Galatheidae
Squat lobsters are crustaceans that are dorsoventrally flattened with long tails held curled beneath their cephalothorax. They belong to the decapod infraorder Anomura, alongside other groups such as hermit crabs and mole crabs. They can be found in oceans worldwide, ranging from near the surface to deep sea hydrothermal vents. There is even one species that occupies caves above sea level. With over 900 species described in around 60 genera, some squat lobster species form dense aggregations either on the sea floor or in the water column, and a few are commercially fished.
Both main groups of squat lobsters share similar features in their morphology. While they resemble true lobsters to some extent, they are somewhat flattened horizontally and are generally smaller, ranging in length from 0.7 to 3.5 inches. Squat lobsters exhibit variations in their postorbital carapace length, ranging from 90 millimeters in the case of Munidopsis aries to just a few millimeters in certain species of Galathea intermedia and Uroptychus. Like other decapod crustaceans, a squat lobster's body can be divided into two main regions: the cephalothorax (consisting of the head, or cephalon, and the thorax) and the abdomen, or pleon. The pleon is only partially flexed under the cephalothorax, and the cephalothorax is longer than it is wide, giving the squat lobster a morphological appearance between that of a lobster and a crab.
The cephalothorax is composed of 19 body segments, although the divisions are not clearly visible and can be deduced from the paired appendages. These include two pairs of antennae, six pairs of mouthparts (mandibles, maxillae, maxillules, and three pairs of maxillipeds), and five pairs of pereiopods. The cephalothorax is protected by a thick carapace, which may extend forward to form a rostrum in some squat lobster species. The carapace's characteristics vary widely among squat lobsters, from being vestigial in Chirostylus to being wide and serrated in certain genera, or long, narrow, and flanked with "supraorbital spines" in others. The carapace surface often exhibits varying degrees of ornamentation, with the presence of bristles, or setae, being common. In some members of the Galatheidae and Munididae families, these setae can even display iridescence. A pair of compound eyes project from the front of the carapace on stalks. These eyes consist of ommatidia with square facets, conforming to the typical "reflecting superposition" eye structure. Deep-sea species often possess reduced eyes and limited movement of the eyestalks. Close to the eyes, there is frequently a row of setae forming "eyelashes" in the families Munididae and Galatheidae.
The mouthparts consist of six pairs of appendages—three posterior cephalic appendages and the first three pairs of thoracic appendages. Although traditionally thought to primarily handle food, the mouthparts actually have a more complex movement pattern that allows them to perform various functions such as prey and sediment gathering, sediment transfer, and sediment sorting/particle rejection. In Munida Sarsi, the complexity of movement and functional scheme increases the farther the mouthparts are located from the mouth.
The most noticeable appendages of squat lobsters are the pereiopods, with the first pair being the largest. These pereiopods, known as "chelipeds," end in a claw or chela, and can be more than six times the length of the body. However, sexual dimorphism can be observed in some groups, with females having relatively shorter chelipeds. The following three pairs of pereiopods are slightly smaller and lack claws but are otherwise similar. They are primarily used for walking. The fifth pair of pereiopods is considerably smaller than the preceding pairs and is inconspicuously positioned under the carapace. Each of these pereiopods ends in a tiny chela and is believed to be involved in cleaning the body, particularly the gills, which are housed in a cavity protected by the carapace.
The pleon comprises six somites, each bearing a pair of pleopods, and terminates in a telson. The first somite is narrower than the subsequent somites, and the last pair of pleopods is modified into uropods located on either side of the telson. Typically, the pleon is curled under the thorax, so only the first three somites are visible from above. The structure of the pleopods differs between males and females. In females, the first one or two pairs are absent, and the remaining pairs are uniramous and possess long setae to which the eggs can be attached. In males, the first two pairs are transformed into gonopods and are responsible for transferring the spermatophore to the female during mating, although the first pair is often missing. The rest of the pleopods in males can resemble those of females or may be reduced or entirely absent. Both sexes have biramous uropods.
While the concept of carcinisation has primarily been explored with regard to outer morphology, it also affects internal anatomical features. The use of micro-computer tomography and 3D reconstruction has revealed anatomical differences within Galatheoidea, particularly in the ventral vessel system when compared to porcelain crabs. Carcinisation has also resulted in the loss of the caridoid escape reaction, leading to changes in gonads and pleonal neuromeres in squat lobsters.
