Isopod

The order Isopoda is used to refer to the group of crustaceans which mainly includes the woodlice and its relatives. Belonging to the phylum Arthropoda, these isopods dwell in the sea, fresh water or on land. Over 10,000 species of the order isopod are found worldwide, out of which, around 4,500 species are found in marine environments, i.e., on the seabed and 500 species in freshwater. The remaining  5,000 species are found to dwell on land. The order is divided into eleven suborders. The word Isopoda is derived from the Greek words iso- (meaning "equal") and -pod (meaning "foot"). The fossil record of isopods dates back to the Carboniferous period, which is about 300 million years ago, when the isopods lived in shallow seas. 

This page will describe the order Isopoda and its characteristics. Students can refer to this page to have a clear understanding on Isopod meaning, their classification and behaviour.

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All the isopod species have rigid and segmented exoskeletons. They also consist of two pairs of antennae along with seven pairs of jointed limbs on the thorax. There are a total five pairs of branching appendages on the abdomen which are used by the isopods during respiration. The female isopods brood their young ones in a pouch under their thorax.

The isopods are known for their varied feeding habits, some survive on dead or decaying plant and animal matter, while the others are grazers or filter feeders. Few of the isopod species are predators and sometimes are internal or external parasites (mostly of fish). The aquatic isopod species mostly dwell on the seabed or bottom of freshwater bodies, while some of them can swim for a short distance. The terrestrial species move around by crawling and are mostly found in cool and moist places. It has also been found that some of the species has the ability to roll themselves into a ball as a source of defence mechanism or for conserving moisture.

Characteristics of Isopods

Some of the general characteristics of the Isopods are:

1. They are classified under the arthropods and have a chitinous exoskeleton and jointed limbs.

2.  Isopods are typically flattened dorsoventrally. They have varying body colour, which can be mostly grey to white, or in some cases red, green, or brown.

3. Isopods also have varying size, which ranges from just 0.3mm (some Microcerberidae species) to nearly 50 cm (the deep sea Bathynomus spp.). 

4. Isopods lack an obvious carapace (shell), which is reduced to a "cephalic shield" covering only the head. This means that the gill-like structures, which in other related groups are protected by the carapace, are instead found on specialised limbs on the abdomen.

5. They have a series of overlapping, articulated plates on the dorsal (upper) surface of their body which give protection and provide flexibility. 

6. The body structure of an isopod consists of a head (cephalon), a thorax (pereon) with eight segments (pereonites), and an abdomen (pleon) with six segments (pleonites), some of which may be fused.

7. The head of an isopod is fused with the first segment of the thorax to form the cephalon.

8. Two pairs of unbranched antennae can be found in this species. The first pair being vestigial in land-dwelling species. 

9. The eyes are compound and unstalked and the mouthparts include a pair of maxillipeds and a pair of mandibles (jaws) with palps and spine-like movable appendages.

10. There are seven free segments of the thorax, each bearing a pair of unbranched pereopods (limbs), which are used for locomotion.

Evolution of Isopods

The first fossil record of Isopods was discovered during the Carboniferous period (of the Paleozoic) which was around 300 million years ago. These species were primitive and short-tailed and were originally members of the suborder Phreatoicidea, which consisted of the marine organisms and had a cosmopolitan distribution. Today, these members of suborder Phreatoicida are widely distributed populations that are found in freshwater environments of South Africa, India and Oceania, the greatest number of species being in Tasmania. Asellota, Microcerberidea, Calabozo Idea and the terrestrial Oniscidea are some of the similar suborders which includes the primitive and short-tailed organisms.

The short-tailed isopods are found to have a sedentary lifestyle on or under the sediments of seabeds and consist of a short pleotelson and stylus-like uropods. The long-tailed isopods, on the other hand, have a long pleotelson and broad lateral uropods which are used for the swimming purpose. They are considered to be much more active than the short-tailed isopods as they have the ability to launch themselves off the seabed and can swim for short distances. The short-tailed forms of isopods were assumed to have driven from the shallow seas due to the increased predatory pressure from the marine fishes, which were their main predators. Also, the development of the long-tailed species have also resulted in a competition that forced the short-tailed forms to move into these surfaces. Therefore, as a result, the short-tailed forms are now mostly restricted to deep sea, freshwater, groundwater and dry land. Isopods belonging to the suborder Asellota are by far the most species-rich group of deep sea isopods.

Classification of Isopods

Isopods are placed under the group Peracarida, which consists of organisms that are grouped together due to a common feature. This common feature includes the presence of a special chamber below the thorax for brooding their young ones. The isopods have a cosmopolitan distribution with more than 10,000 species which are further classified into 11 suborders. 

Out of the 10,000 species of isopods, nearly 4,500 species are found in marine environments and on the seafloor. Around 500 species are found in freshwater, while the remaining 5,000 species form the terrestrial woodlice, which are included under the suborder Oniscidea.

The order Isopoda is further divided into the following 11 suborders:

1. Asellota - It contains the superfamilies Aselloidea, Stenetrioidea, Gnathostenetroidoidea and Janiroidea, All these superfamilies mainly include the freshwater isopods, marine and deep sea members.

2. Calabozoida - It is a small suborder consisting of two marine species of the family Calabozoidae and one freshwater species belonging to the family Brasileirinidae.

3. Cymothoidae - Includes over 2,700 species of marine isopods. They are mostly carnivorous or parasitic, belonging to the family Gnathiidae. They also include the former suborder Anthuridea, i.e., a group of worm-like isopods with very long bodies.

4. Limnoriidea - They are mainly the tropical isopods, some of which are herbivorous.

5. Microcerberidea - They are the tiny, worm-like isopods that dwell between particles on the bed of freshwater and shallow marine habitats.

6. Oniscidea - They consist of over 4,000 species of woodlice isopods inhabiting the forests, mountains, deserts and the littoral zone.  They are the semi-terrestrial and terrestrial isopods that are fully adapted for life on land.

7. Phoratopidea - It consists of a single marine species known as Phoratopus remex, having unique characteristics.

8. Phreatoicidea - They are known to resemble Amphipods and include the freshwater isopods which are limited to South Africa, India, Australia and New Zealand.

9. Sphaeromatidae - They are the Benthic isopods mostly found in the southern hemisphere, consisting of respiratory pleopods inside a branchial chamber. This suborder also includes part of the formerly recognised suborder Flabellifera.

10. Tainisopidea - Includes the freshwater isopods which are found in the relictual environment.

11. Valvifera - They are a large group of benthic and marine isopods having respiratory pleopods inside a branchial chamber found below their abdomen.

The members of the suborder Asellota are predominantly found in the deep sea and have undergone large adaptive radiation in that environment. Isopods belonging to the genus Bathynomus are considered to be the largest. Some of these large species are even fished commercially as human food in Mexico, Japan and Hawaii.

Some of the isopod groups have developed as a parasitic lifestyle, most specifically as external parasites of fish. This, as a result, can damage or kill their hosts causing a significant economic loss to commercial fisheries. In reef aquariums, these parasitic isopods can also develop as a pest and can endanger the fish, possibly injuring the aquarium keeper as well. 

Some of the Isopod members belonging to the family Cirolanidae suck the blood of fish, while those belonging to the family Aegidae, consume the blood, fins, tail and flesh of the fish, thereby killing the fish in the process.

Isopods - Food Habits and Nutrition

Isopods are known for their varying food habits and feeding system. They can be detritivores, browsers or carnivores (including predators and scavengers). While, some of them are also found to be parasites and filter feeders. Each of the isopod species, can therefore, occupy one or more of these feeding niches.  But mostly, the aquatic and marine isopod species are known to be parasites or filter feeders.

The terrestrial species, on the other hand, are in general found to be herbivorous, with the majority of woodlice feeding on moss, bark, algae, fungi and decaying material. In most of the marine isopods which are feeding on wood, the cellulose is found to be digested by some enzymes secreted in the caeca. For example, Limnoria lignorum, digs a hole into the wood and feeds on the mycelia of fungi, which as a result, increases the nitrogen in its diet. Land-based wood-borers mostly house symbiotic bacteria in the hindgut which aid in digesting cellulose. 

Isopods are known for having a simple gut lacking a midgut section. Instead, they have caeca which are connected to the back of the stomach, where the process of absorption takes place. The food which is sucked into the esophagus is passed by peristalsis into the stomach, where the food material gets processed and filtered. This process can be mostly found in the blood-sucking parasitic species of Isopods. The structure of the stomach varies in each isopod species. In most of the cases, it consists of a dorsal groove into which indigestible material is channelled. It also consists of a ventral part which is connected to the caeca where the process of intracellular digestion and absorption takes place. All the indigestible materials are passed on through the hindgut and get eliminated through the anus, which is located on the pleotelson. There are numerous adaptations to this simple gut, but these are mostly correlated with their diet rather than the taxonomic group.

Locomotion of Isopods

The marine and freshwater isopods are entirely benthic, unlike the amphipods. As a result, they get a little chance to disperse to new regions and therefore, many isopod species are endemic to restricted ranges. The primary means of locomotion found in the isopod is crawling, and some species bore into the seabed, the ground or timber structures. Most terrestrial isopod species conceal/ hide under objects in crevices or under the bark and are often slow-moving. The semi-terrestrial sea slaters (Ligia spp.) can run rapidly on land and many terrestrial species can roll themselves into a ball when threatened, a feature that has evolved independently in different groups and also in the marine sphaeromatidae. Some members of the Flabellifera can swim to a limited extent and have their front three pairs of pleopods modified for this purpose, with their respiratory structures limited to the hind pleopods.

Reproduction in Isopods

In most of the Isopod species, the sexes are separate and have a little sexual dimorphism. But some of these species are found to be hermaphroditic and some parasitic forms of isopods also showed large differences between the sexes.  

The Cymothoidans are considered to be protandrous hermaphrodites, which initially start their life as males and later changes their sex as per adaptability. The Anthuroideans, on the other hand, undergo a reverse process, i.e., they are protogynous hermaphrodites who are born female and undergo changes later. Some Gnathiidans males are sessile and live with a group of females.

The male isopods have a pair of penises, which may be fused in some species. The sperm is transferred to the female isopod species through the modified second pleopod which receives it from the penis and is then inserted into a female gonopore. This sperm is stored in a special receptacle, which is a swelling on the oviduct close to the gonopore. Fertilisation only takes place when the eggs are shed soon after a moult, at which time a connection is established between the semen receptacle and the oviduct.

The females lay up to 100 eggs, which are brooded by them in the marsupium (a chamber formed by flat plates known as oostegites) under the thorax. This marsupium is filled with water even in terrestrial species. The eggs hatch in the form of mancae, which is a post-larval stage resembling the adult except for the absence of the last pair of pereopods. The lack of a swimming phase in the life cycle is a limiting factor in isopod dispersal and may be responsible for the high levels of endemism in the order. An adult isopod differs from other crustaceans in the moulting process. In isopods, the moulting occurs in two stages known as "biphasic moulting". In the first stage, they shed the exoskeleton from the posterior part of their body and shed the anterior part later in the second stage. The giant Antarctic isopod Glyptonotus antarcticus is an exception and moults in a single process.