Flat fish flying in the water.
Recently I visited an aquarium and encountered with these “flying” fish, rays and skates. They looks the same to me: flat, kite-like fish.
In fact, rays and skates are dorsoventrally flattened fish that belong to the same superorder Batoidea, which contains four orders. Skates are in Rajiformes, while rays are in Myliobatiformes, Rhinopristiformes, Torpediniformes. Historically, due to morphological reasons, some rays were classified as skates, and vice versa.
Furthermore, rays, skates, and their close relatives, sharks, are considered to be within a subclass of (primitive) fish called Elasmobranchii in the class Chondrichthyes. Chondrichthyes contains the cartilaginous fish. From wikipedia: They are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. In a word, these fish have soft skeletons 1. Furthermore, Elasmobranchs have five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil. Sometimes it is hard to differentiate rays and skates from sharks, as illustrated in this short video. However, a rule of thumb is that, sharks have gill slits on the side, rays and skates have theirs on the bottom.
Now the diffeences between rays and skates. The major difference is in the reproductive strategies: Rays are viviparous while skates are oviparous. The skate eggs are a little peculiar, because they are released in hard rectangular cases nicknamed “mermaid’s purses”. Morphologically, skates typically have a prominent dorsal fin while the dorsal fin is absent or greatly reduced in rays. Most rays are kite-shaped with whip-like tails having stinging spines (hence “stingray”) while skates have fleshier tails and lack spines 2. While some rays have stings to protect them, the electric rays have kidney-shaped electric organs that are visible through the skin on the dorsal surface to generate and discharge a strong electric current to stun prey and for defense from potential predators. As for skates, they rely on thorny projections on their backs and tails to for protection. For more differences between rays and skates, see here.
It is said that rays are generally much larger than skates. Nevertheless, the variance of the sizes of rays is huge. The largest ray (Manta birostris) reaches up to 9 meters in width, while the smaller ray (Narcine bancroftii) is only 20 cm. The sizes of skates are in between. The largest skate (Dipturus batis) reaches up to 2.5 meters in length, while the smaller skate (Raja straeleni) reaches to a maximum total length of 68 cm.
The last interesting thing I would like to mention is the osmoregulation of cartilaginous fishes, i.e. the passive regulation of the osmotic pressure of an organism’s body fluids for the water-salt balance. For adaptation to high-salinity marine environments, cartilaginous fishes adopt a unique urea-based osmoregulation strategy. Their kidneys reabsorb nearly all filtered urea from the primary urine, and this is an essential component of urea retention in their body fluid.
Due to urea retention, the meat of cartilaginous fishes is usually poisonous, due to the high content of urea and sometimes trimethylamine oxide. Nevertheless, human managed to process the meat, e.g. via fermentation, and eat without costing their lives. For example, hongeo-hoe from Korea is made from skate and emits a very strong ammonia-like odor. And hákarl from Iceland is made from sharks and has a strong ammonia-rich smell and fishy taste. The ammonia produced during fermentation actually helps the preservation of the food. I personally have tried hákarl, which tasted good if the ammonia smell is accepted. I like the texture.
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