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Smooth Butterfly Ray

The Smooth Butterfly Ray (Gymnura micrura) is a species of cartilaginous fish in the Gymnuridae family. They are a member of the Order Myliobatiformes, which contains 10 total families. Its natural habitats are shallow seas, subtidal aquatic beds, estuarine waters, and coastal saline lagoons. They are distinguished by having a body that is compressed, pectoral fins that are wider than their length, and having a diamond shape; giving them their common name of 'butterfly rays'.

Smooth butterfly ray
Conservation status
Data Deficient (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Order:	Myliobatiformes
Family:	Gymnuridae
Genus:	Gymnura
Species:	G. micrura
Binomial name
Gymnura micrura (Bloch & J. G. Schneider, 1801)

Taxonomy

Gymnura is derived from greek roots and is translated into 'naked tail' (Gymnos=Naked, Oura=Tail). They belong to a monophyletic group of Batoid fish. This group contains over 500 other elasmobranch fishes which includes electric rays, sawfishes, guitarfishes, skates, and stingrays. They are a part of Order Myliobatiformes which are characterized by their pectoral fins being widely expanded and fused to their heads. The family Gymnuridae contains 2 genera which encompasses 12 different species.

Biology

Appearance

These are broad-diamond shaped rays with a short tail that has low dorsal and ventral fin folds. The tail has 3 to 4 dark lines that are referred to as crossbars. The edges of the disc concave. The caudal fin is never present and a variable number of tubercles can be found on larger specimens. The smooth butterfly rays have disc widths nearly twice the size of their body lengths and are very flat-bodied (compressed). The width of the rays are between 16 and 22 centimeters when they are born, they are about 50 cm when mature for a female and about 42 cm for a male. Females are bigger than their male counterparts; this allows them to carry larger embryos and a greater abundance of embryos. They have a maximum size of 120 cm.

Dorsal view of a Smooth Butterfly Ray

Lifecycle

These rays invest a large amount of energy into reproduction and only give birth to a few offspring; however, they give birth on a yearly basis. They use internal fertilization which is the process of the male inserting his claspers into the females Cloaca to fertilize the eggs. The offspring takes between two to four months to develop inside the mother. They use aplacental uterine viviparity and the young are histotrophs.

Behavior and ecology

The ventral side is lightly colored while the dorsal side is variable in color. The ventral side is usually white but can contain a rusty or bronze coloration. The dorsal side can be grey, light green, brown, and also not uniform in color. They tend to use countershading to blend in with the bottom of their environments in order to hide from predators and to catch prey.

The foraging strategy that these rays use is dependent on the abundance of prey in their environments. They either use opportunistic feeding where they eat what is available, or they use specialized feeding where they eat a specific organism. They tend to feed on larger prey items and swallow them whole; then they enter a long digestion time period where they feed very little or not at all. They prey mainly on Teleost and crustaceans, but have also been noted to consume bivalve muscles and polychaetes. They use a structure called the lateral line canal that contains ampullae of lorenzini that is located on their ventral side to detect prey. Their upper jaw consists of 6 to 120 teeth and the lower jaw has 52 to 106 teeth; each jaw containing 6 to 8 simultaneously functioning rows.

They are hunted by larger predators, such as sharks. The great hammerhead (Sphyrna mokarran) specializes in feeding on these butterfly rays and are their main predator in some areas.

Locomotion

They alter their swimming habits depending on where they are swimming in the water column. They tend to change between an Undulation pattern and an Oscillation pattern. They use small amplitude undulations of their fins when they are swimming along the bottom, but switch to an oscillatory approach when they are swimming freely in the water. When swimming freely in the water column, they use a quick, powerful downstroke to increase their speed; which means they move their fins down and then quickly back up. They pause after each stroke and then repeat.

Distribution and habitat

They are found in the western and eastern parts of the Atlantic ocean (Maryland to brazil) and the Gulf of Mexico. Most commonly found in neritic waters but are known to enter brackish estuaries and hypersaline lagoons. They live in a range from the continental shelf to 40 meters deep in tropical and warm waters. They prefer habitats that have either sandy or muddy bottoms.

Relation to humans

The dorsal spine on the tail is absent, therefore no stinging can occur to harm humans.

This species does not have a sufficient amount of information known about them, but there are no current conservation measures set in place. They are usually takes as bycatch but generally released back into the waters alive. Australia, Europe, and some parts of Asia do consume this species of rays and consider them to be food.