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Lay & E. T. Bennett, 1839
The spotted ratfish (Hydrolagus colliei) is a chimaera found in the north-eastern Pacific Ocean. Often seen by divers at night in the Pacific Northwest, this cartilaginous fish gets its characteristic name from a pointed rat-like tail. The ratfish lays leathery egg cases on the bottom of muddy or sandy areas which are often mistaken by divers as something inanimate. While mainly a deep-water species, it occurs at shallower depths in the northern part of its range. The generic name, Hydrolagus, comes from the Greek words ὕδωρ, meaning water, and λαγώς/λαγῶς, meaning hare, and the specific name honors Alexander Collie, who was a ship surgeon and early naturalist. The spotted ratfish is common in much of its range, not typically eaten by humans (the flesh is bland with an unpleasant aftertaste) and is not commercially caught.
The spotted ratfish has a very distinct appearance compared to unrelated fish species. The female is up to 38 inches (97 cm) long, much bigger than the male. These fish have a smooth and scaless skin that is a silvery-bronze color, often with sparkling shades of gold, blue, and green. The speckled white spots along their back contribute to their name. Dark edges outline both the caudal and dorsal fins, whereas, the pectoral fins have a transparent outline. The ratfish’s pectoral fins are large and triangular, and extend straight out from the sides of their bodies like airplane wings. They have a venomous spine located at the leading edge of their dorsal fin, which is used in defense. It does not present a serious danger to humans, but can cause painful wounds and has been known to kill harbor seals that ate spotted ratfish (caused by the spine penetrating vital tissue in the stomach or esophagus after the ratfish was swallowed). The tail of the ratfish constitutes almost half of their overall length and closely resembles a pointed rat-like tail. The body of this fish is supported by cartilage rather than bone. It has a duckbill shaped snout and a rabbit-like face. The mouth is small and contains one pair of forward directed, incisor-shaped teeth in the bottom jaw and two pairs in the top jaw. Unlike sharks who have sharp teeth that are easily replaceable, the spotted ratfish teeth are plate shaped, mineralized and permanent which assist them in grinding their prey. Like many bony fishes but unlike its sister group, the elasmobranchii, the upper jaw of the chimaera is fused with the skull. Although their jaws are soft and mouths are relatively small, they have the largest biting force and jaw leverage found within the holocephali which supports their ability to consume the prey that they do. One of their most mesmerizing features is their large emerald green eyes which are able to reflect light, similar to eyes of a cat.
Distribution and habitat
The spotted ratfish can be found in the north-eastern Pacific Ocean, ranging from Alaska to Baja California with an isolated population in the Gulf of California. They are abundant in much of their range. They be found most commonly off the Pacific Northwest. The range of depths in which this fish is found extends from 0 to 913 m (0–2,995 ft) below sea level, but it is most common between 50 and 400 m (160–1,310 ft). Spotted ratfish typically live closer to the shore in the northern part of the range than in the southern, but it is also known as shallow as 30 m (98 ft) off California. Spotted ratfish tend to move closer to shallow water during the spring and autumn, then to deeper water in summer and winter. For most of the year they prefer temperatures between 7.2 and 8.9 °C (45–48 °F), but seasonally they do move into slightly warmer water. Spotted ratfish can most commonly be found living near the bottom of sand, mud or rocky reefs of the ocean floor. Unlike most of its relatives, which are entirely restricted to deep waters, the spotted ratfish has been held in public aquaria. It has also been bred in such aquaria, where two of the main issues are the requirement of low light and temperature (generally kept at 8–12 °C or 46–54 °F).
The spotted ratfish swims slowly above the seafloor in search of food. Location of food is done by smell. Their usual hunting period is at nighttime, when they move to shallow water to feed. Spotted ratfish are particularly drawn to crunchy foods like crabs and clams. Besides crabs and clams, the spotted ratfish also feeds on shrimp, worms, small fish, small crustaceans, and sea stars. Species known to prey on the spotted ratfish include soupfin sharks, dogfish sharks, Pacific halibut, pinnipeds and pigeon guillemots.
Like some sharks, spotted ratfish are oviparous. Their spawning season peaks during the spring to autumn months. During this time, the female releases up to two fertilized eggs into sand or mud areas of the seabed every 10 to 14 days. The extrusion process can last anywhere from 18 to 30 hours and the actual laying can last another four to six days. The egg sack is leather-like, five inches long, and has a filament connected to it which is used to attach it to the ocean floor when it is let go by the mother. It is not unheard of to see a female ratfish swimming around her newly laid eggs, in hopes of preventing predators from finding them. Development of the egg can take up to a year, which can be dangerous because the eggs are sometimes mistaken for inanimate objects by divers. When the young finally hatch, they are about 5.5 inches (14 cm) in length and grow, reaching 11.8 inches (30 cm) in length their first year.
Male spotted ratfish have multiple secondary sexual characteristics, which include paired pelvic claspers, a single frontal tentaculum, and paired pelvic tentacula. The pelvic claspers are located on the ventral side of the fish. They protrude out from the pelvic fins and are responsible for the movement of sperm to the oviduct of the female. The interior of the pelvic clasper is supported by cartilage and separates into two branches, ultimately ending in a fleshy lobe on the posterior end.
The cephalic clasper (tentaculum), possessed by the male, is a unique club-like organ not found in any other vertebrates. The cephalic clasper is located on the head of the fish, just anterior to the eyes. The tip of the retractable organ is fleshy and lined with numerous small, sharp barbs. In order for the male to stay attached during courtship, the clasper has been observed to clamp down on the pectoral fin of the female. Additional evidence for this type of usage has been found in the form of scars and scratches on the dorsal side of females. The significantly smaller body size of males, which is a sexually dimorphic characteristic, may be a contributing factor to this type of mating behavior.
The ratfish prefers to maintain a safe distance from divers, and are usually not aggressive. However, if they feel their territory has been invaded, the ratfish is able to inflict a mildly toxic wound with their dorsal fin spine. As they swim, the ratfish perform barrel rolls and corkscrew turns, as if they are flying. Ratfish swimming using large pectoral fins, and this has often been termed aquatic flight given the resemblance to a bird.
Albino Puget Sound ratfish
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