INTRODUCTION TO ISOPODA

Isopods are paradoxical for being simultaneously extremely obscure and also extremely charismatic. Ask any random person off the street what an isopod is and they would likely not know, yet within a smaller subset of people isopods are very popular to the point of creating a large invertebrate keeping industry around them. However, outside a very niche group of scientists, the only isopods that most people know to exist are the terrestrial isopods (Oniscidea) and giant isopods (Bathynomus). This is only a small fragment of the full diversity of isopods, with a fascinating array of fantastic and sometimes outright bizarre forms lost in a maze of endless scientific articles. The goal of this guide is to bring this unknown diversity out of extreme obscurity and to allow more people to explore the rich and utterly bizarre isopod fauna of North America.

TAXONOMY

Taxonomy is the study of sorting earth’s biosphere into discrete categories (or boxes), with the general idea of closer-related organisms sharing lower boxes. The main goal of taxonomy is to achieve monophyletic groups, where a group contains a common ancestor and all of its descendants, in contrast to paraphyletic groups, where a common ancestor and only some of its descendants are included, or polyphyletic groups, where the grouped organisms are seemingly random with no shared common ancestor (unless you look really far back). Taxonomy rides on descriptions of organisms (i.e. a descriptive science), which are a list of traits that the organism has. Traits can be divided into plesiomorphies, which are traits ancestral to a group, and apomorphies, which are derived traits that occur in a group, with an apomorphy defining a group called a synapomorphy. What traits are considered plesiomorphies and apomorphies is based on the context it is being used. For example, a chitinous jointed exoskeleton is a plesiomorphy for isopods, but in the context of all arthropods it is an apomorphy. A good example of a synapomorphy in Isopoda is biphasic molting (the front and back halves molt at different times), as it an important defining characteristic of the order. Sometimes a synapomorphy of a group will become lost in a highly derived subgroup; the presence of only 1 pair of uropods can be considered a synapomorphy for Isopoda, but they have reverted back to pleopods in Anuropidae and completely lost in Paravireia.
Taxonomy is currently split into two major directions: classical Linnaean Taxonomy, which groups organisms in hierarchical sets of nesting boxes called taxa (sing. taxon), and more modern cladistics, which divides organisms into discrete sets of nesting groups called clades. Typically, a combination of the two systems is used, with clades being given a taxonomic name to facilitate the understanding of a group. In Isopods, the largest clade that contains all isopods is currently set at the order level, with major subordinate clades being placed at the suborder level.

[drawings of concepts]

Unfortunately, taxonomy is very far from being a perfect science, and isopods are a prime example of this. Many taxa within Isopoda are very poorly defined, with many classic taxa likely being paraphyletic or even polyphyletic. The Taxonomic Guide attempts to organize our current knowledge of the relationships between different groups of isopods avaliable in current literature, but it is very likely that this organization will change dramatically in the future.

SUBORDERS

The order Isopoda is currently arranged into 12 suborders. They range in size from the monospecific Phoratopidea (only known from 2 specimens of 1 species off the coast of Australia) to Oniscidea, which contains almost a fourth of the total global Isopod diversity. Most of the suborders are generally easily recognizable from each other, with charismatic gestalts that allow for them to be instantly recognizable from each other in most cases, and even where differentiation is more difficult (i. e. Asellota vs Microcerberidea), specific characters can still facilitate identification. A short summary of each are provided below:

• Phreatoicidea Stebbing, 1893 (Grigslaters): at least 113 species in about 6 families found only in freshwater environments in Australia, New Zealand, South Africa and India. These have a very odd appearance for an isopod, with pereonite-like pleonites, a laterally compressed body, free ring-like coxal plates and other features that make them resemble amphipods more than isopods, although their only 1 pair of uropods separates them from the former. Despite the modern distribution, this suborder is not a Gondwanan endemic, with fossils indicating a former cosmopolitan distribution in both fresh and marine waters. The modern taxa may be a Gondwanan branch of the former diversity of this suborder, though, with genera on some continents being closer-related to ones on other continents than genera on their own continent.

• Asellota Latreille, 1802 (Asellotes): over 2300 species in 4 superfamilies and an undetermined amount of families worldwide in marine environments and bipolar-temperate in freshwater environments. Members of Asellota are generally flattened with few free pleonites and a huge telson with the uropods set at the tip, but this basic bauplan is twisted in often spectacular ways. Members of this suborder can resemble less like isopods and more like dream entities, especially in the massive abyssal radiation in Janiroidea. Interestingly, many presumably basal groups (the exact arrangement is still undetermined) are entirely freshwater, indicating that freshwater branches may survive better when new radiations push out older taxa from their former prime.

• Microcerberidea Lang, 1961 (Cerberuses and Pillslaters): about 50 species in 2 families with a patchy worldwide distribution. The two families in this suborder share the same exact basic bauplan as Asellota and from a technical standpoint can be somewhat difficult to separate from the latter. However, differentiating the two suborders is vastly easier when the two families of Microcerberidea are take to account: Atlantasellidae consists of two bizarre conglobulating isopods endemic to caves under Bermuda and Hispaniola, while Microcerberidae consists of extremely elongate interstitial and groundwater isopods. Some Asellote families (most noticeably Lepidocharontidae and Angerliera in our area) look somewhat similar to the latter but always have some differentiating features at the family level.

• Calabozoidea Van Lieshout, 1983 (Slaterlice): only 3 species in 2 families known, all endemic to tiny areas in freshwater subterranean caves in South America. Members of this suborder have an assorted mash of traits from both Asellota + Microcerberidea and Oniscidea, sharing the pleon reduction and huge shield-like telson of the former duo and the reproductive structures and mouthparts of the latter. Due to this, the exact placement of this suborder is unknown, and will likely remain unknown until a genetic study can be done with this suborder.

• Oniscidea Latreille, 1802 (Woodlice and allies): almost 4000 species in 5 major clades and an indeterminate number of families worldwide. This huge suborder contains all but 4 species of terrestrial isopods on the planet (the other 4 are narrow-ranged Phreatoicideans endemic to Australia), with a number of odd traits that have arisen due to their terrestrial habits. Oniscideans are unique in having their antenna 1 reduced to a minute structure not visible to the naked eye, a complex system of scales along the underside of their body that acts as a water transport system (and also allows them to drink out of their uropods!) and the modification of the pleopods in many groups into true lungs (although this is not universal).

• Tainisopidea Brandt & Poore, 2003 (Water-Earwigs): about 7 species in 1 family, entirely endemic to Western Australia (the state). Members of this suborder are odd looking, with effectively tri- or quadriramous pleopods (3-)4-5 and unusual ♂ genitalia along many traits that align them with both the Sphaeromatidea-Valvifera clade and the clade containing Phoratopidea, Limnoriidea, (part of) Cymothoida and Epicaridea. Tainisopidea likely represents a very early branch somewhere near the bases of those two clades, although which clade it is the closest to remains unknown.

• Valvifera Sars, 1882 (Valvetails): over 600 species in 11 families worldwide. This suborder is the smallest major suborder, having less species than some isopod families, but members are often common and conspicuous in shallow water environments, especially in temperate and polar regions. Valviferans can easily be told from other groups by the unique “double-door” uropods that completely enclose the branchial chamber. The presence of a branchial chamber, where the underside of the telson is domed, among other traits unites this suborder witn Sphaetomatidea

• Sphaeromatidea Wägele, 1989 (Seapills and Trisobites): over 800 species in 7 families worldwide. Members of this suborder are generally very compact and either flattened (Seroloidea) or domed (Sphaeromatoidea), united by similar mouthparts and specific details on the appendages, along with the same branchial chamber as in Valvifera. Both superfamilies have a strong center of diversity in the southern hemisphere, specifically around the Southern Ocean and Australasia. While Sphaeromatidae is represented by a fairly well-developed diversity in North America, Seroloidea barely reaches the region with only 4 species. Both superfamilies are highly charismatic, with the Seroloids closely resembling trilobites (to the point of being used as modern day analogs in some studies) and the Sphaeromatoids often being common and conspicuous in shallow water environment.

• Phoratopidea Brandt & Poore, 2003 (Oarsman): only 1 species (Phoratopus remex) known from the continental shelf south of Australia. The single member of this suborder is extremely enigmatic, only being known from two specimens. It has a unique pereopod morphology, with greatly reduced dactyls and flattened pereopods 3-4, and isn’t clearly in any other suborder currently recognized. Outside of its unusual morphology, this suborder is extremely enigmatic and not much else can be said about it.

• Limnoriidea Brandt & Poore, 2003 (Gribbles and allies): about 70 species in 3 families worldwide. This small suborder is mostly defined from mouthparts and is otherwise fairly similar to Cymothoida morphologically. The main family in Limnoriidea, Limnoriidae, are also known as Gribbles and are infamous for causing structural damage in seaside wooden structures constructed by humans, although most species mainly bore into driftwood or seagrass stems. The other 2 families are tiny, obscure and endemic to the Indopacific

• Cymothoida Wägele, 1989 (Wormpods and Fisheaters): over 2000 species in about 15 families worldwide. This huge suborder is divided into 2 very distinct superfamilies, Anthuroidea and Cymothooidea, and is tenuously held together by details in the mouthparts. Genetic evidence suggests that Cymothoida is not a natural group, with Cymothooidea likely consisting of a bizarre basal grade of isopods with entire subfamilies nestling within commonly accepted genera, although taxon sampling for genetic studies remain low and likely rely on less ideal stretches of genetic code for phylogeny in isopods. The Anthuroids are extremely long and slender and highly modified for a fossorial ecology, with the uropods and telson modified into a complex flower-like digging apparatus that allows them to dig both forwards and backwards. The Cymothooids on the other hand are usually squat and ovate and are highly predatory or parasitic, often inducing a painful bite when picked up by naturalists.

• Epicaridea Latreille, 1825 (Eldritch Isopods): about 900 species in at least 13 families worldwide. Members of Epicaridea are parasites on crustaceans are are so heavily derived for this mode of life that many cease to resemble isopods. Epicarideans also have a bizarre 4-part life cycle involving a free-living first stage (the epicaridium), a ectoparasitic stage on copepods (the microniscus), another free-living stage (the cryptoniscus) and a final adult parasitic stage usually on decapods but also often on other malacostracans. If a cryptoniscus arrives at an occupied host, it modifies slightly into a male, but if the host is unoccupied, the cryptoniscus grossly enlarges and distorts into a female barely recognizable as an isopod. Females in the most commonly studied family (Bopyridae) resemble squashed distorted isopods, although many members of the superfamily Cryptoniscoidea reduce their bodies so much that they are effectively reduced to a large ovary with an anchoring organ, while members of the family Entoniscidae distort their appendages and other body parts to such an extreme degree that females are completely and utterly incomprehensible. Many Epicarideans also modify their hosts to some degree, varying from swelling the carapace in gill-inhabiting bopyrids to the classic crustacean castration in some endoparasites to forming a protective sheath around their body formed from their host’s own bloodcells in Entoniscidae.

In addition, the suborder Flabellifera was recognized until fairly recently, when it was divided up into multiple suborders. The majority of species of Flabellifera went to the new suborders Sphaeromatidea and Cymothoida, while small fragments also going to Phoratopidea and Limnoriidea. The loose grouping is still useful on a gestalt ground to help narrow down to a currently accepted suborder, with former flabelliferans being recognized by the presence of a tailfan created from laterally articulating uropods and a wide telson.

[ILLUSTRATION OF SUBORDERS]

Sources

Brandt, A., & Poore, G. C. (2003). Higher classification of the flabelliferan and related Isopoda based on a reappraisal of relationships. Invertebrate Systematics, 17(6):893-923.

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Published: Apr 1, 2023