The sea cockroaches or chitons are marine mollusks characterized by presenting an oval body, depressed dorsoventrally and protected by eight imbricated plates, a thick belt and a flat and wide foot that allow it to adhere strongly to the substrate that can be a rock or the shell of another organism.
These mollusks were previously classified within the defunct Amphineura group, but are now recognized as a class (Polyplacophora). They are distributed worldwide, but are more abundant and diverse in the rocky intertidal zone of tropical waters.
Most of the species are dioecious, with external fertilization, in the water column or in the paleal furrow of the female and the eggs develop in the water column. Development is indirect and consists of a trochophore larva, but lacks a velíger larva..
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The body is oval and dorsoventrally depressed, covered dorsally by eight plates (very rarely seven) called cerramas, which are arranged in an imbricate manner. The presence of these plates is what gives rise to the name of the group, Polyplacophora or carrier of many shells.
The edge of the mantle is very thick and covers laterally or totally the plates forming the belt, which is covered by a delicate cuticle that can be smooth or ornamented with scales, thorns or calcareous spicules..
The foot is muscular, flat and very wide, occupying a large part of the ventral surface of the body. This foot, together with the belt, are adapted to create a vacuum and adhere strongly to the substrate. The foot is also used for movement.
Cephalization is little marked in this group and the organisms lack tentacles and eyes, although the latter are present during the larval stage..
Sea cockroaches lack the crystalline stylet, a rod-shaped matrix of proteins and enzymes, which aids in the digestion process and is common in other classes of mollusks..
The gills are numerous and are found in number ranging from 6 to 88 pairs, arranged in rows in the paleal cavity on each side of the body. The total number of gills can vary depending not only on the species but also on the size of the organism.
Another characteristic of this group is the absence of the velíger larva, one of the larval stages that characterize mollusks in general..
Sea cockroaches belong to the phylum Mollusca and were first described by Carlos Linneo in 1758. In traditional taxonomy, these organisms were located in the Amphineura class, however, currently this taxon is not valid..
By eliminating the Amphineura class, the Polyplacophora, Solenogastres and Caudofoveata that were contained in it were elevated to the category of class.
The class Polyplacophora was erected by Henri Marie Ducrotay de Blainville in 1816 and currently has about 800 described species located in the subclass Neoloricata and the orders Chitonidae and Lepidopleurida, while the subclass Loricata is considered synonymy of Polyplacophora.
Sea cockroaches are exclusively marine organisms, there is no species that has managed to adapt to brackish or fresh waters. They live attached to hard substrates, such as rocks or shells of other organisms.
Most species inhabit the rocky intertidal zone, where they can withstand long periods of exposure to air, or in the subtidal zone. However, there are also some species that inhabit deep waters.
Chitons are distributed worldwide from warm tropical waters to cold waters.
To feed, sea cockroaches use their radula, an organ in the shape of a belt or ribbon armed with rows of teeth. The anterior teeth are used and later discarded or displaced by another group of teeth with an offset like that of a conveyor belt.
Some teeth are hardened by a substance called magnetite, which makes them harder than steel. Depending on the species, it feeds by scraping the film of microalgae that grows on the surface of the rocks where it lives, from pieces of algae or from colonies of sessile animals such as bryozoans..
It can also feed on sponges, while others can feed on the microfauna that grow on rocks. There are even some species that feed on tree trunks that have sunk and rest on the great ocean floors. Most of the species with this type of diet belong to the genera Ferreiraella, Nierstraszella Y Leptochiton.
At least three genera of sea cockroaches (Placiphorella, Loricella Y Craspedochiton) are predators of amphipods and other organisms. Members of the genus Placiphorella use their front end, which is raised and bell-shaped, to catch prey.
Most species of sea cockroaches are dioecious or gonochoric organisms, that is, they have separate sexes. Only two species of the genus Lepidochitonia they are hermaphrodites, L. fernaldi Y L. cavern.
Chitons lack copulatory organs and fertilization is generally carried out in the water column, after both sexes release the gametes into the sea. In these cases, the fertilized eggs are small and develop in the water column until the trochophore larva hatches..
Few species deposit their eggs in a mass or row of mucus that fixes to the substrate, among the species that have this reproductive strategy are, for example Chryptochiton stelleri Y Callochiton achatinus.
In other species, fertilization occurs in the female's paleal cavity. In these cases, the female can take care of the eggs in said cavity, releasing the trochophor larvae once hatching occurs, or they can retain them even longer and release them when they are in the final stage of their development..
The species Calloplax vivipara it was called by that specific epithet as it was believed to be indeed a viviparous species and was for a long time the only species with this type of known reproductive strategy. However, later studies managed to show that it was another species that conserved the young in the paleal cavity..
The eggs of sea cockroaches have a typical spiral cleavage that leads to the trochophore larva, which hatches and continues its development outside the egg, but without acquiring any exogenous food, but instead feeds on the accumulated yolk, that is, they are lecithotrophic species.
The trochophore larva then transforms into a juvenile, bypassing the velíger stage.
This is the largest species of polyplacophore that currently exists, being able to reach up to 36 cm in length and more than two kilograms in weight. Apart from its large size, it is easily distinguishable from other species of chitons because the mantle completely covers the shell plates. Its color varies from reddish brown to orange.
It is a nocturnal organism that feeds on microalgae that scrapes from the surface of rocks, as well as some macroalgae such as Ulva Y Laminaria.
Cryptochiton stelleri It lives in the low intertidal zone and in the subtidal zone of rocky coasts. Its distribution covers the North Pacific, from California to Alaska in North America, the Kamchatka Peninsula and southern Japan in Asia, including the Aleutian Islands..
This species has few natural enemies, among which are the snail Lurid ocenebra, the starfish Pisaster ochraceus, some species of octopus and otters. Researchers have estimated its longevity at about 40 years.
Cryptochiton stelleri It is used as food by the inhabitants of some original North American tribes, as well as by Russian fishermen. However, its taste, smell and texture are not considered very pleasant..
Species commonly known as ghost chiton or phantom chiton because it easily goes unnoticed due to its color pattern that allows it to blend in with the rocks where it lives..
This species can reach up to 7 cm in length and has very thick, eroded or granulated plates and generally colonized by barnacles. The belt is thickly covered with calcareous spicules. The greenish-brown coloration with white spots is similar to the coloration of the stones where it lives.
This species is typical of the islands of the Caribbean Sea up to Trinidad. In the American continental territory it is distributed from Florida (USA) to Venezuela, including Mexico, Honduras, Colombia, among others..
The foot of the organisms of this species is considered edible in the islands of the Caribbean Sea and is also used as bait for fishing.
The coloration, despite the common name, is generally uniform green or brown, and blue or greenish-blue coloration is less frequent. It lives in the intertidal zone and can generally be found in tidal pools.
It is also common in estuaries, where it lives among the shells of oysters and other bivalves or among rocks. It is also capable of surviving in muddy areas. It can also survive in lightly polluted areas.
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