EEL SHARK: CHARACTERISTICS, HABITAT, REPRODUCTION, BEHAVIOR - BIOLOGY - 2025

The eel shark (Chlamydoselachus anguineus) is an elasmobranch of the order Hexanchiformes and the family Chlamydoselachidae. This species is also known as a lizard shark due to its morphological characteristics.

This group of sharks is currently the most primitive known. The two representative species of the genus Chlamydoselachus are considered living fossils due to their archaic characteristics within sharks.

Front view of the Eel Shark By © Citron

This shark has an elongated body similar to the body of an eel, which is why they receive that common name. In addition, it has a terminal and non-ventral mouth as in most sharks today. It also has frilled interbranchial septa, which is why they are also known as a frilled shark.

On the Japanese coasts, C. anguineus is particularly abundant between December and July, when bycatch increases significantly. Its distribution is cosmopolitan in the Pacific and Atlantic oceans, but the records cover only some patches of these oceans.

Although very little is known about its biology in general, it is very likely that the species is not very tolerant of exploitation and exploitation. They are very rare sharks to see and have a very low incidence in gillnets and longlines in deep sea fishing activities.

Eel sharks appear to breed throughout the year. This is due to the fact that the temperature and food availability conditions of the areas they occupy do not vary significantly during the year.

Adult females are usually slightly larger than males. The largest female recorded in Japanese seas measured 1.96 meters. The weight of the species varies between 5.5 kg in adult males of 1.5 meters to 15.4 kg in adult females 1.8 meters in length with embryos.

General characteristics

This shark has many characteristics similar to the genus Cladoselachus, an ancient extinct shark. The species was described from specimens from Sagami Bay by Garman in 1884.

This primitive shark is characterized by having the shape of an eel. It has six gill slits and the lower ends of the first slits communicate with each other in the throat.

The dorsal fin is small and lobe-shaped. This originates at the height of the pelvic fins and extends behind the origin of the anal fin, which is larger than the dorsal fin. The pectoral fins are small and paddle-shaped. On the other hand, the caudal fin has a very weak ventral lobe and does not have an subterminal notch.

The largest sizes reported for the eel shark are 1.6 meters for males and 1.96 meters for females.

All teeth are tricuspid in both jaws. The tooth arrangement and morphology of Chlamydoselachus is very similar to Cladoselache, an extinct Devonian shark. Because of this, Chlamydoselachus has the most primitive teeth among sharks.

The spine of these sharks is incompletely segmented, and the vertebrae are poorly calcified. The coloration of this cartilaginous fish can vary from dark brown to brownish gray, the ventral surfaces usually have a lighter coloration.

Habitat and distribution

Like other deep-sea sharks, it has a wide, patchy distribution in the Atlantic and Pacific oceans.

In the eastern Atlantic region, the eel shark has been recorded in the Arctic of Norway, the British Isles, the Iberian Peninsula, the island of Madeira and some areas of North Africa. On the other hand, in the western Atlantic it has been recorded in the eastern United States, Suriname, French Guiana and Guyana.

Distribution of the eel shark BY Chris_huh

Records of this species have also been made on the mid-Atlantic crest north of the Azores islands. In the Pacific Ocean there are records in Australia (New South Wales, Tasmania and Victoria), New Zealand, Japan, Taiwan, Peru, Chile, the Californian coasts of the United States and the Hawaiian Islands.

The eel shark is a bathydemersal and benthopelagic species, which indicates that the marine ecosystems it occupies are very close to the seabed, both on continental shelves and on large underwater slopes.

The depth range recorded for this species is between 120 and 1570 meters. However, it is most frequently reported between 270 and 1280 meters. In the following video you can see its morphology:

Taxonomy

The genus currently has two living species C. anguineus and C. africana and approximately seven species in the fossil record. Some extinct species are Chlamydoselachus bracheri, C. gracilis, C. goliath, C. fiedleri, C. lawleyi, C. thomsoni, and C. tobleri.

The southern African frilled shark, C. africana, is distinct from C. anguineus. C. africana has smaller sizes, maximum 1.2 meters approximately and also has a more restricted distribution in southern Africa in both the Atlantic and Indian Oceans. Females and males have smaller maturation sizes than C. anguineus.

Chlamydoselachus anguineus

Among other externally inconspicuous differences, both species differ internally due to structural differences in the chondrocranium, the vertebral valve counts, and the number of vertebrae. In addition, they also present differences in the radial counts of the pectoral fins.

On the other hand, both species of sharks have different eating habits, with C. africana being a specialized predator in other sharks such as Galeus polli, which usually consume whole. It has a very distensible stomach in which complete specimens of elasmobranchs such as Apristurus manis have been found.

So far there are no known shared localities between both species. The monophyly of the order Hexanchiformes is well supported.

State of conservation

Currently this species is in the category of "least concern" according to the IUCN.

Despite this, and due to its apparent low abundance and a possible intrinsic sensitivity to overexploitation, careful monitoring of the fisheries should be established, monitoring the incidence of capture of this species to guarantee the stability of its populations in the future.

One of the concerns that exist today for the conservation of the eel shark and other species of sharks that inhabit deep waters is the extent of fishing activities in these regions.

Both the geographical expansion of these activities and the increase in the depth and range of influence of these activities, can influence high levels of capture of this species of shark.

A large part of the individuals that are captured in these ways are discarded. A small proportion is used for the production of fish meal or for the consumption of meat.

Fortunately, in many of the places where this species occurs, there are limitations with the depths in which fishing can be carried out and they also have an effective management of fishing techniques (Australia, New Zealand, Europe).

The long gestation period of this shark, which is the longest for any known vertebrate species, should be taken as the main argument in the activities proposed to conserve the species.

Reproduction

It is an ovoviviparous species. This shark appears to segregate by size and reproductive stage. Males mature below 1.1 meters, and it is common for them to mature from sizes between 73 and 97 cm.

On the other hand, the females reach sexual maturity between 1.4 and 1.5 meters in total length. Males have testicular activity throughout the year and females do not have a well-defined reproductive season. Females' ovulation intervals appear to extend for about two weeks.

The size of the litters ranges between 2 and 10 individuals. Females with up to 15 embryos have even been registered. Despite this, the average litter size is six individuals.

Late-stage embryos can receive nutrients from the mother. It has been determined that embryos only develop in the right uterus, the left being non-functional due to the size of the liver.

Length of pregnancy

The gestation period is very long and slow, the embryos only grow 1.4 cm per month. It is estimated that the maturation time of embryos can last between two years and three and a half years. It is possible that this species is able to stop or pause the development of embryos according to the availability of resources and environmental conditions.

The size of the pups at birth varies between 40 and 60 cm in total length and an approximate weight of 380 gr.

Nutrition

Analysis of the stomach contents of 139 specimens caught with shrimp trawls and bottom gillnets in Japan resulted in a highly specialized diet. This shark's teeth are adapted to prevent prey from breaking free from its jaws.

Most prey comprise decapod cephalopods (squid), constituting about 60% of the diet.

In total, they feed on more than ten species of squid, including Onychoteuthis borealijaponica, O. banksi, Sthenoteuthis oualaniensis, and several species of the genera Gonatus, Histioteuthis, Chiroteuthis, and Mastigoteuthis. The most frequent species of squid in the diet is the common squid Todarodes pacificus.

Jaw detail of the Nesnad eel shark

In addition to this, but to a lesser extent, they are capable of consuming varied teleost fish, which constitute around 10% of the diet. The fish they consume have not been identified due to their state of degradation in the stomachs of these sharks.

Other reports indicate that remains of other small sharks of the genus Apristurus were recorded in an eel shark of approximately 1.6 meters, which constitutes the only case of use of this species from another shark.

Power frequency

Many of the specimens examined for the determination of the diet do not present anything in their stomachs because it is inferred that they have low feeding rates due to the scarce availability of resources in the deep waters they occupy.

On the other hand, this species may have a rapid rate of digestion of soft parts, because most of the time only hard parts such as the vertebrae of fish and the beaks of squid are found in the stomach.

Behavior

The presence of relatively slow bathypelagic squid species (Chiroteuthis and Histioteuthis) and epipelagic squid with high swimming capacity (O. borealijaponica, S. oualaniensis and T. pacificus) indicate that the eel shark can use shallow water areas.

However, they have never been observed in their feeding activities. For this reason, the incidence of epipelagic squid species in their diet may be due to the sinking of specimens of these species to deep water after spawning activities.

On the other hand, it is not really known whether these sharks are agile enough to catch relatively fast species of squid in shallow water. Your hunting strategy is probably like a snake-eel-like envestment.

It has been pointed out that this species can regurgitate stomach contents once they are captured, due to the low incidence of stomachs with content in diet analyzes.

It is likely that when captured or threatened to dispose of the contents of their stomachs to flee more quickly, however, this behavior has not been observed.

Other behaviors

Few eel sharks are caught between the months of August and November, when the water temperature, up to a depth of 100 meters, rises above 15 ° C. The increase in temperature seems to prevent these sharks from moving into shallower waters, limiting their observation.

As a result of this, sharks can migrate to deeper areas or to colder latitudes.

A large number of individuals of this species have been recorded with tail fin injuries. They generally lose the end of it as a result of copulation. Sharks of this species bite each other to maintain position during mating.

Many of these injuries are also attributed to negative interactions such as predation by other shark species. The latter has been pointed out by some authors, as the teeth of the eel shark are not capable of inflicting some of the strongest damage seen in their tail fins.

References

1. Bustamante, C., Bennett, MB, & Ovenden, JR (2016). Genetype and phylogenomic position of the frilled shark Chlamydoselachus anguineus inferred from the mitochondrial genome. Mitochondrial DNA Part B, 1 (1), 18-20.
2. Castro, JI (2010). The sharks of north America. Oxford University Press.
3. Ebert, DA, & Compagno, LJ (2009). Chlamydoselachus africana, a new species of frilled shark from southern Africa (Chondrichthyes, Hexanchiformes, Chlamydoselachidae). Zootaxa, 2173 (1), 1-18.
4. Kobayashi, K., Tomonaga, S., & Tanaka, S. (1992). Idetification of a second immunoglobulin in the most primitive shark, the frill shark, Chlamydoselachus anguineus. Developmental & Comparative Immunology, 16 (4), 295-299.
5. Nakaya, K., & Bass, AJ (1978). The frill shark Chlamydoselachus anguineus in New Zealand seas. New Zealand Journal of Marine and Freshwater Research, 12 (4), 397-398.
6. Smart, JJ, Paul, LJ & Fowler, SL 2016. Chlamydoselachus anguineus. The IUCN Red List of Threatened Species 2016: e.T41794A68617785. http://dx.doi.org/10.2305/IUCN.UK.2016-1.RLTS.T41794A68617785.en. Downloaded on 06 December 2019.
7. Tanaka, S., Shiobara, Y., Hioki, S., Abe, H., Nishi, G., Yano, K., & Suzuki, K. (1990). The reproductive biology of the frilled shark, Chlamydoselachus anguineus, from Suruga Bay, Japan. Japanese Journal of Ichthyology, 37 (3), 273-291.
8. Tanaka, K., Shiina, T., Tomita, T., Suzuki, S., Hosomichi, K., Sano, K. & Tanaka, S. (2013). Evolutionary relations of Hexanchiformes deep-sea sharks elucidated by whole mitochondrial genome sequences. BioMed research international, 2013.