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An egg case or egg capsule, often colloquially called a mermaid's purse, is the casing that surrounds the eggs of
oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s,
skate Skate or Skates may refer to: Fish * Skate (fish), several genera of fish belonging to the family Rajidae * Pygmy skates, several genera of fish belonging to the family Gurgesiellidae * Smooth skates or leg skates, several genera of fish belongi ...
s and
chimaera Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively. At ...
s. Egg cases typically contain one embryo, except for
big skate The big skate (''Beringraja binoculata'') is the largest species of skate (family Rajidae) in the waters off North America. They are found along the Pacific Coast from Alaska to Baja California, typically from the intertidal zone to a depth of ...
and
mottled skate The mottled skate (''Beringraja pulchra'') is a species of skate in the family Rajidae. An inhabitant of shallow coastal waters, it is found in the northwestern Pacific Ocean off Korea, Japan, and China. This species grows to long and has a di ...
egg cases, which contain up to 7 embryos.
Oviparity Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...
is completely absent in the superorder
Squalomorphii Squalomorphii is a superorder of cartilaginous fishes, generally characterized by lacking traits such as an anal fin, nictitating membrane, or suborbital shelves in the cranium. Also called squalea, or squalean sharks. There are about 163 living ...
.


Description

Egg cases are made of
collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whol ...
protein strands, and are often described as feeling rough and leathery. Some egg cases have a fibrous material covering the outside of the egg case, thought to aid in attachment to substrate. Egg cases without a fibrous outer layer can be striated, bumpy, or smooth and glossy. With the exception of bullhead shark eggs, egg cases are typically rectangular in shape with projections, called horns, at each corner. Depending on the species, egg cases may have one or more tendrils.


Development

Shortly after internal fertilization, the fertilized ovum enters the partially formed egg case located in the oviduct. After the ovum enters, the rest of the egg case forms around it. Shortly after the egg case finishes developing, it is deposited outside the body; common locations include kelp forests and rocky seafloors. Egg cases are typically produced in pairs, each with one fertilized embryo inside, with the exception of a few species that produce egg cases with more than one viable embryo. Gestation can take anywhere from a few months to over a year. After a period of development, typically a week or two, small slits open on each side of the egg case to aid water flow. The embryo fans its tail constantly to promote exchange with surrounding water.


Sharks

Oviparity in sharks can be categorized as single or retained. With single oviparity, the egg cases are extruded soon after fertilization. With retained oviparity, eggs are kept within the oviduct for a period of time before depositing outside of the body as an unhatched egg case. It is thought that
viviparity Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...
is the ancestral condition for sharks, and that it evolved through the elongation of retention time of retained oviparity. Oviparous sharks are known to regularly produce unfertilized eggs when kept in captivity without males.


Bullhead sharks

Bullhead shark egg cases are shaped like an
auger Auger may refer to: Engineering * Wood auger, a drill for making holes in wood (or in the ground) ** Auger bit, a drill bit * Auger conveyor, a device for moving material by means of a rotating helical flighting * Auger (platform), the world's f ...
, with two spiral flanges. This allows the egg cases to become wedged in the crevices of rocky sea floors, where the eggs are protected from predators; however, some bullhead sharks deposit their eggs on sponges or seaweed. Hatchlings are considered large for sharks, reaching over 14 cm in length by the time they leave the egg case. Bullhead shark eggs typically hatch after 7 to 12 months, depending on the species. The Female Japanese bullhead shark has been known to deposit their eggs in one location along with other females, called a "nest". The egg case of the
Mexican hornshark The Mexican hornshark (''Heterodontus mexicanus'') is a bullhead shark of the family Heterodontidae. This shark is grey-brown in color, with black spots scattered on the fins and body. It has a cylindrical trunk, conical head, and small spiracles ...
features a tendril and more rigid flanges, suggesting that egg case design of this species is evolving towards anchoring with tendrils and away from wedging into crevices. As a member of the order Heterodontiformes, the whitespotted bullhead sharks is thought to be oviparous, but egg cases have never been observed.


Carpet sharks

The bamboo sharks (Hemiscylliidae) and the zebra shark (Stegostomatidae) lay eggs on the bottom, while the other carpet sharks give live birth. The egg cases are oval and covered with adhesive fibers that serve to secure them to the bottom.


Ground sharks

Some
catshark Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
s (Scyliorhinidae) and the
finback catshark The finback catsharks are a small family, the Proscylliidae, of ground sharks. They can be found in warm seas worldwide and are often the most numerous and common shark in tropical regions. They are generally less than 1 m in length, and are slow ...
s in the genus ''
Proscyllium The genus ''Proscyllium'' is a small genus of finback catsharks in the family Proscylliidae. Species * '' Proscyllium habereri'' Hilgendorf, 1904 (graceful catshark) * '' Proscyllium magnificum'' Last A last is a mechanical form shaped l ...
'' are the only members of their order that lay eggs. The egg cases of catsharks are purse-shaped with long tendrils at the corners that serve to anchor them to structures on the
sea floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
. The size of egg cases vary; those of the
small-spotted catshark The small-spotted catshark (''Scyliorhinus canicula''), also known as the sandy dogfish, lesser-spotted dogfish, rough-hound or morgay (in Scotland and Cornwall), is a catshark of the family Scyliorhinidae. It is found on the continental shelves ...
or lesser spotted dogfish, '' Scyliorhinus canicula'', are around long, while those of the
greater spotted dogfish The nursehound (''Scyliorhinus stellaris''), also known as the large-spotted dogfish, greater spotted dogfish or bull huss, is a species of catshark, belonging to the family Scyliorhinidae, found in the northeastern Atlantic Ocean. It is general ...
, ''S. stellaris'', are around . That excludes the four long
tendril In botany, a tendril is a specialized stem, leaf or petiole with a threadlike shape used by climbing plants for support and attachment, as well as cellular invasion by parasitic plants such as '' Cuscuta''. There are many plants that have tend ...
s found in each corner, which assist in anchorage. Egg cases from rays vary in that they have points rather than tendrils. The colours and shapes of egg cases also vary greatly from
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
to species.


Skates

The
skate Skate or Skates may refer to: Fish * Skate (fish), several genera of fish belonging to the family Rajidae * Pygmy skates, several genera of fish belonging to the family Gurgesiellidae * Smooth skates or leg skates, several genera of fish belongi ...
s (Rajidae, Arhynchobatidae, Anacanthobatidae) are the only rays that are
oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...
. Females lay egg cases onto the sea floor after fertilization occurs
in utero ''In Utero'' is the third and final studio album by American rock band Nirvana. It was released on September 21, 1993, by DGC Records. After breaking into the mainstream with their second album, ''Nevermind'' (1991), Nirvana hired Steve Albin ...
. While in utero, a protected case forms around the
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
which is called the egg case.Ebert, David A., Davis, Chante D. (2007). "Descriptions of skate egg cases (Chondrichthyes: Rajiformes: Rajoidei) from the eastern North Pacific". ''Zootaxa'' 1393: 1-18. Studies have been done where egg cases were removed from gravid females to ensure proper identification in regard to skate species. Egg cases have distinguishable characteristic traits that are unique to that species, thus making it a great tool for identifying a skate. The two most distinguishable features on the egg case are the keel and the absence or presence of a fibrous covering. A keel runs laterally along both sides of the outer edge of the egg case; it is a flexible structure. Keels will also run the length of the horns on some skate species. Some egg cases have broad keels (greater than 10% of the maximum egg case width) while others have narrow keels (less than 10% of the maximum egg case width). Many egg cases are covered with a layer of fiber; some will have a fine layer while others have a thick layer.


Big skate

Big skate egg cases are larger than most other skate egg cases; typically ranging from 210 to 280 mm in length and 110 to 180 mm in width. Big skates egg cases are approximately 15% of the overall length of the female skate. The egg case is very smooth and lacks external fibrous material. This egg case can be easily identified from all others in that it is the only one to have a steep ridge; giving the case a convex shape. The keel on the egg case is considered very broad; representing 30–33% of the width of the egg case. Big skates are one of only two skates known to have multiple embryos inside an egg case; up to 7 embryos have been found inside a single case. But most big skate egg cases contain 3–4 embryos.Ebert, D.A., Smith, W.D., and Cailliet, G.M. (2008). "Reproductive biology of two commercially exploited skates, ''Raja binoculata'' and ''R. rhina'', in the western Gulf of Alaska". ''Fisheries Research'', 94:48-57.


Longnose skate

The
longnose skate Longnose skate can refer to several species of rays (fish): * '' Beringraja rhina'', from the northeast Pacific * '' Dentiraja confusa'', found off southeastern Australia * '' Dipturus oxyrinchus'', from the northeast Atlantic and Mediterranean ...
, ''Raja rhina'', is considered a larger skate species; reaching a maximum size range of 145 cm total length. Although their egg cases are smaller than that of the big skate, their cases are also considered large; ranging 93–102 mm in length. Egg cases contain a single embryo. Longnose skate egg cases found in the field are brown in color. The external side is covered with a fibrous material, which is thicker on the top side and thinner on the bottom side of the case. The case is smooth underneath the fibrous material.


Chimaeras

The egg cases of
chimaera Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively. At ...
s are spindle- or bottle-shaped with fins on the sides. They are laid on the bottom of the sea floor. Chimaeras (subclass
Holocephali Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primitiv ...
), some
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s, and skates are among the 43% of known Chondrichthian species to exhibit
oviparity Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...
.Fischer, Jan, Martin Licht, Jürgen Kriwet, Jörg W. Schneider, Michael Buchwitz, and Peter Bartsch. "Egg capsule morphology provides new information about the interrelationships of chondrichthyan fishes." Journal of Systematic Palaeontology 12.3 (2013): 389-99. However, there are some key morphological differences that are specific to chimaeras. The holocephalan egg capsule, or egg case, has a bulbous center flanked laterally by flattened collagen tissue. The flattened collagen tissue joins on the anterior end of the egg capsule to form a tail. Sharp projections located on the anterior and posterior end of the egg case serve to better secure the egg case in between rocks, as well as protection against potential predators.


Threats

Predation on egg cases is thought to be a major source of mortality for developing oviparous sharks, skates and chimaeras. In general, predation is the leading cause of mortality for marine fish eggs, due to their abundance and high nutritional value. Parental care ends when the egg case is released from the body, so the embryo relies on its tough, leathery exterior as its only source of protection. Some gastropods are known to feed on egg cases by boring into the exterior. Sharks are also common predators of egg cases.


References


External links

*http://www.glaucus.org.uk/Mermaid.htm {{DEFAULTSORT:Egg Case (Chondrichthyes) Cartilaginous fish Fish reproduction Ichthyology Articles containing video clips