Roly-Poly Lifestyles

Marilyn Schotte

Although you may think that you don’t and continues to grow as scientists dis- know what isopods are, in fact you’ve cover new ones. probably crossed paths with them on Like other , isopods pos- many occasions. You may know the ter- sess two pairs of antennae, mandibles, restrial isopods by other names; what three body parts (head, thorax, and ab- people call sow bugs, pill bugs, roly- domen), and compound eyes. Yet, they polies, or, in the British Isles, woodlice, are distinguished from their relatives by are all isopods. These lower-profile rela- their unstalked, or sessile, eyes and their tives of crabs, lobsters, shrimp, and seven pairs of legs; most other crus- other crustaceans inhabit , for- taceans have five pairs of legs (although ests, caves, grasslands, greenhouses, city adult isopods in the advanced family sidewalks, and even your backyard. You Gnathiidae have only five leg pairs). The can find them in nearly every environ- seven leg pairs of isopods are similar in ment on earth. Marine live in structure and function, and it’s this sim- brackish water and in oceans at depths ilarity that gives rise to the name of the ranging from the intertidal to the deep class — iso meaning “same or equal” and sea. Freshwater isopods live in streams, poda, “foot.” lakes, wells, hot springs, and subterra- An isopod starts life as an egg in a nean groundwaters. The list of species mass carried in the brood pouch on the currently numbers more than 10,400 underside of the female’s abdomen.

The marine isopod Cirolana kokoru is a carnivorous scavenger of intertidal rocky shores in New Zealand. Photograph courtesy Niel L. Bruce, © NIWA.

22 WINGS Living in conditions about as far removed from oceans as any isopod gets, the social-living wood louse ( reaumuri) ex- cavates burrows up to a foot (thirty centimeters) deep to survive arid- ity. Photographed in by Edward S. Ross.

Here the immature forms of most groups matures to reproductive age and is ready undergo direct development. Because to be mated. they do not go through a larval stage, The fossil record shows that isopods they emerge looking like miniature existed 325 million years ago in the Pa- adults. In some marine forms, there is leozoic era. Modern species in the sub- parental care after hatching. Parents order Phreatoicidea appear to be large- (primarily mothers) guard the newly ly unchanged since that time and likely hatched young and seek out the pro- represent an early offshoot from the or- tection of sponges or the holdfasts of iginal ancestral line. Phreatoicids, famil- kelp where they make burrows, raise a iarly known by the affectionate scien- brood, and fight off intruders. Parental tific mispronunciation “friar tucks,” care is not restricted to aquatic forms; were once marine but are found now the desert isopod Hemilepistus creates a only in freshwater refugia in Australia, single-family burrow in the sand to Tasmania, and South Africa. Isopods are house its offspring. thought to have originated in shallow Not all guarding behavior is neces- marine waters and subsequently to have sarily parental in nature. In several spe- radiated into the deep sea, and then to cies of the brackish-water Iais, have colonized land as the only truly it is common to find an adult male car- air-breathing crustaceans. The suborder rying an immature female beneath his Oniscidea contains all of the terrestrial abdomen by clasping her with his mod- isopods that can exist independent of ified legs. But this is not parental care — bodies of water; they are able to do so it’s mate guarding. The male is keeping because they can breathe air through this young female to himself until she leaf-like appendages on their abdomens.

FALL 2006 23 The common pill bug () is a regular inhabitant of gardens across Europe and North America. When disturbed it rolls into a tight ball for protection. Photographed in California by Edward S. Ross.

Wherever they are found, isopods Bathynomus of deep Caribbean waters lead lives similar to those of their more can reach a length of nearly twenty distant relatives, the insects. The more inches (half a meter). Bathynomus is a primitive aquatic and terrestrial groups predator and has been observed attack- tend to be plant-feeders or scavengers ing and eating fish when hauled up to- on plant matter. The advanced subor- gether with them in fishing nets. ders whose members live mainly in ma- Marine isopod families are found in rine environments include , all latitudes of the world’s oceans and carnivores, predators, commensals, and are adapted for living in sponges, on highly modified parasitic forms. It has algae, inside seagrasses, on coral reefs, in even been suggested (though not prov- mangrove roots, in coral rubble, and in en) that some isopods may serve as pol- sediment. Species in the family Cymo- linators, spreading the pollen of sea- thoidae are ectoparasitic and live exter- grasses as they crawl over the plants. nally on or in the mouths or gill cham- Marine species show the greatest di- bers of fish. The suborder Epicaridea are versity in morphology and size. The the most highly modified group and are smallest are the parasitic forms, as small exclusively parasites of other crusta- as a fiftieth of an inch (half a millime- ceans, including other isopods. ter) in length. One whole suborder, the In the family Bopyridae, the female Microcerberidea, contains only tiny ma- is grossly inflated and distorted, with rine species, approximately a sixteenth greatly reduced appendages; the male of an inch (a millimeter and a half) is tiny in comparison and clings to the long, that live between grains of sand underside of the female’s body. These or in sediment. In contrast, species of types are host-specific and live on the

24 WINGS surfaces or in the branchial chambers of states, possibly a result of the transport crabs, hermit crabs, and shrimp. When of their ancestors into harbors by hu- the larva-like forms emerge, they attach man immigrants. to an intermediate host. After molting Freshwater isopods include several they seek out an appropriate host. The families, some of which are very re- first larva to arrive on the host develops stricted in their distribution. One sub- into a female, which is then able to hor- order distinguished by its unique seg- monally control the development of the mentation was first found in wells in second larva to arrive so that it becomes northern Venezuela. These blind ani- a male. Other smaller epicarideans are mals, isolated for millennia from other internal parasites, like the superfamily isopods, display very unusual morphol- Cryptoniscinae, in which the female is ogy, characterized by reduced first and reduced to a sac, generally without ap- second abdominal segments as well as a pendages. These feed on the blood of completely fused posterior appendage. their hosts, which are marine crusta- They are, in fact, so different from other ceans such as ostracods, barnacles, cu- isopods that systematics placed them by maceans, and amphipods. themselves in a new suborder, Calabo- The Mediterranean region, and the zoida. Another isolated species, a fresh- Balkan Peninsula in particular, is the water cave species discovered in eastern epicenter of terrestrial isopod diversity. Brazil in 1998, also belongs to this un- Because a number of dedicated Euro- usual suborder. pean specialists have collected and clas- Endemism, or restriction in distrib- sified in this area over the past ution to one particular locality, is ex- 150 years, its “hotspot” status may be tremely high in isopods that live in hot real or partly an artifact of collecting ef- springs in the southern United States fort. Still, new species continue to be de- scribed in this region. This is not the case for North America, however. So far, only about 115 different terrestrial iso- pods (oniscideans) have been found in the United States (excluding Hawaii) and Canada. This could be due to lack of specialists, absence of collecting ef- fort, paucity of fauna, or maybe all three. About two-thirds of this number are considered native to North America and are usually found in caves or on seashores. Many of the most common oniscideans in the United States (includ- Known as a rock louse or sea slater, Ligy- ing our backyard pill bugs) are widely da occidentalis is the only marine isopod regularly found above the high-tide line distributed and have probably been on rocky shores in California. This is an introduced from Europe. A majority of inhospitable zone, heavily influenced by these introduced species are concen- wave splash and salt spray. Photograph by trated in the northeast and northwest Edward S. Ross.

FALL 2006 25 and Mexico. Each spring or well may generally bottom-dwellers that move by contain just one species, which may be crawling. Therefore, the ranges of aquat- found nowhere else in the world. The ic species tend to be restricted, although ancestors of these isopods likely were there are cosmopolitan marine types marine and lived in the embayment such as Idotea that can float around on that covered most of Mexico and parts rafts of seaweed. Others have been of Texas and New Mexico during the found in places far from their origins, Mesozoic era. As the waters receded dur- possibly because they were transported ing the Cretaceous period, the climate in the ballast water of oceangoing ships. became drier. Stranded, they may have This lack of mobility can, due to adapted to freshwater environments loss of habitat or pollution, lead to en- and later some adapted even to the in- dangered or threatened status for some imical conditions of hot springs. species. Worldwide, there are forty-two Although some isopods can paddle isopods on the IUCN Red List of Threat- around briefly and a few can tread the ened Species, up from twenty-seven in water columns of the ocean by means 1988. Eight of these are listed as endan- of very long appendages, isopods are gered, and seven are designated as crit-

Listed as threatened under the Endangered Species Act, the Madison Cave isopod (Antrolana lira) is restricted to deep lakes in a single cave in Virginia. Photograph © 1993 by Susan Middleton and David Liittschwager.

26 WINGS ically endangered. In the United States there are eight Red List species; six are freshwater cave isopods from Virginia, Kentucky, Oklahoma, and Tennessee, and two are found only in hot springs in Texas and New Mexico. The last of these is the Socorro iso- pod (Thermosphaeroma thermophilum). Also protected as endangered under the Endangered Species Act, this isopod is a modern conservation success story. Be- cause of habitat destruction, it went ex- tinct in the wild, but a captive popula- tion at the University of New Mexico enabled scientists to restock the one thermal spring in which it had lived, lo- cated in Socorro County, New Mexico. In 1989 the state received a federal grant to develop a recovery plan and build a permanent habitat, where the isopod is A female sow bug produces eggs that she a local attraction and enjoys some pro- carries in a pouch beneath her body until they hatch. After hatching, the young sow tection. Still, as habitats shrink and pol- bugs may remain in the pouch for several lution expands, other species will be lost weeks before gaining their independence. to science before they are discovered. Sow bug and young, photographed in A few isopods affect humans in a Tunisia by Edward S. Ross. negative way. For centuries humans have placed wooden pilings in water to carbon compounds. They feed on plant build wharves, docks, and even plat- and matter, breaking down de- forms for cities. Such isopods as Lim- tritus and organic material, returning noria tripunctata (also known as gribbles) nutrients to the soil or substrate, be- have been chewing on wood for much coming food of larger animals in the longer. Shipworms and gribbles are the process. Completely beneath our usual main reasons why pilings must be treat- awareness, they instinctively carry on ed with creosote. These isopods are sig- the myriad actions and interactions that nificant pests in places such as Venice, help maintain the web of all life. Italy, where they have contributed to the instability of the city’s foundation. Why are isopods worthy of our at- Marilyn Schotte is a museum specialist in tention or even our acknowledgment? the Department of Invertebrate Zoology at Oblivious to human beings, the isopods the Smithsonian Institution, National Mu- under the garbage can or in the wood- seum of Natural History. She is a world pile (just like their relatives in the sea) expert on marine isopods, and a co-author are busily engaged night and day in the of Guide to Marine Isopod Crustaceans of immense job of recycling nutrients and the Caribbean.

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