Hexatrygonidae

Diversity

The family Hexatrygonidae consists of one genus containing only one species. These marine stingrays are unique in that they have six pairs of gill openings and six gill arches, rather than five. They also have a distinctive triangular long snout, confluent with the rest of the disc created by the pectoral fins. They have a functional sting, but no reports of negative effects on humans were found. Little information was found regarding Hexatrygonidae, but stingrays in general are viviparous (see Development and Reproduction), and carnivorous.

Geographic Range

Sixgill stingrays live in the western Pacific from Hong Kong to Japan, and off the coast of South Africa. They are found near Australia over the continental slope.

• Biogeographic Regions
• pacific ocean pacific ocean
  • native native

Habitat

Hexatrygonidae is a marine family. Further information regarding habitat was not found.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

Physical Description

Sixgill stingrays (Hexatrygonidae) are characterized by their six pairs of gill openings, which have gill arches with well-developed filaments. They are unique among rays in that their spiracles (respiratory openings) are closed with an external valve flap, rather than an internal valve. The spiracles are large and far behind the eyes. The snout is long and triangular, measuring over a third of the total disc length. The snout, which is confluent with the rest of the disc, is translucent, depressed (thin), and some investigators suggest that it may be used as an electroreceptive organ. The nostrils are set wide apart, as are the eyes. The mouth is broad and contains many small, blunt teeth. The disc is longer than it is wide, and has smooth skin with no denticles or thorns: “an unusually flabby ray.” Sixgill stingrays have no dorsal fin. They do have small pelvic fins and a long, low caudal fin that reaches the tip of the tail. The tail is short, slender, not whip-like, and bears one or two serrated stinging spines. These rays are brownish pink, with a pale snout and dark caudal fin. Adults can measure up to 2 m long.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry
• venomous venomous

Development

No information was found on development in Hexatrygonidae. They are, however, likely to share the developmental characteristics of related ray families such as Urolophidae . This would mean that they invest much of their reproductive energy into relatively few young over a lifetime, bearing only a few young each year. Most rays, develop inside the mother’s uterus in a system called aplacental uterine viviparity. In this arrangement, developing embryos receive most of their nutriment from a milky, organically rich substance secreted by the mother’s uterine lining. An embryo absorbs this substance, called histotroph, by ingestion, or through its skin or other specialized structures. Although no specific information was found regarding sixgill stingrays, in general rays’ eggs are small and insufficient to support the embryos until they are born. The first stage of development for most rays does happen inside tertiary egg envelopes that enclose each egg along with egg jelly. The embryo eventually absorbs the yolk sac and stalk and the histotroph provides it with nutrition. Development in the uterus usually takes about three months.

Reproduction

Only a few species of elasmobranch (subclass including all sharks and rays) fishes have been observed during courtship and mating. However, all rays have a system that involves internal fertilization, so it can logically be inferred that mating communication between male and female must happen to an extent that allows the male to insert at least one of his two claspers (male reproductive organs that are modifications of the pelvic fins) into the female’s cloaca to deposit sperm. Elasmobranch fishes have relatively complex endocrine (hormonal) systems; based on knowledge of other vertebrates with similar systems, it is likely that females signal to males through chemical or behavioral cues to indicate when their hormonal state is appropriate for mating. In Urolophidae , a family similar in many respects to Hexatrygonidae, researchers found that gland secretions seal the open groove on males’ claspers into a closed tube that protects semen from being diluted before it passes into the female. These secretions coagulate on contact with sea water, help transport sperm into the female, and provide lubrication for clasper insertion.

No information regarding reproduction specifically in Hexatrygonidae was found, but there are some aspects of reproduction common to most rays. Pregnancy usually lasts about three months, generally spanning some period in the spring, summer, and fall. Although pregnancy only lasts a few months, females generally bear young only once a year. Within any given group of rays, individuals appear to go through mating, gestation, and parturition (birth) at the same time as all other females in the group. Rays bear a small number of young at a time, after nourishing the embryos with milky fluid (histotroph) secreted by the uterus (see Development for a description of this system, called aplacental uterine viviparity). In some groups the epithelium, or wall, of the uterus is modified to form trophonemata, elongated villi that extend into the uterine cavity to provide greater surface area for respiratory exchange and histotroph excretion. This advanced system of nourishing young inside the uterus can produce offspring that are relatively large at birth (see Development). According to one investigator, a young ray is rolled up like a cigar during birth, which, along with the lubricating histotroph, facilitates the birth of such proportionally large young. The young ray then unrolls and swims away. Likewise, sting-bearing young are able to pass out of the mother’s body without stinging her because their stings are encased in a pliable sheath that sloughs off after birth.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• viviparous viviparous

No reported evidence of post-birth parental care in Hexatrygonidae was found. After such extended nurturing inside their mothers’ bodies, young rays come into the sea quite able to feed and fend for themselves (see Development and Reproduction).

• Parental Investment
• female parental care female parental care

Lifespan/Longevity

No specific information regarding lifespans in Hexatrygonidae was found, but in general rays, like their relatives the sharks, grow and mature slowly and are long-lived.

Behavior

No information was found regarding the behavior of sixgill stingrays. Some researchers, however, note that the flexible snout tip, which can move both laterally and vertically, may be used to probe for food in sand or mud.

• Key Behaviors
• natatorial natatorial
• motile motile

Communication and Perception

Rays perceive and interact with their environment using sensory channels common to many vertebrates: sight, hearing, smell, taste, and touch. The electrical sensitivity of elasmobranchs seems to exceed that of most other animals. Elasmobranchs are equipped with ampullae of Lorenzini, electroreceptor organs that contain receptor cells and canals leading to pores in the animal’s skin. Sharks and rays can detect the electrical patterns created by nerve conduction, muscular contraction, and even the ionic difference between a body (i.e. of prey) and water. In lab experiments, rays changed their feeding location according to artificially induced changes in the electrical field around them. Other experiments have demonstrated that cartilaginous fishes use electrosensory information not only to locate prey, but also for orientation and navigation based on the electrical fields created by the interaction between water currents and the earth’s magnetic field. Although some rays can produce an electric shock to defend themselves or stun prey, members of the family Hexatrygonidae cannot. They are able, however, to inflict a venomous sting with their tail spine in defense.

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical
• electric electric
• magnetic magnetic

Food Habits

No information was found regarding food habits of sixgill stingrays. Their closest relatives, however, are carnivorous, feeding upon small fishes and invertebrates.

Predation

Ray spines have been found embedded in the mouths of many sharks. The great hammerhead Sphyrna mokarran , in particular, appears to specialize in eating stingrays. It uses its hammer-shaped head to knock a ray to the bottom, and then pins the ray, once again with its head, pivoting around to bite the ray’s disc until the ray succumbs and can be eaten. Sixgill stingrays defend themselves with their venomous sting.

Ecosystem Roles

Sixgill stingrays affect the populations of prey animals such as invertebrates and small fishes. They in turn are eaten by larger fish.

Economic Importance for Humans: Positive

No information was found regarding any human use of sixgill stingrays.

Economic Importance for Humans: Negative

No information was found regarding any negative impacts on humans. Sixgill stingrays do, however, possess one or two venomous spines that could inflict a wound.

• Negative Impacts
• injures humans
  • venomous venomous

Conservation Status

There is currently no known conservation threat to sixgill stingrays.

Additional Links

Encyclopedia of Life

Contributors

Monica Weinheimer (author), Animal Diversity Web.

References

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