Rhodopidae is a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

sea slug Sea slug is a common name for some Marine biology, marine invertebrates with varying levels of resemblance to terrestrial Slug, slugs. Most creatures known as sea slugs are gastropods, i.e. they are Sea snail, sea snails (marine gastropod moll ...

s. Rhodopids are small, meiofaunal organisms with worm-like body plans that differ considerably from a typical gastropod body plan, to the point that ''Rhodope'' was at one point classified as a

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

. They have no shell. Two genera are currently recognized within the family, ''

Rhodope Rhodope may refer to: * Rhodope (mythology), several figures of Greek mythology * Rhodope Mountains, in Bulgaria and Greece * Rhodope (regional unit), of Greece * Rhodope (province), a Roman and Byzantine province * 166 Rhodope, an asteroid * Rhodop ...

'' and '' Helminthope''. ''Helminthope'' has been described as the most worm-like gastropod.

Etymology

''Rhodope'', the

type genus In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearin ...

of the family, is named for

, a figure in

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

Description

Rhodopids have worm-like bodies with no appendages. They often have red, orange, or purple bands, in varying pattern depending on the species, though some species are entirely white. The species range in length from 1.5 to 8 mm, although they are capable of considerably contracting themselves. The length-to-width ratios of the body while extended to crawl is around 9 in ''Rhodope'' and 25 in ''Helminthope''. Their bodies contain

spicules Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes ( ...

below the epidermis that may provide structural support for the animal. The nervous system of rhodopids has accessory ganglia, as in several other lineages of interstitial gastropod.

Ecology

Some rhodopid species are

epibenthic Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone. They are found in temparate and warm oceans worldwide. Rhodopids are the only known predators of

placozoans Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, plac ...

, which make up the primary component of their diet. At least some species, such as ''Rhodope placozophagus'', can subsist entirely on a diet of placozoans; they are apparently not harmed by the toxins present in placozoans.

Evolution

Although their affinities were long unclear, it is now known that rhodopids belong to the basal heterobranch clade Mesoneura. They are not closely related to any other lineage of sea slugs; their closest living relatives are

sea snails Sea snails are slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the absence of a visibl ...

of the family

Murchisonellidae Murchisonellidae is a Taxonomy (biology), taxonomic family (biology), family of sea snails, Marine (ocean), marine opisthobranch gastropod mollusks in the superfamily Murchisonelloidea.Bouchet, P. (2015). Murchisonellidae Casey, 1904. Accessed th ...

, and sea snails of the families Tjaernoeiidae and Parvaplustridae are their next-closest living relatives. Although rhodopids lack a fossil record themselves, murchisonellids have a fossil record dating back to the Triassic, suggesting that rhodopids also date back as far. They may be one of the oldest lineages of slugs. A

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleot ...

analysis suggested that the

stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

of rhodopids may have arisen in the late Paleozoic, with an average age estimate of 281 million years ago and 95% of their age estimates falling within a range of 339 and 217 million years ago.

References

{{Authority control Gastropods Heterobranchia