Kelp Forests: Cathedrals in the Sea
Due to its nocturnal habits, the beautiful Pyjama Catshark (Poroderma africanum) is rarely seen in the wild. Endemic to southern African waters, it is most often encountered by shore-based night divers as they slither across beds of Eklonia kelp to reach deeper water. Sporting a hide of iridescent bronze marked with five to seven thick, chocolate-brown longitudinal stripes, this smallish, attractive species is skittish of divers and quite harmless toward them.
Just the Facts:
Habitat: Intertidal, Rocky Reefs, Kelp Forests
Depth: intertidal - 330 ft (100 m)
Distribution: Southern African, Madagascaran
The Pyjama Catshark is fairly abundant throughout its range. Due to this species’ attractive markings, relatively small size, tolerance of being shipped via air freight, and ability to adapt well to captivity, it is a popular exhibit in public aquaria. But little is known about how the Pyjama Catshark makes a living in its natural environment.
We do know that the Pyjama Catshark rests in caves during daylight hours, becoming active at night. This species lays rectangular, yellowish-green egg cases, each measuring about 1 by 4 inches (3 by 10 centimetres), which are anchored to kelp by long, stringy tendrils at each corner. The Pyjama Catshark exploits a broad spectrum of food sources. Known prey of this shark includes teleost fishes — such as anchovies, gurnards, and hakes — as well as peneid shrimps, mantis shrimps, crabs, squids, octopuses, cuttlefishes, bivalves, and polychaete worms. It also opportunistically scavenges fish offal. The Pyjama Catshark’s ability to take advantage of so many different foods undoubtedly contributes to its abundance in near-shore waters.
Near-shore waters are subject to wide fluctuations in salinity due to river run-off and coastal precipitation. Such variability in dissolved salt concentration demands that coastal creatures be able to passively tolerate fluctuations of their internal salt-to-water balance or actively regulate their internal saltiness. Like other sharks, the Pyjama Catshark maintains an internal osmotic pressure slightly higher than that of the surrounding medium — a condition known as “hyper-osmotic” — by concentrating nitrogenous wastes (principally urea and TMAO) in its tissues.
A classic series of experiments conducted on captive Pyjama Catsharks tested this species’ ability to osmoregulate. Two groups of similar-sized test sharks were exposed to the same range of salinities. A “high intake” group received a given quantity of food twice a week; a “low intake” group received the same quantity of food only once a month. Measurements of blood osmolarity (total osmotic pressure), chloride, and urea concentrations indicated that the “high intake” group maintained the slightly hyper-osmotic profile characteristic of sharks, while the “low intake” group’s blood parameters were distinctly hypo-osmotic. When previously “high intake” sharks were starved over a period of one month, they showed the same response to fluctuating salinities as did the “low intake” group. Because urea and TMAO are waste products of protein metabolism, it stands to reason that well-fed sharks would have higher tissue concentrations of these wastes and thus a higher overall osmotic pressure. From these results, the researcher concluded that a Pyjama Catshark’s ability to osmoregulate depends upon its feeding success.
To ensure feeding success, Pyjama Catsharks opportunistically exploit any abundant food source and employ some remarkable predatory strategies. One of the most amazing of these was only recently discovered.
Pyjama Catsharks have a particular gastronomic fondness for squid. Squids are fast and agile jet-propelled molluscs that school, change color and patterns continually, squirt inky camouflage, have keen eyesight, and can quickly learn to distinguish a shark from a non-threatening teleost. This combination of abilities makes squids very difficult for sharks to catch. Some species of squid mate at night and die immediately after spawning, creating rich pickings for scavengers (as described in the Horn Shark profile, above). But the Chokka Squid (Loligo reynauldii) of southern Africa is less obliging, typically mating during daylight hours and jetting away at high speed after spawning. These squids must therefore be hunted actively and Pyjama Catsharks have developed a clever strategy for doing so.
Recently, breeding behavior of the Chokka Squid was filmed by researchers in Tsitsikamma Coastal National Park, South Africa. Here, the peak breeding season of Chokka Squid occurs from October through December, during which these squids congregate to mate and lay their eggs in huge, communal beds. At the researchers’ site, more than 300 pairs of Chokka Squid shared 20 egg beds up to 30 feet (10 metres) apart. As the female squid uses her arms to attach strands of fertilized eggs to the sea floor, her mate hovers nearby to ward off rival males. To take advantage of this predictable feeding opportunity, normally nocturnal Pyjama Catsharks venture forth in broad daylight to the Chokka Squids’ offshore mating grounds and hide motionless in their egg beds. Among the squid egg cases, the researchers noted 20 Pyjama Catsharks lying in wait with their heads hidden and striped bodies protruding.
The attractive stripes that adorn Pyjama Catsharks help to break up their outline into unrecognizable shapes. This camouflage enables these sharks to hide among Chokka Squid egg strands, putting them in a perfect position to ambush the distracted squids from close range. At Tsitsikamma, the researchers actually observed several Pyjama Catsharks lunge suddenly from their hiding places and attempt to capture female squids as they affixed their strands of eggs. Although at least one squid was grasped and severely injured by an ambushing Pyjama Catshark, no successful predation by the sharks was witnessed. Nevertheless, the ‘squid-mugging’ strategy of Pyjama Catsharks seems likely to enable these adaptable little sharks to take advantage of a rich, seasonally available food source.