Overview

Comprehensive Description

[[ Familiy Rajidae ]]

Key to the Eastern North Pacific Skate Egg Cases

The ENP skate egg cases can be broadly classified into two main groups: those with broad lateral keels (> 10% MAW) and those with narrow lateral keels (<10% MAW). Egg cases in the former group (with broad lateral keels) generally have a surface texture that is finely striated and smooth to the touch. Those in the latter group (with narrow lateral keels) tend to have an egg case surface that is coarse in texture, often with prickles, and is rough to the touch. The only exception in this group is the egg case of R. stellulata that differs in having a finely striated surface beneath a thick fibrous covering.

The key below includes all of the known valid skate species, plus two unidentified species, known to occur from the eastern Gulf of Alaska (ca 59° N, 138° W) southwards to the California Mexico border (ca 32° N 117° W). The validity of two species, B. obtusa (Gill & Townsend, 1897) and B. rosispina (Gill & Townsend, 1897) is questionable and the occurrence of the Alaska Skate, B. parmifera (Bean, 1881) , in the eastern Gulf of Alaska has not been confirmed from this region. The egg cases of B. parmifera , if found to occur in this area, are quite distinct. A description and illustration of the egg case of B. parmifera can be found in Ebert (2005).

1a. Lateral keel width broad,> 10% of maximum egg case width...................................................................2

1b. Lateral keel width narrow, <10% of maximum egg case width.................................................................8

2a. Egg case very large,> 180 mm....................................................................................................................3

2b. Egg case usually smaller, <100 mm...........................................................................................................4

3a. Dorsal surface distinct with two convex ridges; horns <50% ECL; keel width very broad> 30% of MAW .............................................................................................................................................. Raja binoculata ZBK

3b. Dorsal surface without convex ridges; horns> 50% ECL; keel width broad about 15 % of MAW............. ........................................................................................................ Unidentified deepsea skate species “A”

4a. Attachment fibers absent.............................................................................................................................5

4b. Attachment fibers present ............................................................................................................................ 6

5a. Lateral keel width very broad, 17-25% of maximum egg case width; posterior horns about one-half egg case length ............................................................................................................................... Raja inornata

5b. Lateral keel width broad, 12-14% of maximum egg case width; posterior horns about equal to egg case length .............................................................................................. Unidentified deepsea skate species “B”

6a. Fibrous covering absent .................................................................................................. Bathyraja trachura

6b. Fibrous covering present .............................................................................................................................. 7

7a. Fibrous covering thick, woven-like .............................................................................................. Raja rhina

7b. Fibrous covering thin ...................................................................................................... Bathyraja kincaidii

8a. Attachment fibers present ............................................................................................................................ 9

8b. Attachment fibers absent .......................................................................................... Bathyraja spinossisima

9a. Egg case with fibrous covering ............................................................................................... Raja stellulata

9b. Egg case without fibrous covering .......................................................................................... ................... 10

10a. Egg case very large,> 100 mm ................................................................................................................ 11

10b. Egg case smaller, <100 mm .................................................................................. Bathyraja mircotrachys

11a. Surface coarse, striated, with long anteriorly directed prickles giving it a velvety texture, and without a distinct groove demarcating lateral keel and egg case ..................................................... Bathyraja aleutica

11b. Surface very coarse, without anteriorly directed prickles, with a distinct groove demarcating lateral keel and egg case ................................................................................................................. Bathyraja abyssicola

  • David A. Ebert, Chante D. Davis (2007): Descriptions of skate egg cases (Chondrichthyes: Rajiformes: Rajoidei) from the eastern North Pacific. Zootaxa 1393, 1-18: 16-17, URL:http://www.zoobank.org/urn:lsid:zoobank.org:pub:0C16005C-21BC-4252-823E-C83515FCFF28
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[[ Family Rajidae ]]

Discussion

This key is suitable for identifying both wet and dry skate egg cases from southeastern Australia. The key was developed using fresh (extracted from the uteri) egg cases, where correct species identification was assured and egg cases were undamaged. However drying identified fresh specimens showed that the key would still work on dried egg cases. When dry, the keratin-like substance of the egg case becomes brittle and is prone to damage. At room temperature, dried egg cases shrink by about 30% of their original size. This key was also used on previously unknown specimens collected from the beach. It was found if the egg cases were relatively undamaged, identification was possible. Heavily damaged and predated egg cases were unidentifiable.

The egg case of the different species had a variety of characteristics which allowed them to be distinguished. Although egg case body size sometimes overlapped between species, other features could be used to distinguish them, including horn and apron lengths, shape, and presence/absence of a lateral keel and attachment fibres (irrespective of whether the egg case was fresh or not).

Further identifications of other Australian skate egg cases are necessary to elucidate any phylogenic relationships between a skate’s genus and its egg cases. It is interesting that Dipturus cf gudgeri egg cases have horns that are almost fully enclosed in the apron. This feature may be an attribute of a subgenus in the genus Dipturus ZBK (Ishiyama 1958). An egg case with enclosed horns was also found in a Japanese species that belonged to the genus Dipturus ZBK . Ishiyama (1958) and H. Ishihara (Taiyo Engineering, Tokyo, Japan, personal communication) reported some phylogenetic differences in egg cases between genera in Japanese species. This suggests that egg case morphology may help in determining phylogenetic relationships in rajid species, although further work on egg case assemblages is required both in Australia and worldwide.

  • M. A. Treloar, L. J. B. Laurenson, J. D. Stevens (2006): Descriptions of rajid egg cases from southeastern Australian waters. Zootaxa 1231, 53-68: 67-67, URL:http://www.zoobank.org/urn:lsid:zoobank.org:pub:DF293AC9-4368-4381-9FA2-F4486715FF80
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[[ Family Rajidae ]]

Identification key for rajid egg cases collected from skates in southeastern Australian waters

1a Egg case small, ≤ 85 mm BL ........................................................................................ 2

1b Egg case medium to large, ≥ 90 mm BL ....................................................................... 5

2a Lateral keel absent; dorsal and/or ventral surfaces of egg case covered with dense fibroids .......................................................................................................................... 3

2b Lateral keel present; dorsal and ventral surfaces of egg case not covered with dense fibroids ....................................................................................... Dipturus sp. A (Fig. 3)

3a Posterior horns shorter than body width; egg case> 40 mm BL .................................. 4

3b Posterior horns at least 2.7 times maximum body width; egg case ≤ 40 mm BL ......... ................................................................................................. Pavoraja nitida (Fig. 10)

4a Ventral surface of egg case covered with dense fibroids; egg case narrow at posterior end; posterior apron 1.5 times length of anterior apron ............. Dipturus cerva (Fig. 6)

4b Ventral surface of egg case not covered with dense fibroids; posterior apron length almost 3 times the length of anterior apron .......................... Dipturus lemprieri (Fig. 8)

5a Dorsal and ventral surfaces of egg case covered with dense fibroids; posterior horns shorter than body width ................................................................................................. 6

5b Dorsal and ventral surfaces of egg case not covered with dense fibroids; posterior horns at least 1.4 times maximum body width ........... Amblyraja cf hyperborea (Fig. 2)

6a Lateral keel prominent,> 18% of maximum body width; aprons fringed ...................... ................................................................................................ Dipturus whitleyi (Fig. 9)

6b Lateral keel discrete, <14% of maximum body width; aprons not fringed .................. 7

7a Egg case large, ≥ 160 mm BL; posterior horns enclosed in apron .................................. ............................................................................................ Dipturus cf gudgeri (Fig. 7)

7b Egg case medium, ≤ 120 mm BL; posterior horns extending out of apron .................... ..................................................................................................... Dipturus sp. B (Fig. 4)

Note: Dipturus sp. L was not included in this key as only partially formed egg cases were examined.

  • M. A. Treloar, L. J. B. Laurenson, J. D. Stevens (2006): Descriptions of rajid egg cases from southeastern Australian waters. Zootaxa 1231, 53-68: 57-57, URL:http://www.zoobank.org/urn:lsid:zoobank.org:pub:DF293AC9-4368-4381-9FA2-F4486715FF80
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[[ Family Rajidae ]]

Skates (Family: Rajidae ) are oviparous; a reproductive mode that enables females to encapsulate oocytes in morphologically structured shells that suit their environment. Generally, one oocyte from each ovary passes through the oviduct to the oviducal gland. The fertilised ovum then enters the egg case, which is generally one third to half formed. The remainder of the egg case then forms around the ovum. Two egg cases are produced (one in each oviduct) and deposited consecutively onto the substrate. The embryo develops inside the egg case and is nourished from the nutrients of the yolk until most of the yolk is absorbed. The neonate is then strong enough to leave the egg case (Hamlett and Koob 1999). In Australia, there are at least 43 skate species (10 genera) with most exhibiting a high level of endemicity (Last and Yearsley 2002). Several nomenclature changes have occurred in the Australian skate fauna recently with some species remaining undescribed. This may result from rajids forming one of the largest groups of elasmobranchs and having a high species diversity along with morphological conservatism (McEachran and Dunn 1998).

To date, there are several published identification keys or descriptive morphological studies on skate egg cases (Clark 1922; Breder and Nichols 1937; Breder and Atz 1938; Ishiyama 1958; Hitz 1964; Templeman 1982; Stehmann and Merrett 2001; Ebert 2005), although few exist for Australian species (Whitley 1938; Whitley 1944). Despite Whitley’s studies, there is still confusion with identifications of most Australian skate egg cases.

Egg cases have specific characteristics that aid in species identification. Differences between species have been widely noted, although generic characters may also exist. Identifying egg cases can lead to more knowledge about the adaptive differentiation of a species and may shed light on the taxonomic status of many species (Ishiyama 1958).

  • M. A. Treloar, L. J. B. Laurenson, J. D. Stevens (2006): Descriptions of rajid egg cases from southeastern Australian waters. Zootaxa 1231, 53-68: 53-54, URL:http://www.zoobank.org/urn:lsid:zoobank.org:pub:DF293AC9-4368-4381-9FA2-F4486715FF80
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Physical Description

Diagnostic Description

Description

Benthic rays occurring in all oceans, from Arctic to Antarctic waters and from shallow coastal shelfs to abyssal regions; rare in tropical shallow waters or near coral reefs; some species enter brackish waters. Disc quadrangular to rhomboidal. Mouth transversed to arched, with numerous teeth. Five pairs of ventral gill slits. Tail very slender, with lateral folds, usually 2 reduced dorsal fins and a reduced caudal fin. Electric organs weak, developed from caudal muscles. Skin prickly in most species, the prickles often in a row along midline of dorsal. Oviparous; eggs in a horny capsule with four long tips. Skates feed on other benthic organisms. Skate wings are considered good eating.
  • MASDEA (1997).
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© WoRMS for SMEBD

Source: World Register of Marine Species

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Evolution and Systematics

Functional Adaptations

Functional adaptation

Electric organs generate volts: rajid skates
 

Electric organs of rajid skates help defend against predators or stun prey by generating electricity.

     
  "Much weaker voltages are generated by other fish, including bottom-dwelling marine rajid skates, whose electric organs are in their tails. (Shuker 2001: 53)
  Learn more about this functional adaptation.
  • Shuker, KPN. 2001. The Hidden Powers of Animals: Uncovering the Secrets of Nature. London: Marshall Editions Ltd. 240 p.
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© The Biomimicry Institute

Source: AskNature

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:2,794Public Records:949
Specimens with Sequences:2,450Public Species:67
Specimens with Barcodes:2,066Public BINs:61
Species:134         
Species With Barcodes:111         
          
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Source: Barcode of Life Data Systems (BOLD)

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Barcode data

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Source: Barcode of Life Data Systems (BOLD)

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Rajidae

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Wikipedia

Skate

Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. More than 200 species have been described, in 27 genera. The two subfamilies are Rajinae (hardnose skates) and Arhynchobatinae (softnose skates).

Genera[edit]

Skates versus stingrays[edit]

Mermaid's purse, or the protective egg case of a skate
Skates swim with their pectoral fins
Comparison of skates and stingrays
Superficially, skates and stingrays appear somewhat similar.
Skate
Stingray
However, there are fundamental differences.
CharacteristicSkatesRaysSources
ReproductionSkates are oviparous, that is they lay eggs. Their fertilized eggs are laid in a protective hard case called a mermaid's purse.Rays are viviparous, that is, they bear their young inside their bodies and give birth to them alive.[2]
Dorsal finDistinctMissing or vestigial[2]
Pelvic finsFins are divided into two lobesFins have one lobe[3]
TailFleshy tails which lack spinesWhip-like with one or two stinging spines[2][3]
ProtectionRely on "thorny projections on their backs and tails for protection from predators"Rely on their stinging spines or barbs for protection[2]
TeethSmall"Plate-like teeth adapted for crushing prey"[2]
SizeUsually smaller than raysUsually larger than skates[2]
ColourOften drab, brown or gray (but not always)Often boldly patterned (but not always)[3]
HabitatOften deep water (but not always)Often shallow water (but not always)[3]

Conservation[edit]

Skates have slow growth rates and, since they mature late, low reproductive rates. As a result, skates are vulnerable to overfishing and appear to have been overfished and are suffering reduced population levels in many parts of the world. The barndoor skate, Raja laevis, is currently listed with the IUCN as vulnerable due to being severely overfished.[4] However, population data are lacking to determine the exploitation of the big skate at this time.[citation needed]

In 2010, Greenpeace International added the barndoor skate, bottlenose skate, spotback skate, and maltese skate to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries."[5]

Gallery[edit]

See also[edit]

  • Jenny Haniver, a fake sea monster created from a skate corpse
  • Mokpo city of South Korea is famous for its skate cuisine.

References[edit]

  1. ^ Froese, Rainer, and Daniel Pauly, eds. (2009). "Rajidae" in FishBase. January 2009 version.
  2. ^ a b c d e f Ichthyology: Ray and Skate Basics Florida Museum of Natural History. Retrieved 21 March 2013.
  3. ^ a b c d Skate or Ray? ReefQuest Centre for Shark Research. Retrieved 212 March 2013.
  4. ^ Florida Museum of Natural History
  5. ^ Greenpeace International Seafood Red list
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