Egg case (Chondrichthyes): Difference between revisions

{{Use dmy dates|date=MaySeptember 20132022}}

An '''egg case''' or '''egg capsule''', often colloquially called a '''mermaid's purse''', is the casing that surrounds the [[egg (biology)|egg]]s of [[Oviparity|oviparous]] [[Chondrichthyes|chondrichthyans]]. Living chondricthyans that produce egg cases include some [[shark]]s, [[Skate (fish)|skate]]s, and [[chimaera]]s. Egg cases typically contain one embryo, except for [[big skate]] and [[mottled skate]] mammaegg cases, which contain up to 7 embryos.<ref name="EbertEbert2" /> [[Oviparity]] is completely absent in the superorder [[Squalomorphii]].<ref name=":1" /><ref name="1st ed" /> Egg cases are also thought to have been produced by some extinct chondricthyan groups, such as [[hybodonts]] and [[Xenacanthida|xenacanths]].

Egg cases 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|doi=10.1242/jeb.92.1.337 |url=http://jeb.biologists.org/content/jexbio/92/1/337.full.pdf|format=PDF}}</ref> and are often described as feeling rough and leathery.<ref name=":2" /> Some egg cases have a fibrous material covering the outside of the egg case, thought to aid in attachment to substrate.<ref name="EbertEbert2" /><ref name=":3">{{Cite journal|lastlast1=Treloar|firstfirst1=M.A.|last2=Laurenson|first2=L.J.B.|last3=Stevens|first3=J.D.|date=2006|title=Description of Rajid egg cases from southeastern Australian waters|url=|journal=Zootaxa|volume=1231 |pagespage=53 |viadoi=10.11646/zootaxa.1231.1.3 }}</ref> Egg cases without a fibrous outer layer can be striated, bumpy, or smooth and glossy.<ref name="EbertEbert2" /><ref name=":3" /> With the exception of [[bullhead shark]] eggs, egg cases are typically rectangular in shape with projections, called horns, at each corner.<ref name="EbertEbert2" /><ref name=":3" /> Depending on the species, egg cases may have one or more tendrils.<ref name=":2" />

Shortly after internal fertilization, the fertilized ovum enters the partially formed egg case located in the oviduct.<ref name=":3" /> After the ovum enters, the rest of the egg case forms around it.<ref name=":3" /> 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.<ref name="EbertEbert2" /><ref name=":3" />

Oviparity in sharks can be categorized as single or retained.<ref name=":1">{{Cite book|title=Biology of Sharks: Second Edition|lastlast1=Carrier|firstfirst1=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 [[viviparity]] 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|lastlast1=Carrier|firstfirst1=J.C|last2=Musick|first2=J.A.|last3=Heithaus|first3=M.R.|publisher=Taylor & Francis Group|year=2004|isbn=|location=|pages=270}}</ref>

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>

Bullhead shark egg cases are shaped like an [[Auger (drill)|auger]], with two spiral flanges. This allowallows the egg cases to become wedged in the crevices of rocky sea floors, where itthe iseggs are 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=31–50}}</ref> Hatchlings are considered large for sharks, reaching over 14&nbsp;cm 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" /> FemaleThe female [[Japanese bullhead shark]]<nowiki/>s havehas 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|whitespotted bullhead sharks]]<nowiki/>s is thought to be oviparous, but egg cases have never been observed.<ref name=":2" />

The [[Skate (fish)|skate]]s (Rajidae, Arhynchobatidae, Anacanthobatidae) are the only [[Batoidea|rays]] that are [[oviparous]].<ref name="Florida Museum of Natural History">{{cite web|url=http://www.flmnh.ufl.edu/fish/education/questions/Biology.html#purse|publisher=Florida Museum of Natural History|title=Most Commonly Asked Questions}}</ref> Females lay egg cases onto the sea floor after fertilization occurs [[Uterus|in utero]]. While in utero, a protected case forms around the [[embryo]] which is called the egg case.<ref name="EbertEbert2">Ebert, David A., Davis, Chante D. (2007). "Descriptions of skate egg cases (Chondrichthyes: Rajiformes: Rajoidei) from the eastern North Pacific". ''Zootaxa'' 1393: 1-18.</ref><ref name="Zipcodezoo">{{cite web|website=Zipcodezoo.com|url=http://zipcodezoo.com/Animals/R/Raja_binoculata/|title=''Raja binoculata'' (Big Skate, Big Skate)|archiveurlarchive-url=https://web.archive.org/web/20080908085320/http://zipcodezoo.com/Animals/R/Raja_binoculata|archivedatearchive-date=8 September 2008 |deadurlurl-status=yesdead}}</ref> Studies have been done where egg cases were removed from gravid females to ensure proper identification in regard to skate species.<ref name="EbertEbert2"/> 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).<ref name="EbertEbert2"/> Many egg cases are covered with a layer of fiber; some will have a fine layer while others have a thick layer.

Big skate egg cases are larger than most other skate egg cases; typically ranging from 210 to 280&nbsp;mm in length and 110 to 180&nbsp;mm in width.<ref name="Ebert2">Ebert, David A., Davis, Chante D. (2007). "Descriptions of skate egg cases (Chondrichthyes: Rajiformes: Rajoidei) from the eastern North Pacific". ''Zootaxa'' 1393: 1-18.</ref><ref name="Zipcodezoo2Zipcodezoo">{{cite web|url=http://zipcodezoo.com/Animals/R/Raja_binoculata/|title=''Raja binoculata'' (Big Skate, Big Skate)|website=Zipcodezoo.com|archiveurl=https://web.archive.org/web/20080908085320/http://zipcodezoo.com/Animals/R/Raja_binoculata|archivedate=8 September 2008|deadurl=yes}}</ref> Big skates egg cases are approximately 15% of the overall length of the female skate.<ref name="Ebert2" /> The egg case is very smooth and lacks external fibrous material.<ref name="Ebert2" /> 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.<ref name="Ebert2" /> The keel on the egg case is considered very broad; representing 30–33% of the width of the egg case.<ref name="Ebert2" />

All known [[Chimaera|chimaeras]] produce egg cases.<ref>{{Cite journal |last1=García-Salinas |first1=Pablo |last2=Gallego |first2=Victor |last3=Asturiano |first3=Juan F. |date=2021-07-23 |title=Reproductive Anatomy of Chondrichthyans: Notes on Specimen Handling and Sperm Extraction. II. Sharks and Chimaeras |journal=Animals |language=en |volume=11 |issue=8 |pages=2191 |doi=10.3390/ani11082191 |issn=2076-2615 |pmc=8388383 |pmid=34438648 |doi-access=free }}</ref> The egg cases of [[chimaera]]schimaeras are spindle- or bottle-shaped with fins on the sides. They are laid on the bottom of the sea floor. Chimaeras (subclass [[Holocephali]]), some [[shark]]s, and [[Skate (fish)|skates]] are among the 43% of known [[Chondrichthyes|Chondrichthian]] species to exhibit [[oviparity]].<ref name=":0">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.</ref> However, there are some key [[Morphology (biology)|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.<ref name=":0" /> 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.<ref name=":0" /> 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.<ref>{{Cite book|title=The Biology of Sharks and Rays|last=Klimley|first=A. Peter|publisher=The University of Chicago Press|year=2013|isbn=0226442497978-0226442495|location=Chicago, Illinois|pages=286–288}}</ref>

Predation on egg cases is thought to be a major source of mortality for developing oviparous sharks, skates, and chimaeras.<ref name=":4">{{Cite journal|lastlast1=Lucifora|firstfirst1=L.O.|last2=Garcia|first2=V.B.|date=2004|title=Gastropod predation on egg cases of skates (Chondrichthyes, Rajidae)|url=|journal=Marine Biology|volume=145|pages=917917–922|doi=10.1007/s00227-922004-1377-8|vias2cid=53508039 }}</ref> In general, predation is the leading cause of mortality for marine fish eggs, due to their abundance and high nutritional value.<ref name=":4" /> 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.<ref name=":4" /> Some gastropods are known to feed on egg cases by boring into the exterior.<ref name=":4" /> Sharks are also common predators of egg cases.<ref name=":3" />