Egg case (Chondrichthyes)

Egg case of a skate.

An egg case or egg capsule, colloquially known as a mermaid's purse or devil's purse, is a casing that surrounds the fertilized eggs of some sharks, skates, and chimaeras. They are among the common objects which are washed up by the sea. Because they are lightweight, they are 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 pots, and on the sea floor during surveys using an ROV.

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.

Egg cases are deposited in pairs on the sea floor, and hatchlings are believed to emerge within 9 months.^[1] Gestation can take longer, up to 12 months or more for deep sea catsharks which lay their eggs in very cold water. If an egg case is still moist (not dried out) and has no visible hole, it is probably still occupied.

Skates are oviparous organisms^[2] 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.^[3][4] Studies have been done where egg cases were removed from gravid females to ensure proper identification in regard to skate species.^[3] 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).^[3] Many egg cases are covered with a layer of fiber; some will have a fine layer while others have a thick layer. Below are pictures of egg cases belonging to the Big skate, Raja binoculata. Another skate egg case belonging to the roughtail skate is shown.

Big skate

Fish egg cases are not always small. Here, a crested bullhead shark feeds on the egg case of a Port Jackson shark

Raja binoculata

The big skate is known to be the largest skate in the eastern North Pacific Ocean.,^[5][6] Their maximum size has been noted as 244 cm total length (TL),^[5] but the largest confirmed size is 203.9 cm TL.^[7]

• 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.,^[3][4]
• The egg case is very smooth and lacks external fibrous material.^[3] 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.^[3]
• The keel on the egg case is considered very broad; representing 30-33% of the width of the egg case.^[3]
• 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.^[7]
• Big skate have a 70 mm difference between the smallest and largest egg cases;^[3] 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.^[3]
• 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 (See image below).

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  Egg case of the Big skate, Raja binoculata

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  Four embryos inside a big skate egg case

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  Big skate egg cases; Egg case on left is fresh and older ones are to the right

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  Roughtail egg case Bathyraja trachura

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  The eggcase of a Raja species, probably Raja undata

Other skate species

The Longnose skate, Raja rhina, is considered a larger skate species; reaching a maximum size range of 145 cm TL.^[7] Although their egg cases are smaller than that of the big skate, their cases are considered large too; ranging 93–102 mm in length.^[3]

• Egg cases contain a single embryo.
• Longnose skate egg case's smallest to largest size in length varies by only 9 mm;^[3] 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.^[3]

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.^[3]
• 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.^[3]
• The egg cases are smooth and lack external fibrous material.^[3] The keels are very wide, and a great characteristic for identifying it to this species.^[3]

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.^[3]
• 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.^[3]
• The presence of egg cases can assist researchers in gaining knowledge about skate distribution,^[3] 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.^[3]

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  A mermaid's purse from a dogfish found on Trefor beach, North Wales

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  Empty mermaid's purse from a lesser spotted dogfish. Found on the shore at Anglesey

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  A Port Jackson shark's egg case. The screw-like helical flanges help the mother shark to secure the egg within rock crevices

See also

Sharks portal

• http://www.glaucus.org.uk/Mermaid.htm
• http://www.fishbase.org
• http://www.csgc.ucsd.edu

References

1. ^ http://www.arkive.org/big-skate/raja-binoculata/range-and-habitat.html.
2. ^ http://www.flmnh.ufl.edu/fish/education/questions/Biology.html#purse Florida Museum of Natural History, Most Commonly Asked Questions.
3. ^ ^{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.
4. ^ ^{a b} Zipcodezoo.com http://zipcodezoo.com/Animals/R/Raja_binoculata/.
5. ^ ^{a b} FishBaseSearch Common Name Big skate and Longnose skate.FishBase: A Global Information System on Fishes. 2001. Web 09 November, 2009 http://www.fishbase.org/home.htm.
6. ^ Discoverylife.org. http.//www.discoverylife.org/mp/20q?search=Raja+binoculata.pdf. date?.Web 07 October, 2009.
7. ^ ^{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.