Positive buoyancy is not exhibited by non air breathing aquatic animals, according to a new study. Oil filled livers instead of gas filled swim bladders are relied by sharks instance to increase their buoyancy, but are regarded as either negatively or neutrally buoyant.
Deep-sea sharks have large, oil-filled livers and are neutrally buoyant in their natural habitat, but this has not been confirmed.
Researchers used accelerometer-magnetometer data loggers to empirically determine the buoyancy status of two species of deep sea sharks (bluntnose sixgill sharks, Hexanchus griseus and a prickly shark, Echinorhinus cookei) in their natural habitat thereby measuring their swimming performance. Both species of deep sea sharks showed similar diel vertical migrations; they swam at depths of 200-300 at night and deeper than 500 m during the day. Ambient water temperature was around 15 degree Centigrade at 200-300 m but below 7 degree centigrade at depths greater than 500 m.
Deep-sea sharks showed higher swimming efforts in descent than ascent to maintain a given swimming speed during the vertical movements. This assisted them to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks were in fact positively buoyant in their natural habitats.
This was especially beneficial for hunting in stealth, but could also be used in evening upward migrations where the muscle temperatures were at their lowest, and swimming was the slowest, after spending a day in deep and cold water. Positive buoyancy could be rampant in fish conducting vertical migration everyday in deep sea habitats but had not been independently corroborated.
Deep-sea sharks were captured off Kane’ohe bay (Oahu, Hawaii, 21°31’N, 157°46’W) by using demersal fishing lines baited with fish scraps. Lines were set in the evening in depths of 300 m and were retrieved the following morning. Captured sharks were brought beside a 7 m boat, where they were tail-roped and inverted to induce tonic immobility.