Sphaeroma serratum is approximately one centimeter large marine isopod that belongs to a family of seapills (Sphaeromatidae). In danger it can roll its arched body into a tight ball. Individuals can be differently colored. They can be flecked, bright yellow to dark gray or even reddish brown. The males are a bit larger than the females. 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.

The cephalothorax includes an acron and six anterior body segments, which are fused into a uniform functional unit. The head bears compound eyes and two pairs of filiform antennae (antennae I and II). Antennae II are somewhat longer than the antennae I, but do not exceed one half of the length of the body. Antennae function in mechano- and chemoreception. 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 as thoracic appendages.

The pereon is composed of six clearly delimited segments (pereomeres), each carrying a pair of appendages (pereopods). All pereopods are alike in the form of rod-like walking legs.

The pleon is comprised of six segments (pleomeres) and a telson. Due to the fusion of pleomeres the segmentation of pleon is obscured. The first pleomere is small, inconspicuous and hidden beneath the tergite of the last pereomere. The second and third pleomere are fused into a uniform unit. The last three pleomeres are fused with telson into a uniform pleotelson. The pleon thus appears to consist of just two large segments. The five pairs of appendages on the pleon (pleopods) are biramous and phyllopodous and function as gills, respirate the marsupium and aid in swimming. In males the second pair is modified as gonopods, which are used to transfer the sperm. The last sixth pair of appendages on the pleon are uropods, which are flattened flap-like with a serrated outer edge on the outer branches.

S. serratum is widely distributed. It can be encountered in the Mediterranean and Black sea, along the European and American coasts of Atlantic and also along the south African coasts. It inhabits seashores in the intertidal zone where it hides under stones and in crevices. A very similar Lekanesphaera hookeri lives in brackish water of salt marshes, shallow lagoons and estuaries.

S. serratum is a detritivore. It feeds on decaying organic material and biofilm of fungi and bacteria growing on the rocky substrate.

The amazing diversity of coloration between individuals of S. serratum is not completely explained yet but is probably a result of natural selection. The pigmentation has two roles. It provides protection against the UV radiation which is intense in the habitat of S. serratum. The pigmentation also acts as a camouflage allowing the animal to hide in its environment and thus protect it against predation. Different types of coloration in S. serratum are genetically determined, and the abundance of different coloration types varies between different habitats and geographic locations. However, the coloration is also extremely plastic and can be modified in accordance with changes in the environment. With redistribution of the pigment in melanophores the S. serratum can change its coloration in a matter of hours and adapt to a new environment. The coloration can be changed also during the course of the day. The individuals are darker during the day and lighter during the night. These daily changes follow the circadian rhythm and occur also if the animal is constantly in the dark.

In defense against predators, besides the conglobation (rolling into a ball) and camouflage, the S. serratum employs two other mechanisms. It is a good and fast swimmer, and it aggregates in large groups with other conspecifics which reduces the risk of predation of an individual. Good swimming also contributed to its wide distribution across the globe. They swim with ventral side up, due to their high center of gravity.

As in other isopod crustaceans, the embryonic development takes place in a marsupium on the ventral side of the female’s body. After the parturial molt of a female, a marsupium is developed at the ventral side of her pereon. Into the marsupium the female lays fertilized eggs, and it provides a safe environment for the development of offspring. The marsupium is basket-shaped and is formed by oostegites, which are leaf-like outgrowths at the coxae of the second to fourth pair of pereopods.