The Pacific hagfish (Eptatretus stoutii) is a species of hagfish. It lives in the mesopelagic to abyssal Pacific Ocean, near the ocean floor. It is a jawless fish and has a body plan that resembles early Paleozoic fishes. They can excrete copious amounts of slime in self-defense.
Description The Pacific hagfish has a long, eel-like body, but is not closely related to eels. Maximum body lengths of 63 cm (25 in) have been reported; typical length at maturity is around 42 cm (17 in). It is dark brown, gray or brownish red, often tinted with blue or purple. The belly is lighter and sometimes has larger white patches. It has no true fins, but there is a dorsal fin-fold. The head, as in all agnathans, does not have jaws, and the sucker-like mouth is always open, with 8 barbels around it. There are no visible eyes. Hagfish also only have one nostril, which is located above the mouth.
Starting about one quarter of their body length from the front are 10–14 gill pores. Hagfish have loosely fitted, slimy skins, and are notorious for their slime-production capability. When disturbed, they ooze proteins from slime glands in the skin that respond to water by becoming a slimy outer coating, expanding into a huge mass of slime. This makes the fish very unsavory to predators, and can even be used to clog the gills of predatory fish. Pacific hagfish can create large amounts of slime in just minutes. The slime is notoriously difficult to remove from fishing gear and equipment, and has led to Pacific fishermen bestowing the nickname of 'slime eel' on the species. Slime production in hagfish is also an energetically costly process, and regeneration has been shown to take a long time. The limiting factor in producing new slime is the replacement of threads of slime proteins. Studies have shown that the time to complete regeneration is roughly 24–28 days. Slime can still be ejected before this period is finished, but with less material than when performed with full glands. The eyes of the Pacific hagfish are very basic compared to other vertebrates. Research on Eptatretus stoutii eye tissues has shown that the eyes of hagfishes do not represent a basal form of the vertebrate eye. Instead, hagfish eyes are likely a regressed form of a more complex, basal vertebrate eye that was present in the common ancestor of modern vertebrates. The Pacific hagfish also has a transparent window of skin stretched over the eye, which appear as white spots on the fish. The internal structures of the eye (retina and photoreceptors) are also more complex than previously thought. The increased eye complexity found in Eptatretus stoutii may relate to their ecology, as some species within this genus have been observed exhibiting predatory hunting behavior.
Similar species Eptatretus deani has its first gill pore closer to the head. Lampetra tridentata has visible eyes and 7 gill pores. Myxine limosa has gill pouches with one exterior connection.
Taxonomy The genus Eptatretus means "seven perforations", referring to the seven gill apertures of E. cirrhatus, another hagfish within the genus. The epithet stoutii is in honor of Arthur B. Stout, former surgeon and corresponding secretary of the California Academy of Sciences. The Pacific hagfish confused scientists at first because Carl Linnaeus mistakenly classified the organism as an "intestinal worm".
Distribution and habitat The Pacific hagfish occurs in the Northeast Pacific Ocean from southeast Alaska to Baja California, Mexico. It inhabits fine silt and clay bottoms on the continental shelves and upper slopes at depths from 16–966 metres (52–3,169 ft). The species appears to be abundant within its range. The Pacific hagfish also was discovered off the coast of Costa Rica in 2015, which extends the southern part of their range by roughly 3500 kilometers than was previously thought. It is unclear whether this was a recent species expansion, or if this section of the population has not been sampled.
Biology
Reproduction Hagfish fertilize their eggs externally after the female has laid them. On average females lay about 28 eggs, about 5 mm (0.20 in) in diameter, which are carried around after they have been fertilized. Females will however try to stay in their burrows during this period to ensure the protection of their eggs. Pacific hagfish also have a sex ratio of roughly 1:1. This species also exhibits slight sexual dimorphism, with females growing to slightly smaller sizes than males. E. stoutii is iteroparous, and spawns on an irregular time schedule. These cycles can take up to 2–3 years before new spawning . Pacific hagfish juveniles also exhibit hermaphroditism, and do not become differentiated into separate sexes until after maturity.
Diet and feeding While Pacific hagfish likely take polychaete worms and other invertebrates from the sea floor, they are also known to enter dead, dying or inhibited large fish through the mouth or the anus, and feed on their viscera. The diet of other hagfish species includes shrimps, hermit crabs, cephalopods, brittle stars, bony fishes, sharks, birds and whale flesh, but specific information about the Pacific hagfish diet is lacking. The Pacific hagfish's skin can absorb amino acids. Additionally, hagfish do not actively drink water, as their internal salt concentration must match the surrounding seawater. The feeding apparatus of hagfish is unique among fishes and is primarily composed of soft tissues and a prominent dental plate. This contrasts with bony fish, which have multiple bones to provide rigidity in translating biting force. Despite this, hagfish are still capable of bite forces similar to jawed vertebrates. The dental plate consists of two symmetrical halves that open and close laterally. This use of this plate in practice resembles a grasping and tearing motion and relies on muscles that run posteriorly from the head. Since hagfish do not have bones to attach their muscles to, the collective term for this feeding musculature is called a hydrostat. In addition, juveniles do not have a larval stage and resemble smaller adults. This means that juveniles feed in the same way as adults, but are only capable of taking smaller amounts of food.
… excerpt ends here. Continue reading the full article.
