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[[File:Shark egg case video, Bristol Aquarium, Nov 2014.ogv|thumb|Baby active inside egg case.]]
[[File:Shark egg case video, Bristol Aquarium, Nov 2014.ogv|thumb|Baby active inside egg case.]]


An '''egg case''' or '''egg capsule''', colloquially known as a '''mermaid's purse''' or '''devil's purse''', is a casing that surrounds the [[fertilization|fertilized]] [[egg (biology)|egg]]s of some [[shark]]s, [[Skate (fish)|skate]]s, and [[chimaera]]s. They are made of [[collagen]] protein strands.<ref>{{cite journal|last=Evans|first=David H.|title=The egg case of the oviparous elasmobranch, Raja Erinacea, does osmoregulate|journal=Journal of Experimental Biology|volume=92|date=June 1981|url=http://jeb.biologists.org/content/jexbio/92/1/337.full.pdf|format=PDF}}</ref>
An '''egg case''' or '''egg capsule''' is a casing that surrounds the [[egg (biology)|egg]]s of oviparous [[shark]]s, [[Skate (fish)|skate]]s, and [[chimaera]]s. They are made of [[collagen]] protein strands.<ref>{{cite journal|last=Evans|first=David H.|title=The egg case of the oviparous elasmobranch, Raja Erinacea, does osmoregulate|journal=Journal of Experimental Biology|volume=92|date=June 1981|url=http://jeb.biologists.org/content/jexbio/92/1/337.full.pdf|format=PDF}}</ref>

Because they are lightweight, they are commonly washed up by the sea, often found at the [[strandline]], the farthest point of the high tide. The egg cases that wash up on beaches are usually empty, the young [[fish]] having already hatched out. Furthermore, egg cases are often found in commercial fishing gears like nets and [[fishing pot|pot]]s, and on the sea floor during surveys using an [[Remotely operated underwater vehicle|ROV]].


==Sharks==
==Sharks==
[[File:Heterodontus portusjacksoni egg.jpg|thumb|upright=0.8|Egg case of a [[Port Jackson shark]]]]
[[File:Heterodontus portusjacksoni egg.jpg|thumb|upright=0.8|Egg case of a [[Port Jackson shark]]]]
Oviparity in sharks can be categorized as single or retained.<ref name=":1">{{Cite book|title=Biology of Sharks: Second Edition|last=Carrier|first=J.C|last2=Musick|first2=J.A.|last3=Heithaus|first3=M.R.|publisher=Taylor & Francis Group|year=2012|isbn=|location=|pages=296}}</ref> With single oviparity, the egg cases are extruded soon after fertilization.<ref name=":1" /> 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.<ref name=":1" /> It is thought that oviparity is the ancestral condition for sharks, and that it evolved through the elongation of retention time of retained oviparity.<ref name="1st ed">{{Cite book|title=Biology of Sharks: First Edition|last=Carrier|first=J.C|last2=Musick|first2=J.A.|last3=Heithaus|first3=M.R.|publisher=Taylor & Francis Group|year=2004|isbn=|location=|pages=270}}</ref>
Among the 9 extant [[shark#Taxonomy|orders of sharks]], egg-laying is found only in the following groups:


Oviparous sharks are known to regularly produce unfertilized eggs when kept in captivity without males.<ref>{{Cite web|url=http://www.aquariumofpacific.org/blogs/comments/egg_id|title=Egg Identification|last=|first=|date=2008|website=Aquarium of the Pacific}}</ref> Oviparity is completely absent in the superorder [[Squalomorphii]].<ref name=":1" />
===Bullhead sharks===
===Bullhead sharks===
Bullhead shark egg cases are shaped like an [[Auger (drill)|auger]], with two spiral flanges. This allow the egg cases to become wedged in the crevices of rocky sea floors, where it is protected from predators; however, some bullhead sharks deposit their eggs on sponges or seaweed.<ref>{{Cite web|url=https://www.floridamuseum.ufl.edu/fish/discover/species-profiles/heterodontus-francisci/|title=Heterodontus francisci|last=Buch|first=Robert|date=|website=Florida Museum|access-date=}}</ref><ref name=":2">{{Cite journal|last=Compagno|first=Leonard|date=2002|title=Sharks of the World|url=|journal=FAO Species Catalogue for Fishery Purposes|volume=Vol 2|pages=31-50|via=}}</ref> Hatchlings are considered large for sharks, reaching over 14cm in length by the time they leave the egg case.<ref name=":2" /> Bullhead shark eggs typically hatch after 7 to 12 months, depending on the species.<ref name=":2" /> Female [[Japanese bullhead shark]]<nowiki/>s have been known to deposit their eggs in one location along with other females, called a "nest".<ref name=":2" /> The egg case of the [[Mexican hornshark]] 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. <ref name=":2" /> As a member of the order Heterodontiformes, the [[whitespotted bullhead shark]] id thought to be oviparous, but egg cases have never been observed.
The egg cases of [[bullhead shark]]s (Heterodontidae) have spiral flanges that allow them to be wedged inside crevices or into sand.


[[File:Chiloscyllium punctatum egg sunshine international.jpg|thumb|upright=0.8|Egg case of a [[brownbanded bamboo shark]]]]
[[File:Chiloscyllium punctatum egg sunshine international.jpg|thumb|upright=0.8|Egg case of a [[brownbanded bamboo shark]]]]

Revision as of 07:01, 19 February 2018

Egg case of a skate
Baby active inside egg case.

An egg case or egg capsule is a casing that surrounds the eggs of oviparous sharks, skates, and chimaeras. They are made of collagen protein strands.[1]

Sharks

Egg case of a Port Jackson shark

Oviparity in sharks can be categorized as single or retained.[2] With single oviparity, the egg cases are extruded soon after fertilization.[2] 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.[2] It is thought that oviparity is the ancestral condition for sharks, and that it evolved through the elongation of retention time of retained oviparity.[3]

Oviparous sharks are known to regularly produce unfertilized eggs when kept in captivity without males.[4] Oviparity is completely absent in the superorder Squalomorphii.[2]

Bullhead sharks

Bullhead shark egg cases are shaped like an auger, with two spiral flanges. This allow the egg cases to become wedged in the crevices of rocky sea floors, where it is protected from predators; however, some bullhead sharks deposit their eggs on sponges or seaweed.[5][6] Hatchlings are considered large for sharks, reaching over 14cm in length by the time they leave the egg case.[6] Bullhead shark eggs typically hatch after 7 to 12 months, depending on the species.[6] Female Japanese bullhead sharks have been known to deposit their eggs in one location along with other females, called a "nest".[6] The egg case of the Mexican hornshark 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. [6] As a member of the order Heterodontiformes, the whitespotted bullhead shark id thought to be oviparous, but egg cases have never been observed.

Egg case of a brownbanded bamboo shark

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

Egg cases of a catshark

Some catsharks (Scyliorhinidae) and the finback catsharks in the genus Proscyllium 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 size of egg cases vary; those of the small-spotted catshark or lesser spotted dogfish, Scyliorhinus canicula, are around 5 centimetres (2 in) long, while those of the greater spotted dogfish, S. stellaris, are around 10 centimetres (4 in). That excludes the four long tendrils 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 to species.

Skates

Egg case of a big skate

The skates (Rajidae, Arhynchobatidae, Anacanthobatidae) are the only rays that are oviparous.[7] Females lay egg cases onto the sea floor after fertilization occurs in utero. While in utero, a protected case forms around the embryo which is called the egg case.[8][9] Studies have been done where egg cases were removed from gravid females to ensure proper identification in regard to skate species.[8] 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).[8] Many egg cases are covered with a layer of fiber; some will have a fine layer while others have a thick layer.

Big skates

The big skate is known to be the largest skate in the eastern North Pacific Ocean.[10][11] Their maximum size has been noted as 244 cm total length (TL),[10] but the largest confirmed size is 203.9 cm TL.[12]

  • The fresh egg cases of the big skate, like the one shown below, were taken from dead gravid females that were caught in commercial groundfish fisheries.
  • Their 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.[8][9]
  • The egg case is very smooth and lacks external fibrous material.[8] 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.[8]
  • The keel on the egg case is considered very broad; representing 30-33% of the width of the egg case.[8]
  • Egg cases, in utero, are found in pairs.
  • 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.[12]
  • Big skate have a 70 mm difference between the smallest and largest egg cases;[8] this is uncommon in most skate species. Research is currently being conducted on whether or not a relationship exists between egg case length and the overall size of the female big skate. This may answer questions like "why does a larger disparity exist between the minimum and maximum sizes of egg case length of the big skate versus the other skate species"?
  • Big skates egg cases are approximately 15% of the overall length of the female skate.[8]
  • The color of the egg case is olive green when fresh, and it turns to a dark brown as it ages. Dried egg cases often turn black and wrinkle up.

Other skate species

The longnose skate, Raja rhina, is considered a larger skate species; reaching a maximum size range of 145 cm TL.[12] Although their egg cases are smaller than that of the big skate, their cases are considered large too; ranging 93–102 mm in length.[8]

  • Egg cases contain a single embryo.
  • Longnose skate egg case's smallest to largest size in length varies by only 9 mm;[8] this is common for most skate species which have a 1–16 mm spread in egg case length.
  • 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.[8]

The roughtail skate, Bathyraja trachura, is a medium-size skate, with a maximum size of 91 cm TL.

  • Egg cases contain a single embryo.
  • Roughtail skate egg cases are of medium size, previously recorded as having a maximum size up to 78 mm in length.[8]
  • This egg case, shown on the measuring strip depicting centimeter measurements, exceeds the maximum recorded size and measures 85 mm in length. This egg case was found in the field; caught in commercial fishing gear.
  • The Roughtail skate egg case's smallest to largest size in length varies by 16 mm in length.[8]
  • The egg cases are smooth and lack external fibrous material.[8] The keels are very wide, and a great characteristic for identifying it to this species.[8]

Applications

Relevance of knowing skate egg case identification features and size:

  • Knowledge of egg case morphology can be used to identify which skate species it belongs to, as well as aid in finding new skate species.[8]
  • Researchers have been able to identify skate species, not previously known to reside in a geological area, by finding egg cases not belonging to all verified known species living within that region.[8]
  • The presence of egg cases can assist researchers in gaining knowledge about skate distribution,[8] behavior of the species and their preferred habitat, such as nursery areas.
  • These identification features aid in narrowing down the choice of the likely possible species, and in many cases makes it easier to accurately identify a species.[8]

Chimaeras

Egg case of a Cape elephantfish

The egg cases of chimaeras are spindle- or bottle-shaped with fins on the sides. They are laid on the bottom of the sea floor. Chimaeras (subclass Holocephali), some sharks, and skates are among the 43% of known Chondrichthian species to exhibit oviparity.[13] However, there are some key morphological differences that are specific to chimaeras. The spotted ratfish chimaera is also known as Hydrolagus colliei. There are currently 11 holocephalan fossil species that were found throughout the United States, Canada, Germany, and Russia.[13] 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.[13] 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.[14]

References

  1. ^ Evans, David H. (June 1981). "The egg case of the oviparous elasmobranch, Raja Erinacea, does osmoregulate" (PDF). Journal of Experimental Biology. 92.
  2. ^ a b c d Carrier, J.C; Musick, J.A.; Heithaus, M.R. (2012). Biology of Sharks: Second Edition. Taylor & Francis Group. p. 296.
  3. ^ Carrier, J.C; Musick, J.A.; Heithaus, M.R. (2004). Biology of Sharks: First Edition. Taylor & Francis Group. p. 270.
  4. ^ "Egg Identification". Aquarium of the Pacific. 2008.
  5. ^ Buch, Robert. "Heterodontus francisci". Florida Museum.
  6. ^ a b c d e Compagno, Leonard (2002). "Sharks of the World". FAO Species Catalogue for Fishery Purposes. Vol 2: 31–50. {{cite journal}}: |volume= has extra text (help)
  7. ^ "Most Commonly Asked Questions". Florida Museum of Natural History.
  8. ^ a b c d e f g h i j k l m n o p q r s t Ebert, David A., Davis, Chante D. (2007). "Descriptions of skate egg cases (Chondrichthyes: Rajiformes: Rajoidei) from the eastern North Pacific". Zootaxa 1393: 1-18.
  9. ^ a b "Raja binoculata (Big Skate, Big Skate)". Zipcodezoo.com. Archived from the original on 8 September 2008. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  10. ^ a b "FishBaseSearch Common Name Big skate and Longnose skate". FishBase: A Global Information System on Fishes. 2001. Retrieved 9 November 2009.
  11. ^ "Raja binoculata" (PDF). Discoverylife.org. Retrieved 7 October 2009.
  12. ^ a b c 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. doi:10.1016/j.fishres.2008.06.016
  13. ^ a b c 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.
  14. ^ Klimley, A. Peter (2013). The Biology of Sharks and Rays. Chicago, Illinois: The University of Chicago Press. pp. 286–288. ISBN 0226442497.
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