Blotchy Swell Shark - Biology and Ecology

Biology and Ecology

Like other members of its genus, when threatened the blotchy swell shark is capable of rapidly inflating its stomach with water or air. This allows the shark to wedge itself inside a rocky crevice, becoming extremely difficult to remove. This species is an opportunistic, highly voracious predator; one recorded female 1 m (3.3 ft) long had 10 fish about 20 cm (7.9 in) long and 15 squid about 15 cm (5.9 in) long in her stomach. Predominantly piscivorous, this species is known to prey upon hagfish and at least 50 species of bony fishes, including fast-swimming types that inhabit open water; significant prey species include the mackerel Scomber japonicus, the sardine Sardinops melanostictus, the filefish Thamnaconus modestus, and the hakeling Physiculus japonicus. Unusually for such a small shark, it also feeds on at least 10 species of cartilaginous fishes, including lantern sharks, catsharks (particularly the cloudy catshark, Scyliorhinus torazame, and its eggs), the electric ray Narke japonica, and skates (including their eggs). It also cannibalizes smaller members of its own species. Cephalopods, mostly the squid Doryteuthis bleekeri and the cuttlefish Sepia spp., are also frequently taken, while crabs, shrimp, and isopods are occasionally consumed. The dietary composition of juveniles varies notably from place to place.

The blotchy swell shark is oviparous, and reproduction proceeds throughout the year with no obvious seasonal cycling. Adult females have a single functional ovary, on the right, and two functional oviducts. The species is thought to be relatively prolific, as the ovary contains numerous ova at various stages of development. Pairs of eggs are laid at a time, one per oviduct. Females have been documented producing eggs even after years without male contact, suggesting that they may be able to store sperm. The purse-shaped egg capsules are relatively large and thick, measuring around 12 cm (4.7 in) long and 7 cm (2.8 in) across. The capsule surface is smooth with lengthwise striations, and opaque cream in color with yellow margins. Long, coiled tendrils extend from the four corners of the capsule. When the embryo is 11 cm (4.3 in) long, the external gills have been lost, the dermal denticles have begun to develop, and light brown saddles are present. The eggs take roughly one year to hatch; newly emerged sharks measure 16–22 cm (6.3–8.7 in) long. From a series of captive rearing experiments, Sho Tanaka reported that hatchling sharks grew in length by up to 0.77 mm (0.03 in) per day. Males and females attain sexual maturity at 86–96 cm (34–38 in) and 92–104 cm (36–41 in) long respectively; the growth rate after maturity is very low. Known parasites of this species include the nematode Porrocaecum cephaloscyllii and the leech Stibarobdella macrothela.