Squat lobster

Squat lobsters are crustaceans with a flattened top-to-bottom shape and a long tail that stays curled under their body. They belong to two superfamilies, Galatheoidea and Chirostyloidea, within the decapod group Anomura, which also includes hermit crabs and mole crabs. These animals live in oceans all over the world, from shallow waters down to deep-sea hydrothermal vents, and one species even inhabits caves above sea level. Over 900 species have been described, spread across about 60 genera. If the family Aeglidae is included as squat lobsters, that number exceeds 1,000 species in 61 genera, and they can be found at altitudes up to 3,600 meters above sea level. Some species gather in dense groups, either on the seafloor or floating in the water, and a few are caught commercially.

In terms of appearance, the two main groups share most features. They look somewhat like true lobsters but are flatter and generally smaller, measuring from 18 to 89 millimeters (0.7 to 3.5 inches) in length. Their postorbital carapace length—measured from the eye socket to the rear edge—ranges from 90 millimeters (3.5 inches) in *Munidopsis aries* down to just a few millimeters in *Galathea intermedia* and some *Uroptychus* species. Like other decapods, their bilaterally symmetrical body has two main parts: the cephalothorax (head and thorax combined) and the pleon (abdomen). The pleon is only partly bent under the cephalothorax, and the cephalothorax is longer than it is wide, making squat lobsters a morphological middle ground between a lobster and a crab.

The cephalothorax consists of 19 body segments, though the divisions are not obvious and are best identified by the paired appendages. From front to back, these are two pairs of antennae, six pairs of mouthparts (mandibles, maxillae, maxillulae, and three pairs of maxillipeds), and five pairs of pereiopods. A thick carapace covers the cephalothorax and may extend forward in front of the eyes to form a rostrum, which varies greatly among species—it is tiny in *Chirostylus*, wide and often serrated in some genera, and long, narrow, and flanked with "supraorbital spines" in others. The carapace’s surface ornamentation also varies widely, and it almost always has at least a few setae (bristles), which can be iridescent in some Galatheidae and Munididae. A pair of compound eyes on stalks projects from the front of the carapace; these eyes are made of ommatidia with square facets, typical of the "reflecting superposition" eye type. Many deep-sea species have reduced eyes and limited eyestalk movement. In the families Munididae and Galatheidae, a row of setae near the eyes often forms "eyelashes."

The mouthparts are six pairs of appendages: three from the back of the head and the first three from the thorax. While once thought only to handle food, they actually perform complex movements for tasks like gathering prey and sediment, transferring sediment, and sorting or rejecting particles. In *Munida sarsi*, the farther the mouthparts are from the mouth, the more complex their movement and function become.

The most noticeable appendages are the pereiopods, with the first pair being the largest. These end in a claw (chela) and are called chelipeds; they can be over six times the body length, though some groups show sexual dimorphism, with females having shorter chelipeds. The next three pairs of pereiopods are smaller than the chelipeds and lack claws but are otherwise similar, used for walking. The fifth pair is much smaller, held inconspicuously under the carapace, and ends in a tiny claw, likely used for cleaning the body, especially the gills inside a cavity protected by the carapace.

The pleon has six segments, each with a pair of pleopods, and ends in a telson. The first segment is narrower than the rest, and the last pair of pleopods are modified into uropods that flank the telson. The pleon is usually curled under the thorax, so only the first three segments are visible from above. Pleopod shape differs between sexes. Females lack the first one or two pairs, while the remaining pairs are uniramous with long setae for attaching eggs. Males have the first two pairs formed into gonopods for transferring spermatophores during mating; the first pair is often missing. The remaining pleopods may resemble those of females, be reduced, or be absent. In both sexes, the uropods are biramous.

Carcinisation—the evolutionary process of becoming crab-like—has been studied in terms of outer morphology, but changes in body shape also affect internal anatomy. Micro-computer tomography and 3D reconstruction have revealed anatomical differences within Galatheoidea, such as variations in the ventral vessel system between porcelain crabs and squat lobsters. Carcinisation also led to the loss of the caridoid escape reaction, which shifted the position of the gonads and pleonal neuromeres in squat lobsters.

Regarding development, fecundity (number of eggs) increases when eggs are smaller and the parent’s body is larger. This leads to longer incubation times and, as a result, greater egg volume. The trend is toward more eggs of smaller size.