Isopods of genus Monolistra belong to a family of seapills (Sphaeromatidae). They are from one half to two centimeters large, have an arched body and are white or yellow due to the lack of pigment. They are capable of conglobation (rolling into a ball) and usually have the dorsal side of the body ornamented with intriguing protrusions in the form of bumps or curved spines of different lengths. The different species differ also in body size and uropod shape. Their body consists of a cephalothorax, pereon and pleon (abdomen). The cephalothorax and pleon are relatively small, with pereon being the most prominent body region. Pleon is usually highly arched.

The cephalothorax includes an acron, five head segments and one additional segment. All the segments are fused into a uniform unit. The eyes are completely reduced. The head bears two pairs of antennae (antennae I and II). The second antennae are a bit longer than the first antennae but never exceed one half of the body length. The antennae have a chemo- and mechanosensory functions. The mouthparts comprise a pair of mandibles, a pair of maxillae I, a pair of maxillae II and a pair of maxillipeds. Evolutionary, the maxillipeds originate from the first thoracic segment.

The pereon consists of seven clearly delimited segments (pereomeres), each carrying a pair of appendages (pereopods). All the pereopods are similar in appearance and serve as walking legs. Only males of some species have subchelate second pair of pereopods.

The pleon is comprised of six posterior body segments (pleomeres) and a telson. Due to the fusion of some pleomeres not all borders between them are visible and the pleon appears three-segmented. The first pleomere is free but hidden beneath the tergite of the seventh pereonic segment. The second and third pleomere are fused into a uniform structure. The last three pleomeres and a telson are fused into a uniform pleotelson. The pleon carries five pairs of biramous phyllopodous appendages (pleopods), which serve as gills and respirate marsupium. In males the second pair of pleopods is modified as gonopods for the transfer of sperm during mating. The last, sixth pair of appendages on the pleon are uropods, which are differently shaped in different species. They can be completely reduced, short and stout or long and slender.

The genus Monolistra comprises at least 47 species of troglobitic freshwater isopods, 33 of which already have a formal scientific description. In Slovenia 15 species are present, with one still being undescribed. They are exclusively troglobitic inhabiting underground waters of Dinaric and South Alpine karst. Some species are endemic, limited to just a few caves and thus endangered. Presumably, the genus Monolistra dates back to Miocene when the European continent and the Dinaric Alps were formed. They originate from marine ancestors and some of their relatives, such as Sphaeroma serratum, Lekanesphaera hookeri and Dynamene bicolor, are present also in Slovenian sea. Marine ancestors of the genus Monolistra likely first inhabited surface freshwaters and subsequently colonized the cave habitats. Another troglobitic genus belonging to Sphaeromatidae that inhabits European caves is Caecosphaeroma which comprises two French species.

The isopods of genus Monolistra are detritivores feeding on decaying organic material, fungi and bacteria. They move slowly and do not swim as do their marine relatives.

They developed three main defense mechanisms against predators. The first and the most obvious are the dorsal spines. Species with long spines usually coinhabit waters with the largest predator in Slovenian caves, the olm (Proteus anguinus). The dorsal spines developed in genus Monolistra more than once independently with convergent evolution, due to similar environmental factors. The second defense mechanism is conglobation, which allows the animal to hide its soft ventral side and when rolled into a ball the animal can be quickly carried away from the predator by the water current. The third defense mechanism is aggregation where an individual is less likely to become prey.

The reproduction and lifecycle of the genus Monolistra is relatively unknown. For the species without the dorsal spines, it is known that the males and females exhibit amplexus (precopula) prior to mating, in which the male grasps the female from the dorsal side. In species with spines such amplexus is basically impossible and their mating behavior remains a mystery. In species without dorsal spines the males are generally larger than females, while the situation is reversed in species with dorsal spines. The females carry the fertilized eggs in a marsupium, which is a basket-like structure on the ventral side of the female’s body. The marsupium develops after parturial molt from oostegites, which are leaf-like outgrowths on the coxae of the second to third pair of pereopods. The number of eggs in a batch varies between species and differently sized females. The development of juveniles probably takes longer than in the marine relatives.

Different Monolistra species can coinhabit the same habitats and up to three species have been observed in the same locality. The coinhabiting species differ in size and morphology, while the morphologically similar species do not coinhabit. This indicates that the coinhabiting species occupy different ecological niches.