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Ctenoides mitis
Ctenoides mitis, sometimes referred to as smooth file clam or smooth flame scallop, is a marine species of Bivalvia in the family Limidae, the family of file clams or file shells or flame scallops. Despite the vernacular name “smooth flame scallop”, C. mitis is not closely related to Pectinidae (scallops), being in a different order.

Classification
Kingdom: Animalia

Phylum: Mollusca

Class: Bivalvia

Subclass: Pteriomorphia

Order: Limida

Superfamily: Limoidea

Family: Limidae

Genus: Ctenoidens

Species: C. mitis

Nomenclature
There has been much confusion in the nomenclature and classification of C. mitis, it has been given multiple synonymised names, its genus has been disputed, and it has even been described and discussed as two different species (i.e. Chemnitz (1784) and G.B. Sowerby II (1843)). Some modern authors still disagree with its present status as a species, considering it to be a form, variety, or subspecies of Ctenoides scaber ; however, phylogenic studies support the species status of C. mitis.

The Ctenoides mitis presently accepted by the World Registry of Marine Species was first described by Lamarck in 1807, but by the name of Lima mitis. It was later identified under the genus of Ctenoides, as this genus was described by Mörch (1853).

Synonymised names:
Lima floridanus - Olsson & Harbison, 1953

''Lima glacialis var. b.'' - Lamarck, 1819

Lima mitis (Lamarck, 1807)

Lima tenera - Chemnitz, 1784 (unavailable name, published in a work rejected by ICZN Direction 1)

Lima tenera – G.B. Sowerby II, 1843 (preoccupied by Lima tenera - Turton (1825), which is currently accepted as Limaria hians – W. Turton, 1825)

Etymology
The genus epithet, Ctenoides, illustrates the comb like appearance of the bivalve due to its tentacles (Greek, kteis or ktenos = comb + -oides suffix from εἶδος (eidos) = form  or likeness). While the specific epithet mitis, Latin for “mild” or “gentle”, may refer to the finely scaled radial ribs of the mollusc’s shell; that give the shell a smoother feel than that of the closely related rough file clam Ctenoides scabra.

Description
The calcareous shell is white in colour, with a light brown periostractum (outer protective skin). In shape it is oval, narrowing towards the beak, only slightly inflated (average shell height = 80.2mm) and anteriorly and posteriorly gaping. It is sculped with approximately 90  fine, scaled radial ribs (range 59–149, mean = 88.8 ± 12.5 s.d.) that diverge in the center, and transversely grooved interspaces. It has slight auricles on the sides of the beak and the interior is smooth and shiny. The larval shell (prodissoconch) is 160-170μm in maximum length. The bivalve’s mantle, gills and smooth pallial veil are reddish orange ; the pallial tentacles, characteristics of Limida, are long and predominantly white, though colorations of light orange or white with red bands have been found, the latter coloration possibly only occurring in juveniles. The eyes are as in C. scaber, bright orange-pink and 18 per side (in 51.3mm specimen).

C. mitis is often misidentified for C. scaber, as the two species are very similar. C. mitis is overall smaller, with a mean body length of 36.5mm (± 17.7mm s.d.), average body volume of 72.5cm3 and average wet body mass of 125g. In contrast to C. scaber, the shell of C. mitis has more numerous, finer radial ribs : C. scaber only has 50 ribs, with wedge shaped spines. The classical distinction between the two species used to be, and still often is, based on C. scaber having red pallial tentacles and C. mitis white pallial tentacles. But, morphological analyses have showed that colour alone cannot be used to distinguish the two molluscs: although most C. mitis specimen have solid white tentacle’s, two other colour variations exist, light orange and white with red bands in jouveniles. C. scaber mainly has dark orange to red tentacles, but white tentacles may also occur. A potential difference in the prevalence of white tentacles in C. mitis may be the result of fewer carotenoids in the diet, the organic pigment that results in the red coloration.

A possible evolutionary hypothesis on the different rib number in C. mitis and C. scaber is related to predator defence mechanisms. The bright red coloration of C. scaber may be an aposematism, a warning coloration for distasteful taste; due to the lack of brightly coloured tentacles, C. mitis may have developed a higher shell rib count to increase shell strength. Still, the two species have overlapping distributions and highly similar microhabitats, hence, it is difficult to state what ecological factors may have driven divergent selection between the two species.

Anatomy
The bivalve is a Monomyarian, meaning it has a single posterior adductor muscle; with smaller pedal retractor muscle and ventral retractor muscle. The animal is surrounded by multiple rows of papillose (non-annulated) pallial tentacles, of which the innermost relatively long. The eyes are evenly dispersed at mantle edge, between tentacles, each surrounded by translucent tissue; at the sight of predators, the file clam responds with swimming. The pallial veil is smooth to finely granular, extending from mantle edge inside tentacle rows to the full span opening of the (gasping) shell. Ctenidia are expansive and accompanied by complex aired lips and labial palps. The visceral mass of the mollusk contains a stomach placed dorsally, broadening ventrally to contain wide, 8-shaped intestinal loop. The foot is relatively large, with an unpigmented, transversely ridged “sole” at tip, while the large byssal is at the more dorsal “heel”. Byssal threads originate from the byssal gland. The anal papilla hangs free postereoventrally, behind the adductor muscle.

Swimming
Members of Limidae are epifaunal, meaning they may attach themselves to such surfaces or also range freely over them, by crawling or swimming. In fact, Limidae is amongst the few orders of Bivalvia in which swimming has evolved. Though the energy cost of transport is high, possibly due to cost of carrying shell, some species are even capable of swimming for even extended periods. Such as Limaria fragilis, can swim for periods of 15min or more, and the swimming is mostly aerobic. File shells swim by quickly adducting their valves, to produce posterior propulsive jets of water: this results in a swimming posture where their ventral shell margin stays forward and the dorsal hinge backwards. The thin shell is thought to decrease sinking rate, when the animal stops “valve-clapping”. The long pallial tentacles of the file shells are also used during swimming, by producing thrust or participating in a rowing motion. Still, swimming in Limidae is still not well understood.

The primary function of pallial tentacles is for defense against predation. These tentacles are covered in a sticky, thick mucus, which is distasteful to predatory fish; when attacked, the bivalve is capable of autotomizing its tentacles.

Phylogeny and Evolutionary History
The oldest occurrences of Limidae fossils were found in China and Germany, and date back to the Late/Upper Famennian period, 364.7-360.7 Ma. Following millions of years of adaptive radiations, the Ctenoides genus first appeared 163.5 - 145.0 Ma, in the Oxfordian-Tithonian period in present day Japan. It is less clear when exactly Ctenoides mitis came to be, mainly due to the difficulties in differentiation between the presumably sister species C. mitis and C. scaber. Yet, phylogeny analysis studies support that  C. mitis and C. scaber are recently diverged species. The oldest fossils labelled Ctenoides scaber occurred in the Burdigalian period, 20.44 - 15.97 Ma (Paleobiology Database, 2019), whilst C. mitis’s first stratigraphic occurrences are from the Late Pliocene.

Distribution and Habitats
C. mitis occurs in the Northern Hemisphere, in the tropical and subtropical waters of the Gulf of Mexico and Caribbean sea (see Figure Y). Specimen have been found in North and South Carolina, south-eastern to north-western Florida (including  the   Florida   Keys), Alabama, Louisiana, Texas, Bermuda, Bahamas, Greater and Lesser Antilles, Caribbean, Central and South America (to Venezuela). As of depths, C. mitis range from the shallow subtidal to depths of 53m. Exceptionally, living specimen have also been collected from the complex reef habitat of the West Flower Gardens, northern Gulf of Mexico, at around 140m deep. There have also been records from down to 525m, but there were empty shells or unspecified.

In their adult stage, C. mitis are epifaunal, normally living byssaly attached to some sort of rubbery stratus and swimming only if disturbed. They are found in crevices of reefs and ledges, under or among rocks, and attached to sponges or coral or turtlegrass (Thalassia testudinum).

Ecology
Like most bivalves, Ctenoides sp. are suspension feeders, feeding mainly on phytoplankton (Dukeman et al., 2005). They use their large gills, located within the pallial cavity, to capture food particles suspended in the water. Phytoplankton are rich in β-carotene, which is converted to vitamin A. β-carotene is important in the diet of C. mitis, and results in the red and orange colorations of the mollusc. An essential requirement of bivalves is lipids; lipids have an important role in mollusc reproduction, affecting gamete quality and larvae survival.

Smooth file clams are predated on mainly by predatory fish, starfish and crustaceans. Due to the epifaunal nature of the animal, its byssal threads are very particular in not being thick and strong but very stretchable. It uses the byssal threads only for temporary attachment, not for predatory resistance; stretchy threads of intermediate thickness makes C. mitis able to hang inside crevices, ready to drop and swim away on disturbance.

Relationship with Humans
Due to their relatively large size, colourful mantle and pallial tentacles, and propensity for active swimming when disturbed, file shells have become amongst the most popular bivalves as elements of private and museum shell collections, marine aquaria, scuba dive logs and colour-illustrated  publications   on   living molluscs. However, this aquarium celebrity status has led to overcollecting, with reported annual collections of more than 50000 C. scaber and C. mitis ; the target tends to be C. scaber, but there is difficulty in differentiating it from C. mitis. Due to such ferocious harvesting, the State of Florida has imosed daily bag limits for the aquarium trade of C. scaber.