Deep Sea: the Twilight Zone and Beyond
Named for the dark pattern spiderwebbing over its cream to yellowish-brown back, the Chain Catshark (Scyliorhinus retifer) is among the most attractive members of its family. Known primarily from captures using bottom trawls, this species has also been observed from deep-sea submersibles and maintained in laboratory aquaria, revealing fascinating glimpses of its habitat preferences and details of its reproductive biology.
Just the Facts:
Habitat: Sandy Plains, Deep Sea
Depth: 240-1,800 ft (73-550 m)
Distribution: Western North Atlantic, Caribbean
Geographically, the Chain Catshark is distributed throughout much of the western North Atlantic, from Georges Bank to Nicaragua. In the northern part of its range — from Massachusetts to Cape Hatteras, North Carolina — most specimens are collected from depths of 300 to 500 feet (90 to 150 metres), while in the southern part of its range — from North Carolina southward — most are captured at depths of 750 to 1,475 feet (230 to 450 metres). This pattern suggests that Chain Catsharks prefer a narrow range of temperatures and thus are found at greater depths at lower latitudes. Similarly, the northern extent of the Chain Catshark’s range may be limited by a 48 to 57 degree Fahrenheit (9 to 14 degrees Celsius) temperature regime encountered consistently at the edge of the continental shelf off northern New England.
Direct observation from deep-sea submersibles has revealed that Chain Catsharks spend daylight hours resting on the bottom. While resting, they are usually in contact with some kind of structure — such as lying against the bases of large tube-dwelling anemones or partially concealed beneath piles of various sized boulders. The distinctive reticulated pattern of the Chain Catshark actually helps it blend in among bottom features by breaking up its outline into irregular, unrecognizable shapes — a type of camouflage known as “disruptive coloration”. Intriguingly, Chain Catsharks seem to prefer very different bottom types at different growth stages, with juveniles favoring smooth, sandy or silty substrates while adults prefer rough, hard ones with upright structures. This may be related to different activity focuses, as juveniles may be preoccupied with feeding while adults are more concerned with finding suitable breeding sites.
Reproductive behavior of the Chain Catshark has been observed in intimate detail through the glass walls of laboratory aquaria. Prior to mating, the female is unusually active, swimming with the male almost constantly. During courtship, a Chain Catshark pair often swims in slow, tight circles near the bottom. Suddenly, the male grasps the female, ritualistically biting her gill region, flanks, and tail as often as four times in 30 minutes. Sometimes the female avoids the male’s advances by swimming away; sometimes she gives no indication of avoidance. Intermittently, the courting pair rests side-by-side on the bottom. Eventually, the rough courtship continues, the male biting the tail of the female and holding on determinedly. At first, the female struggles violently, but gradually she becomes submissive — even listless — as the male progressively moves his grasp anteriorly along her flank. The male makes his final bite on the female’s pectoral axil (‘armpit’) and wraps his body around hers. One of his claspers is deflected across the midline of his body and inserted into the female’s cloaca. Locked in this embrace, the mating pair sinks to the bottom, remaining there for 30 seconds or so. Then the male releases his grasp on the female, remaining coiled around his mate until she breaks free and swims off. The male is inactive and unresponsive after copulation, even ignoring other mature females that happen to swim close by. Precopulatory biting has been observed several days after a successful mating, suggesting that Chain Catsharks mate repeatedly.
Female Chain Catsharks are able to store sperm in their shell glands as long as 28 months. As each marble-sized ovum ripens, it is fertilized and packaged in a tough, leathery eggcase as it passes through the shell gland. Each eggcase is more-or-less rectangular in outline, measuring 1 by 3 inches (3 by 7 centimetres), with stringy tendrils extending up to 14 inches (35 centimetres) from each corner. As egg-laying approaches, a pair of these stringy tendrils protrude from the female’s cloaca. She then circles any convenient vertical structure — a deep-sea gorgonian, hydroid, or even a man-made object, such as part of a shipwreck — swimming slowly and nuzzling it repeatedly. The female changes direction often, thereby dragging the trailing tendrils over the structure. Tangling of one or both tendrils stimulates her to swim rapidly around the structure. As the tendril(s) tighten, she pulls vigorously, drawing the eggcase from her oviduct. The female continues circling until the eggcase is secured by the anterior tendrils. After her exertions, the female rests on the bottom nearby, actively pumping oxygen-bearing water over her gills. From several minutes to as long as eight days later, she will use the same or another nearby structure to pull a second eggcase from her oviduct.
Using this ingenious ‘hitching post’ technique, each female Chain Catshark deposits two eggcases at a time, laying one pair every 14 to 18 days. At a water temperature of 53.5 to 55 degrees Fahrenheit (12 to 13 degrees Celsius), development of the eggs takes an average of 256 days. In the mid-Atlantic Bight, Chain Catshark eggcases have been collected from March through August (that is, during all parts of the year it is not too rough to sample the seafloor). More than 300 eggcases have been collected in a single bottom trawl in Baltimore Canyon. A random sample of 78 of these eggcases revealed that the eggs were at all stages of development, from earliest embryo to near-term fetus. This suggests that Chain Catsharks may have no set breeding season, depositing eggcases throughout the year.
When the 4-inch (10-centimetre) long Chain Catshark pups hatch, they are perfect miniatures of their parents, right down to the characteristic webbing pattern on their backs. But, unlike sharks that are born alive, Chain Catshark pups are completely uncoordinated, wriggling madly, bumping into anything and everything — including one another. Despite the chill waters into which they hatched, the pups grow quite quickly — reaching a length of 7 inches (17 centimetres) by the end of their first year and 10 to 12 inches (25 to 30 centimetres) by the end of their second. But, were they not protected by the sheer inaccessibility of their deep-sea nurseries, it is doubtful that enough Chain Catshark pups would live long enough to perpetuate their species.