed in order to hunt together, but also as protection against enemies. Certain swarms consist of pregnant females in search of their nesting grounds, which include the San Francisco Bay.
Females and males usually live in separate swarms. Mixed groups are rare. Males prefer flatter regions while females only return to flat coastal areas to bear their young. Spiny dogfish undertake long migrations, influenced partly by the availability of food and partly by water temperatures, but their migration path has not yet been studied sufficiently.
One tagged animal was found to have migrated 6, km, but the reasons behind this behavior remain purely speculative. Spiny dogfish are usually quite harmless, but a poison is secreted at the base of the spines which may lead to complications in humans suffering from allergies.
Cheryl D. Wilga and Philip J. Motta Conservation and variation in the feeding mechanism of the spiny dogfish Squalus acanthias. Biol 9 : Compagno, L. Sharks of the world. The teeth of the spiny dogfish are small with sharp points that bend outward. The teeth are organized into several rows and are used for grinding, rather than tearing. The first dorsal fin of the spiny dogfish is somewhat larger than the second dorsal fin.
Two large, sharp, mildly poisonous dorsal spines are located in front of each dorsal fin. The spiny dogfish uses its spines defensively by curling up its body and striking at an enemy. Spiny dogfish skin is rough and covered by a tooth-like, scaled surface called dermal denticles.
The skin feels smooth when rubbed with the grain of the denticles, but feels rough when rubbed against the grain. Spiny dogfish are the most abundant and most commonly seen of all sharks in Narragansett Bay.
0コメント