Egg case (Chondrichthyes)
An egg case or egg capsule, often colloquially called a mermaid's purse, is the casing that surrounds the
Description
Egg cases are made of collagen protein strands,[4] and are often described as feeling rough and leathery.[5] Some egg cases have a fibrous material covering the outside of the egg case, thought to aid in attachment to substrate.[1][6] Egg cases without a fibrous outer layer can be striated, bumpy, or smooth and glossy.[1][6] With the exception of bullhead shark eggs, egg cases are typically rectangular in shape with projections, called horns, at each corner.[1][6] Depending on the species, egg cases may have one or more tendrils.[5]
Development
Shortly after internal fertilization, the fertilized ovum enters the partially formed egg case located in the oviduct.[6] After the ovum enters, the rest of the egg case forms around it.[6] 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.[1][6]
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.[6] The embryo fans its tail constantly to promote exchange with surrounding water.[6]
Sharks
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 viviparity 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.[7]
Bullhead sharks
Bullhead shark egg cases are shaped like an
Carpet sharks
The
Ground sharks
Some
The size of egg cases vary; those of the
Skates
The
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.[1][11] Big skates egg cases are approximately 15% of the overall length of the female skate.[1] The egg case is very smooth and lacks external fibrous material.[1] 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.[1] The keel on the egg case is considered very broad; representing 30–33% of the width of the egg case.[1]
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]
Longnose skate
The longnose skate, Raja rhina, is considered a larger skate species; reaching a maximum size range of 145 cm total length.[12] Although their egg cases are smaller than that of the big skate, their cases are also considered large; ranging 93–102 mm in length.[1] 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.[1]
Chimaeras
All known chimaeras produce egg cases.[13] 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.[14] 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.[14] 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.[15]
Extinct chondrichthyans
The egg case genera
Threats
Predation on egg cases is thought to be a major source of mortality for developing oviparous sharks, skates and chimaeras.[17] In general, predation is the leading cause of mortality for marine fish eggs, due to their abundance and high nutritional value.[17] 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.[17] Some gastropods are known to feed on egg cases by boring into the exterior.[17] Sharks are also common predators of egg cases.[6]
References
- ^ a b c d e f g h i j k l m n o Ebert, David A., Davis, Chante D. (2007). "Descriptions of skate egg cases (Chondrichthyes: Rajiformes: Rajoidei) from the eastern North Pacific". Zootaxa 1393: 1-18.
- ^ a b c d Carrier, J.C; Musick, J.A.; Heithaus, M.R. (2012). Biology of Sharks: Second Edition. Taylor & Francis Group. p. 296.
- ^ a b Carrier, J.C; Musick, J.A.; Heithaus, M.R. (2004). Biology of Sharks: First Edition. Taylor & Francis Group. p. 270.
- .
- ^ a b c d e f g h Compagno, Leonard (2002). "Sharks of the World". FAO Species Catalogue for Fishery Purposes. 2: 31–50.
- ^ .
- ^ "Egg Identification". Aquarium of the Pacific. 2008.
- ^ Buch, Robert. "Heterodontus francisci". Florida Museum.
- ^ "Most Commonly Asked Questions". Florida Museum of Natural History.
- ^ "Raja binoculata (Big Skate, Big Skate)". Zipcodezoo.com. Archived from the original on 8 September 2008.
- ^ "Raja binoculata (Big Skate, Big Skate)". Zipcodezoo.com. Archived from the original on 8 September 2008.
- ^
- PMID 34438648.
- ^ a b 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.
- ISBN 978-0226442495.
- S2CID 84827548.
- ^ S2CID 53508039.