During the summer between high school and college I interned at the Mystic Aquarium, in Connecticut. Of all the animals I got to work with the one that I fell most in love with was a small species of shark called the Chain Dogfish (Scyliorhinus retifer).
The problem is these deep-sea fish are very difficult to come by since raising a catch up from 1500 feet will kill most of the animals due to the tremendous differences in pressure. Consider that a mere 33 feet of depth is equivalent to one atmosphere. An atmosphere is a unit of measurement for pressure (originally measured as the pressure at sea level at Paris, France's latitude, for whatever that's worth) and is equal to roughly 15 pounds per square inch. Physics isn't my forte, so here are some basics via Wiki
Doing the math, a fish pulled up from 1500 feet is subjected to over 45 atmosphere changes, which comes out to a negative 630 pounds per square inch. If you were to freedive down to this depth it would crush your lungs. When a fish is brought up from this depth the organs, and especially any gaseous elements they contain, expand, causing hemorrhaging and organ failure. So, basically I gave up any hope of ever owning one of these beauties.
Twenty-five years later a fisherman walks into my education center with a bucket full of mermaid's purses. As we saw in an earlier diary, these are the egg cases laid by oviparous elasmobranchs (egglaying sharks, skates and rays). His dredging equipment pulled up a pile of old netting from the depths and a half dozen of these eggs, pictured below, were attached to it.
Recognizing them immediately, we placed them in their own cool-water aquarium and waited. This is the way to obtain deep-sea sharks; as eggs. When a fish is pulled up from the deep ocean they will rarely survive. The pressure difference simply causes too much damage to the internal organs. However these eggs were freshly laid and contained just yolks and very early embryoes, so there were no internal organs present to be harmed.
Like skates, female chain dogs lay their eggs in pairs. These tan-colored cases are carefully placed to ensure that they are both in a proper current flow to aerate the embryo and secure enough not to be washed away or buried by the sand. Although they naturally prefer to lay them on gorgonian corals, man-made objects like lost fishing gear and abandoned lobster pots make ideal nursery grounds for the young. Since suitable places for the eggs to develop on are scarce commodities in the deep sea, this species will often congregate in large numbers to lay the eggs together. For this reason I have no way of knowing whether my sharks are siblings or not. Since a female normally lays the eggs in pairs, and these hatched within days of each other, they are most likely not.
The embyoes take around eight months to hatch. Once they do the young pups, which are about three inches long, are on their own. There is no parental involvement after the eggs are laid. And yes, baby sharks are referred to as "pups".
I'm not sure who decided on the name "chain dogfish", but it is a partial misnomer. The "chain" part is pretty accurate, since the pattern of black lines on the yellowish background is pretty similar to a chain link fence. But these are members of a shark group known as Cat Sharks, so the "dogfish" (which is, in fact, another grouping of sharks) part is way off. Cat Sharks make up a family of animals called Scyliorhinidae and are named for their oval, cat-like eyes. Most are small. This species reaches a length of only eighteen inches. The fins are fairly large for a fish of its size and, to quote some terms from a yet unwritten diary (if that's possible) that hardly anyone ever votes for, it has a pair of dorsal fins that are placed far back on the body and a typical shark-like heterocercal tail, the definition of which you will learn at some point. (Oh, what the hell. A homocercal tail has both upper and lower lobes equal in length. Sharks have a heterocercal tail where the top lobe is longer than the bottom one.)
Living in darkness, chain dogfish have very poor eyesight. The food, mainly squid and fish, is found by smell. The teeth are tiny and most prey must be small enough to be swallowed whole. Like many marine species that live in total or near darkness, the chain dog is capable of bioluminescence. The skin cells are able to capture photons of light energy and reemit them back into the surrounding water. In this fish's case there appears to be no discernable advantage to being able to do this. PZ Meyers has more.
One downside, on a purely selfish level, to keeping these fish is that being nearly blind they do not form the human-pet bond that many other marine animals form in captivity. It amazes people when I explain this, but most species of saltwater fish will learn to recognize their owner, and this doesn't occur with these cat sharks. To be fair, and not to anthropomorphize these fish, this "bond" is essentially learning "that's the guy who feeds me", but nevertheless I have many animals that greet me in the morning and hide when anyone else approaches their tank. Contrast this with most freshwater hobbyist fish that will rush to the front of the aquarium whenever anyone approaches the tank.
Chain dogs are not fished for commercially, and so are one of the species that illicits one of the most infuriating questions I get as a marine educator: "So, what good are they then?" I won't even dignify that with an answer here.
Other diaries in this series can be found here.
