| Juliidae Temporal range: Paleocene?/Eocene–recent |
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| A live individual of an unidentified species of Juliidae | |
| A drawing of the shell (the exterior of the right valve) of a taxon named "Julia borbonica" | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Heterobranchia Informal group Opisthobranchia |
| Superfamily: | Oxynooidea |
| Family: | Juliidae E. A. Smith, 1885[1] |
| Genera | |
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See text |
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Juliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group.
This family of snails is extremely unusual in that their shells consist of two separate hinged pieces or valves, which are joined by a ligament, and which look nothing like a normal snail shell, but instead look almost exactly like the two hinged valves of a clam, a bivalve mollusk, a very different class of animals.
In the past the Juliidae were known only from fossil shells, and not surprisingly these fossils were therefore interpreted as being the shells of bivalves. Julia, which is the type genus of the family, was named in 1862 by Augustus Addison Gould, who described it as a bivalve genus. Julliidae are known from the Eocene period to the Recent, but they probably first appeared during the Paleocene.[2]
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These bivalved gastropods were for a long time only known from fossils and dead material. Because of this, they had been described as being somewhat atypical bivalves. In the late 19th century they were classified among the bivalves, within the family Mytilidae, the true mussels.[3]
The similarity of the shells of Juliidae to those of bivalves does not mean that these snails are closely related to bivalves; this is an example of convergent evolution.
Up until the mid 20th century, these creatures were still considered to be bivalves. Then finally in 1959, living individuals of one species were collected on the green alga, Caulerpa, in Japan. It was immediately clearly visible that these animals are in fact unusual gastropods with a two-part shell. The first discovered live species of bivalved gastropod was Tamanovalva limax, described by Kawaguti & Baba (1959).[4]
Once the habitat, appearance, and life habits of these very small and inconspicuous animals were understood, in subsequent years researchers were able to find a number of other species and other genera in different parts of the world, also living on various species of Caulerpa.
This family is within the clade Sacoglossa (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005).
The family Juliidae consists of the following subfamilies (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005):
As Tryon (1884)[3] wrote in his description of the genus Julia: the shell is oblong, thick, and cordiform. The valves are closed, the margins entire and the valves are inequilateral. The lunule is deep circular, projecting into the interior of the right valve, the left valve is in the same place furnished with dentiform tubercles. The hinge line is simple and arched. The ligament is external and narrow. There are two muscle scars which are unequal and subcentral.
These animals have two valves, and the soft parts can be completely withdrawn inside the shell.[7] The two valves are usually thin and translucent.
In the genus Tamanovalva there is a protoconch on the apex of the left valve. This is clearly visible in the left valve of Tamanovalva babai.[8]
The body of the live animals is in most cases green (as it is in many sacoglossans), and in many species the individual appears green in totality. This, combined with the very small overall size, makes the animal hard to see on the green algae on which it lives. This ability serves as crypsis, especially as cryptic coloration (camouflage). In two species [1] and [2] the camouflage is even more complete: the mantle of the animal is patterned in a way that very closely resembles the structure of the alga on which it lives.
The empty valves of the shells of these animals are in some cases green, in other cases brownish-green or yellow, and in yet others, colorless. The species Julia zebra has shells that are finely striped with brown and blotched with white. [3]
Species in this family occur in all tropical zone seas and all temperate zone seas.[7]
After hatching out from eggs, the juvenile snails immediately take their place on algae (the larval phase of veliger is extremely short).[9]
Juliidae feed on green algae[7] of the genus Caulerpa. Some species of Juliidae feed only on one species of Caulerpa, others in the family feed on more than one species of this green alga.
The type genus of the family Juliidae is Julia Gould, 1862.
Based on a classification by Jensen (1996),[10] three recent genera were recognized in the family Juliidae[11]: Julia Gould, 1862; Berthelinia Crosse, 1875; Tamanovalva Kawaguti & Baba, 1959.
While Jensen (2007)[12] recognized two recent genera with a few subgenera in the genus Berthelinia. Recent species listed here are based on Jensen (2007)[12], fossil genera and species are based on Le Renard (1996)[2]:
? subfamily
This article incorporates public domain text from reference.[3]