Velvet belly lanternshark

The velvet belly's liver accounts for 17% of its body mass, three-quarters of which is oil, making it nearly neutrally buoyant. To deal with the higher concentrations of heavy metals in the deep sea, the velvet belly has T-cells in its bloodstream that can identify and mark toxic compounds for elimination. These T-cells are produced by a lymphomyeloid gland in its esophagus called a "Leydig's organ", which is also found in some other sharks and rays. In its liver, specialized proteins are also capable of detoxifying cadmium, copper, mercury, zinc, and other toxic contaminants. The velvet belly's bioluminescence is thought to function in counter-illumination, which eliminates the shark's silhouette and camouflages it from upward-looking predators. Its bioluminescence may also serve a social function, such as finding mates or co-ordinating groups, as the pattern is species-specific. The velvet belly is an important food of larger fishes such as other sharks; a major predator of this species is the longnosed skate (Dipturus oxyrinchus).

Numerous parasites are known for this species, and both juveniles and adults often carry heavy parasite loads. Known internal parasites include the monogenean Squalonchocotyle spinacis, the tapeworms Aporhynchus norvegicus, Lacistorhynchus tenuis, and Phyllobothrium squali, and the nematodes Anisakis simplex and Hysterothylacium aduncum. Some of these parasites use the velvet belly's prey as intermediate hosts and are acquired via ingestion, while others use the shark itself as an intermediate host. The barnacle Anelasma squalicola, an external parasite, attaches to the shark's dorsal spine socket and penetrates deeply into the muscle, in the process often providing an attachment site for a second (and rarely a third) barnacle. Infestation by this barnacle reduces its host's fecundity by impairing the development of the reproductive organs.

The velvet belly is ovoviviparous, with the embryos hatching inside the uterus and being sustained by a yolk sac. The reproductive cycle may be two to three years long, with ovulation occurring in early autumn, fertilization in the summer (or possibly in the winter if females are capable of storing sperm), and parturition in late winter or early spring. The gestation period is under one year. The litter size is six to 20, with the number of young increasing with female size. At birth, the young measure 12–14 cm (4.7–5.5 in) long. The shark's bioluminescence develops before birth; the yolk sac is fluorescent before any photophores have formed, suggesting the mother transfers luminescent materials to her offspring. The first luminous tissue appears when the embryo is 55 mm (2.2 in) long, and the complete pattern is laid down by the time it is 95 mm (3.7 in) long. At birth, the young shark is already capable of counter-illumination with 80% of its ventral surface luminescent.

The growth rate of the velvet belly is slow, though faster than some other deep-sea sharks, such as the leafscale gulper shark (Centrophorus squamosus) or the shortspine spurdog (Squalus mitsukurii). Males mature sexually at 28–33 cm (11–13 in) long and females at 34–36 cm (13–14 in) long. The average age at maturity is 4.0 years for males and 4.7 years for females, though four-year-old mature individuals of both sexes have been caught in the wild, along with immature females over eight years old. Males and females eight and 11 years old, respectively, have been caught in the wild; the potential lifespan of this species has been estimated at 18 years for males and 22 years for females.