The tripod fish, (also known as the tripod spiderfish), Bathypterois grallator, is a grinner in the family Ipnopidae found at lower latitudes. Because of its unique way of "standing" on the seafloor, B. grallator is one of the best-known benthic fish, and it is one of the few members of its genus to be directly observed and photographed by submersibles.
Characteristics
Morphology
B. grallator is the largest member of its genus, commonly exceeding a standard length of It is mostly silver or white in color, with darker coloration around its head and gills and a stripe down each side of its body, mirroring the position of a lateral line. Most notably, it has greatly extended fin rays on the lower lobe of its caudal fin and both of its pelvic fins. When feeding, it is often seen standing on the seafloor using these extensions, in a distinctive "tripod" configuration that gives the species its name. These extensions have been observed to be flexible when swimming but stiff when resting, and it is suggested that fluids are pumped into these fins when the fish is standing to make them more rigid. These fin modifications contain no sensory cells and they likely have no further purpose other than supporting the fish as it stands.
Compared to other members of its genus, B. grallator's pectoral fins are relatively small, and it holds them slightly backwards from its body rather than forward into the current. However, its pectoral and caudal fin extensions are very long, reaching up to a meter in length (approximately three times the length of its body). however, independent movement of the caudal fin ray has only been observed in B. grallator. B. grallator does not appear to have a preferred habitat, remaining consistently widespread through most oceans in the mid-latitudes and sharing habitats with other tripod fish, such as Bathypterois longifilis which it has been observed standing in close proximity to. Because B. grallator's population is so isolated, it is difficult to determine the extent to which it is at risk from these factors.
Recent observations from deep-sea ROV surveys suggest that B. grallator may exhibit a more dynamic ecological role than previously described. While it is generally considered a passive sit-and-wait predator, individuals have been observed subtly repositioning in response to gentle bottom currents, potentially improving prey capture efficiency. This behavior may reduce competition with other tripodfish occupying similar depth ranges. Additionally, sediment disturbance caused by fin-supported resting postures has been hypothesized to attract small benthic invertebrates, which may incidentally increase feeding opportunities. It has also been proposed that the species’ elongated fin rays could function in mechanosensory detection of current direction, allowing fine-scale adjustments in body orientation while stationary.
Biology
thumb|Two tripod fish exhibiting filter feeding behavior off the coast of [[Eleuthera, at a depth of 1960 meters.]]
Feeding
Like most members of class Ipnopidae, B. grallator has extremely poor eyesight and no special visual adaptations to help it find food in low-light environments. The ends of B. grallator's pectoral fins are receptive to sensory input and can both feel for prey and direct water into the fish's mouth. In early stages in life, it is primarily a filter feeder, however, as it grows larger, it can also exhibit ambush hunting behavior. B. grallator is extremely well suited to sit-and-wait feeding, and spends much of its life sitting as it waits for prey to drift by. It mates by releasing both sperm and eggs into the water, which can be externally fertilized by another B. grallator. In the event one fish cannot find a mate, it can also self-fertilize, which increases the likelihood of reproduction in the sparsely populated abyssal plain.
Parasitism
Tripod fish are susceptible to parasites. Common parasites that use tripod fish as a host are Steringophorus cf. dorsolineatum, Scolex pleuronectis, Hysterothylacium aduncum, Anisakis sp., and Sarcotretes sp.. Tripod fish contract the majority of their parasites through their diet. When tripod fish feed on things such as crustaceans and zooplankton, they contract parasites that reside within the prey. Tripod fish that feed on prey containing larvae are much more likely to contract a parasite. Parasites within tripod fish help determine the location and depth of tripod fish since prey that contain specific parasites are found at specific locations and depths.