Stomiiformes
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Suborder Gonostomatoidei |
Stomiiformes is an order of deep-sea ray-finned fish of very diversed morphology, including the dragonfish, the lightfish, the hatchetfish, the viperfish or the loosejaws. The order comprises 4 families, more than 50 genera and a total of about 320 species. As usual for benthic fishes, there is no common name for all the species comprised in this order.
The smallest species of this order is Cyclothone pygmaeae of the Mediterranean Sea (1.5 cm for adult specimens), while the largest one is Opostomias micripnis, widely found in the Atlantic, Indian and Pacific Ocean (50 cm for adults). Other species range from less than 2 cm to more than 50 cm in length, as measured from the nose to the tail's tip.
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Distribution and morphology
Members of this order are mostly benthic fishes (i.e. living in deep oceanic waters), even if certain of them are also found in middle waters. Their distribution around the world's oceans is very wide, ranging from subtropical and temperate water bodies upto subartic or even antarctic ones.
These fish have a nightmarish and weird appearance. They all have teeth on the premaxilla and maxilla. Their maxillary ligaments, as well as some muscles and certain bones in the branchial cavity, are specialized in a distinctive way. Most have large mouths extending back past the eyes (Some also have chin barbels). Several fins might be absent, and colors are typically dark brown or black.
How to light up ?
As is the usual case for abyssal sea creatures, all members of the Stomiiformes but one have luminescent organs (photophores), whose structure is characteristic of the order. The gleam emitted can be more or less strong and its color can be light yellow, white, violet or red. The lighting mechanism can be very simple - consisting of small gleaming points on the fish body - or very elaborate, involving lens and refractors.
The commonest disposal is 1 or 2 photophore rows on the lower part of each side of the body. The rows run from the head down to the tip of the tail. Photophores are also present at the tip of chin barbels. order Stomiiformes does not seem to utilize bacteries as lighting source: no bacterium has ever been found in their photophores. However, recent researches have found that a piece of baterium-like ADN in one photophore, which leads to think that these latters contain bacteria whose appearance must have been largely modified.
The light coming from these fish are generally invisible to their preys.
Daily migration
At day, Stomiiformes fish stay in deep waters and measure the intensity of the light that reaches them. By doing so, they manage to stay in the region that always has the same light intensity. When the sun sets, the fish follows the dimming sunlight upto near-surface waters. The upper regions are richer in animal life: small fishes, microplanctons or invertebrates. The fish hunt and feed on these organisms all night long and swim back to deep waters as soon as the sun rises.
This daily migration is well observed at quite a few species of the Stomiiformes. However it is not specific to this fish order, and some larger species of the Stomiiformes - among the largest predator of the oceans - stay in their deep-sea habitat and feed on smaller migrants that return from the surface.
Reproduction
Stomiiformes lay eggs generally in deep seas, but the eggs are light and floats towards the ocean surface. They hatch in surface waters. When the larvae have accomplished metamorphosis and look like adults, they swim down to deep sea again.
Like a lot of benthic fish species, certain Stomiiformes - especially in the genera of Gonostoma and Cyclothone - change their sex during their life. They are born being males, then transform into females.
Gonostomatoidei vs. Photichthyoidei
Primitive specimens of Stomiiformes had thin brownish bodies, with rows of egg-shaped photophores adorning the lower body parts, and mouths with numerous teeth. From this primitive form two evolutive lineages appeared:
On one side, there are the families of Gonostomatidae and Sternoptychidae, forming the suborder Gonostomatoidei. In the first family we find the most primitive forms of Stomiiformes while in the second, it is the inverse.
On the other side, the 2 families of Photichthyidae and Stomiidae, forming the suborder Photichthyoidei. While the Photichthyidae are also primitive and bioluminescent organisms, like certain Gonostomatidae, the group Stomiidae is another example of extreme diversification.
Reference
External link
- FishBase entry for Stomiiformes (http://www.fishbase.org/Summary/OrdersSummary.cfm?order=Stomiiformes)