7 junio, 2024

Sea cockroach: characteristics, habitat, species, reproduction

The sea ​​roaches or chitons are marine mollusks characterized by presenting an oval body, dorsoventrally depressed 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 extinct Amphineura group, but are currently recognized as a class (Polyplacophora). They are distributed worldwide, but are more abundant and diverse in the rocky intertidal zone of tropical waters.

Its size generally ranges from 3 to 12 cm long, although some species can measure up to 40 cm. They generally feed on algae and small animals that they scrape from the substrate using the radula, an organ in the form of a membranous ribbon laterally armed with numerous teeth.

Most of the species are dioecious, with external fertilization, in the water column or in the female’s paleal groove, and the eggs develop in the water column. Development is indirect and consists of a trochophore larva, but lacks a veliger larva.

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Characteristics of sea roaches

The body is oval and dorsoventrally depressed, dorsally covered by eight plates (very rarely seven) called ceramae, which are arranged imbricate. The presence of these plates is what gives rise to the name of the group, Polyplacophora or bearer of many shells.

The edge of the mantle is very thick and laterally or completely covers the plates, forming the belt, which is covered by a delicate cuticle that can be smooth or ornamented with scales, spines, or calcareous spicules.

The foot is muscular, flat and very wide, occupying a large part of the ventral surface of the organism. 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 poorly marked in this group and the organisms lack tentacles and eyes, although the latter are present during the larval stage.

Sea roaches lack the crystalline stylet, a rod-shaped matrix of proteins and enzymes that aids in the digestion process and is common in other classes of molluscs.

The gills are numerous and are found in numbers 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 veliger larva, one of the larval stages that characterize molluscs in general.

taxonomy

Sea cockroaches belong to the phylum Mollusca and were first described by Carlos Linnaeus in 1758. In traditional taxonomy, these organisms were placed in the class Amphineura, however, this taxon is currently invalid.

By removing the class Amphineura, the Polyplacophora, Solenogastres, and Caudofoveata that were contained in it were elevated to class status.

The Polyplacophora class was erected by Henri Marie Ducrotay de Blainville in 1816 and currently has about 800 described species located in the Neoloricata subclass and the Chitonidae and Lepidopleurida orders, while the Loricata subclass is considered synonymous with Polyplacophora.

Habitat and distribution

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 of the species live in the rocky intertidal zone, where they can withstand long periods of exposure to the 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.

Feeding

To feed, sea roaches use their radula, a belt-shaped organ armed with rows of teeth. The anterior teeth are used and subsequently discarded or displaced by another set of teeth with a movement 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 and Leptochiton.

At least three genera of sea roaches (Placiphorella, Loricella and craspedochiton) are predators of amphipods and other organisms. members of the genus placiphorella they use their anterior end, which is elevated and bell-shaped, to catch their prey.

Reproduction

Most species of sea cockroaches are dioecious or gonochoric organisms, that is, they have separate sexes. Only two species of the genus lepidochithonia they are hermaphrodites L. fernaldi and L. cavern.

Chitons lack copulatory organs and fertilization generally takes place 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 hatching of the trochophore larva.

Few species deposit their eggs in a mass or row of mucus that they fix to the substrate, among the species that have this reproductive strategy are for example Chryptochiton stelleri and Callochiton achatinus.

In other species, fertilization occurs in the female’s paleal cavity. In these cases, the female can guard the eggs in the cavity, releasing the trochophore 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 named with that specific epithet because 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 kept its young in the paleal cavity.

The eggs of sea roaches 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 veliger stage.

Featured species of sea roaches

Cryptochiton stelleri

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 chiton species because the mantle completely covers the shell plates. Its coloration varies from reddish brown to orange.

It is a nocturnal organism that feeds on microalgae that it scrapes from the surface of rocks, as well as some macroalgae such as Ulva and laminaria.

Cryptochiton stelleri It lives in the low intertidal zone and in the subtidal zone of rocky shores. Its distribution spans 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 ocenebrathe starfish Pisaster ochraceus, some species of octopus and otter. Researchers have estimated their longevity at about 40 years.

Cryptochiton stelleri It is used as food by the inhabitants of some native North American tribes, as well as by Russian fishermen. However, its taste, smell and texture are not considered very pleasant.

Acanthopleura granulata

Species commonly known as ghost chiton or ghost 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 and eroded or granulated plates and is generally colonized by barnacles. The belt is densely 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.

Chiton glaucus

This species is known as the blue chiton or blue-green chiton. It is one of the most common species in New Zealand, although it is also observable in Tasmania. Its size can reach 55 mm in length. It is characterized by presenting a crest that runs dorsally over the valves and by presenting the belt covered with scales.

Coloration, despite the common name, is generally uniform green or brown, with blue or blue-green coloration less common. It inhabits the intertidal zone and can usually be found in tidal pools.

It is also frequent 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 contaminated areas.

References

RC Brusca, GJ Brusca (2003). Invertebrates. 2nd Edition. Sinauer Associates, Inc.
B.Baur (1998). Sperm competition in molluscs. In TR Birkhead & AP Moller. Sperm competition and sexual selection. Academic Press.
B. Sirenko (2004). The ancient origin and persistence of chitons (Mollusca, Polyplacophora) that live and feed on deep submerged land plant matter (xylophages). Malacological Bulletin, Rome.
EE Ruppert & RD Barnes (1996). Invertebrate zoology. Sixth edition. McGraw-Hill Interamericana.
B.I. Sirenko (2015). The enigmatic viviparous chiton Callopax vivipara (Plate, 1899) (Mollusca: Polyplacophora) and a survey of the types of reproduction in chitons. Russian Journal of Marine Biology.
Hush. On Wikipedia. Retrieved from: en.wikipedia.org.
Gumboot chiton. On Wikipedia. Retrieved from: en.wikipedia.org.

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