There are some new arrivals in the large aquarium at the touch tank on Near Island. They swim with the biggest cod in the tank and are about the same size, but as every little boy is sure to shout out when they see them, they are sharks! This particular species of shark is called a spiny dogfish. “Spiny” for its two venomous spines near the dorsal fin, “dogfish” because they have the peculiar behavior of hunting in packs. Spiny dogfishes hunt for other fish, even other sharks and their “packs” or schools can be made up of large numbers of individuals. Juveniles will hunt in mixed groups, but once the fish are sexually mature, the schools are fish of the same sex and size.
The two individuals at the touch tank were caught in a survey trawl near Kodiak and were carefully taken to their new employment as tourist attraction and educators in the tank. Their kind lives not only in the North Pacific but also in the Atlantic, the Mediterranean and even the Black Sea, and they are great travelers. A spiny dogfish caught and tagged in Newfoundland was caught again in the North Sea, and one individual tagged off the coast of British Columbia was later processed in the fisheries in Japan. In many places of the world spiny dogfish is a commercial food fish, but in our region it is not a big fishery. Besides meat for food, spiny dogfish has been used to make leather from its tough skin and the oils from its liver are a fine lubricant.
Sharks are often considered an evolutionarily primitive fish. However, their adaptations to the challenges of life in the world ocean are highly effective and have kept their kind around for millions of years, much longer than we humans can take credit for. So how are sharks different from other fishes like salmon, herring and rockfish? First, they have no bones; their skeleton is made of cartilage. Secondly, they have no scales; instead their body is covered in rough skin and features denticles to deter a potential predator from closing their mouth over them.
Another cool and attractive feature about these sleek ocean hunters are their big and beautiful eyes. I have not seen it, but I heard that they glow in the dark when a light shines at them, like cat eyes. When the retina reflects the incoming light, the eyes glow in the dark. I imagine, however, that spiny dogfish do not enjoy having a light beam directed at their eyes, because they cannot contract their pupils like most land animals can. It is an adaptation to changing light conditions and when you live in the depths of the ocean for millions of years, too much light is not a concern.
There are some sharks that have to swim constantly to keep water flowing over their gills so that their bodies can get the required oxygen from that water. Spiny dogfish, and their not so distant relatives the rays, have a special system for this purpose: they have water intake openings called spicules. The spicules have a little skin flap and can work like a pump pushing water towards the gills. This enables the fish to breathe while they are lying still at the bottom, waiting for prey or taking a break from their extensive ocean journeys. You can look for the spicules on the sharks in the tank on the side of their heads and a little toward the upper side of their bodies. This location allows them to take in clean water even when the fish settles down on muddy bottom.