Batomorphi is a division
Anatomy
Batomorphs are flat-bodied, and, like sharks, are cartilaginous fish, meaning they have a boneless
skeleton made of a tough, elastic cartilage. Most batomorphs have five
ventral slot-like body openings called
gill slits that lead from the
gills, but the
Hexatrygonidae have six.
[
] Batomorph gill slits lie under the
on the underside, whereas a shark's are on the sides of the head. Most batomorphs have a flat, mantle-like body, with the exception of the
and sawfishes, while most sharks have a spindle-shaped body. Many species of batomorph have developed their pectoral fins into broad flat wing-like appendages. The
anal fin is absent. The eyes and spiracles are located on top of the head. Batomorphs have a ventrally located mouth and can considerably protrude their upper jaw (palatoquadrate cartilage) away from the cranium to capture prey.
The jaws have euhyostylic type suspension, which relies completely on the hyomandibular cartilages for support.
Bottom-dwelling batomorphs breathe by taking water in through the spiracles, rather than through the mouth as most fish do, and passing it outward through the gills.
Reproduction
Batomorphs reproduce in a number of ways. As is characteristic of elasmobranchs, batomorphs undergo internal fertilization. Internal fertilization is advantageous to batomorphs as it conserves sperm, does not expose eggs to consumption by predators, and ensures that all the energy involved in reproduction is retained and not lost to the environment.
All skates and some rays are
oviparous (egg laying) while other rays are
ovoviviparous, meaning that they give birth to young which develop in a womb but without involvement of a placenta.
The eggs of oviparous skates are laid in leathery egg cases that are commonly known as mermaid's purses and which often wash up empty on beaches in areas where skates are common.
Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks and rays when fished.
Habitat
Most species live on the sea floor, in a variety of geographical regions – mainly in coastal waters, although some live in deep waters to at least . Most batomorphs have a cosmopolitan distribution, preferring tropical and subtropical marine environments, although there are temperate and cold-water species. Only a few species, like
, live in the open sea, and only a few live in freshwater, while some batomorphs can live in brackish bays and estuaries.
Feeding
Most batomorphs have developed heavy, rounded teeth for crushing the shells of bottom-dwelling species such as
snails,
clams,
oysters,
, and some
fish, depending on the species. Manta rays feed on
plankton.
Evolution
Batomorphs belong to the ancient lineage of cartilaginous fishes. Fossil
Dermal denticle (tooth-like scales in the skin) resembling those of today's
date at least as far back as the
Ordovician, with the oldest unambiguous fossils of cartilaginous fish dating from the middle
Devonian. A
clade within this diverse family, the
Neoselachii, emerged by the
Triassic, with the best-understood neoselachian fossils dating from the
Jurassic. The oldest confirmed ray is
Antiquaobatis, from the
Pliensbachian of
Germany.
The
clade is represented today by
,
sawfish, rays and skates.
Classification
Molecular evidence refutes the hypothesis that skates and rays are derived sharks.
The
monophyly of the skates, the
Myliobatiformes, and the
has long been generally accepted. Along with Rhinopristiformes, these comprise the four traditionally accepted major batomorph lineages, as in Nelson's 2006
Fishes of the World. However, the exact phylogeny of the major batomorph lineages, internally and with respect to one another, has been subject to diverse treatments. The following cladogram is based on a comprehensive morphological assessment of batomorph phylogeny published in 2004:
However, a 2011 study significantly reevaluated the phylogeny of batomorphs, using Nuclear DNA and mitochondrial DNA from 37 Taxon, representing almost all recognized families and all of the traditional four major lineages. This is a far more numerous and diverse set of sample taxa than in any previous study, producing findings reflected in the cladogram below.
This study strongly confirmed the traditionally accepted internal monophyly of skates, stingrays, and electric rays. It also recovered as Sister taxon to the stingrays, as older morphological analyses had suggested. However, it found the Rhinopristiformes, including the sawfishes and various "guitarfishes", to be paraphyletic, comprising two distinct clades. Referred to as "Guitarfishes 1" and "Guitarfishes 2", the former contains only the Trygonorrhinidae, while the latter contains the remainder of Rhinopristiformes (the families Glaucostegidae, Pristidae, Rhinidae, and Rhinobatidae). In addition, while traditional phylogenies often find electric rays to be the basalmost batomorphs, followed by the Rhinopristiformes, this analysis finds a polytomy between skates, electric rays, and Platyrhinidae at the base of Batomorpha, with weak support for skates being the actual most basal lineage, followed by a clade uniting the electric rays and thornbacks.
The Mesozoic Sclerorhynchoidea are basal or incertae sedis; they show features of the Rajiformes but have snouts resembling those of sawfishes. However, evidence indicates they are probably the sister group to sawfishes.
Eschmeyer's Catalog of Fishes classigies the rays as follows:
-
Order Torpediniformes
-
Order Rhinopristiformes
-
Order Rajiformes
-
Order Myliobatiformes
-
Family Zanobatidae Fowler. 1934 (panrays)
-
Family Hexatrygonidae Phil Heemstra & M. M. Smith, 1980 (sixgill stingrays)
-
Family Dasyatidae D. S. Jordan & Gilbert, 1879 (whiptail stingrays)
-
Family Potamotrygonidae Samuel Garman, 1877 (neotropical stingrays)
-
Family Urotrygonidae McEachran, Dunn & Tsutomu Miyaki, 1996 (American round stingrays)
-
Family Gymnuridae Fowler, 1934 (butterfly rays)
-
Family Plesiobatidae Kiyonori Nishida, 1990 (deepwater stingrays or giant stingarees)
-
Family Urolophidae J. P. Müller & Henle 1841 (round stingrays or stingarees)
-
Family Aetobatidae Louis Agassiz, 1858 (pelagic eagle rays)
-
Family Myliobatidae Bonaparte, 1835 (eagle rays)
-
Family Rhinopteridae D. S, Jordan & Evermann, 1896 (cownose rays)
-
Family Mobulidae Gill, 1893 (mantas or devil rays)
Conservation
According to a 2021 study in
Nature, the number of oceanic sharks and rays has declined globally by 71% over the preceding 50 years, jeopardising "the health of entire ocean ecosystems as well as food security for some of the world's poorest countries".
Overfishing has increased the global extinction risk of these species to the point where three-quarters are now threatened with extinction.
This is notably the case in the Mediterranean Sea - most impacted by unregulated fishing - where a recent international survey of the Mediterranean Science Commission concluded that only 38 species of rays and skates still subsisted.
[Guide of Mediterranean Skates and Rays. Oct. 2022. Mendez L., Bacquet A. and F. Briand.[1]]
Differences between sharks and rays
All sharks and rays are cartilaginous fish, contrasting with bony fishes. Many rays are adapted for feeding on the bottom.
are somewhat between sharks and rays, displaying characteristics of both (though they are classified as rays).
| + Comparison of Elasmobranchii fish |
|
|
| Shape | laterally compressed spindle | compressed (flattened) disc
|
| Spiracles | not always present | | always present |
| Habitat | usually pelagic surface feeders, though are demersal | demersal / pelagic mix | usually demersal bottom feeders |
| Eyes | usually at the side of the head | usually on top of the head
|
| Fish gill openings | on the sides | ventral (underneath) |
| distinct | not distinct
|
| Tail | large caudal fin whose primary function is to provide main forward propulsion | | varies from thick tail as extension of body to a whip that can sting to almost no tail. |
| Fish locomotion | swim by moving their tail (caudal fin) from side to side | Guitar fish and sawfish have a caudal fin like sharks | swim by flapping their pectoral fins like wings |
See also
Footnotes
Bibliography
External links
-
– database of bibliography of living/fossil sharks and rays ( see Chondrichtyes: Selachii) with more than 15 000 listed papers and many download links.
