# Lybia

Lybia is a genus comprising eight species of small marine crabs from the family Xanthidae. Commonly known as boxer crabs and pom-pom crabs, these crabs are famous for their symbiosis with small aquatic invertebrates, particularly sea anemones, which they hold in their claws for defense, and in some cases, feeding. The genus has a diverse distribution, ranging across the Indo-Pacific and existing as early as the Middle Miocene.

## Morphology

Lybia crabs are relatively small, achieving a maximum size of 1-2 cm in adulthood. Each Lybia chelae (claw) is equipped with a set of small hooks, which aid the crab in maintaining its grasp on the anemone. These claws are decidedly specialized for carrying anemones, and this appears to be its only function to the crab; when anemones are absent, Lybia chelae have not been observed in use for burrowing, defence, or prey capture. They are devoid of any of the stereotypical features commonly associated with crab chelae. Thus, food gathering, burrowing, intraspecific relations, and anemone collection must instead be done with the walking limbs of the crab. Chelae are delicate and have no recorded observations of sexual dimorphism.
Size, colouration, and shape of the carapace, chelae, and associated hooks are common diagnostic features for differentiating different species of Lybia.

## Ecology
### Feeding

Different species of crabs have been found to use their symbiont anemones in different manners to aid in food collection and intake. These three strategies are:
Prey stunning: Prey such as small fish or invertebrates are paralyzed using the cnidocytes of the crab's symbiont, notably seen in Lybia tessellata and Lybia leptochelis.
Grasping food: Anemones are used as tools to ease food collection. Lybia edmondsoni has employed this strategy by 'mopping' its anemone so that food is collected on the polyp. The crab then collects food particles from the anemone's surface via its walking limbs; in some cases, the crab will even grasp ingested food from the anemone's pharynx.
Distance: Anemones are held away from the food source, maximizing food access for the crab. Remaining particles collected by the symbiont may then be grasped from the anemone, as seen in Lybia leptochelis.
Food-grasping and anemone-distancing strategies are considered forms of kleptoparasitic interactions. In these interactions, the restriction of food access by the crab causes the anemone to remain at a reduced size, like a "bonsai". Studies have shown that in the absence of this kleptoparasitic interaction, the anemone will grow to considerably larger sizes.

## Distribution

The eight extant species within Lybia are distributed across varying regions of the Indo-Pacific, including Hawaii, Indonesia, Japan, Australia, and Africa. They tend to reside in warm, shallow marine waters, often hidden beneath rocks or coral. The specific range of each species of Lybia is dependent on the distribution of their specific anemone symbiont.

## Taxonomy

Lybia was first classified as a genus by Henri Milne-Edwards in 1834, with Grapse tessellata (later changed to Lybia tessellata) as the type species.
The number of species within Lybia may be contentious. Previously, ten species were documented; two have moved to a different genus and others may not be adequately defined. The species Tunebia hatagumoana and Tunebia tutelenia, once part of Lybia, now form the genus Tunebia. Additionally, it has been suggested that Lybia leptochelis and Lybia pugil may be synonyms due to their morphological similarities. These ten species, with the addition of the teddy bear crab (Polydectus cupulifer) of another genus, form the subfamily Polydectinae.
The genus Lybia currently contains eight documented species: :
| Species | Image | Known Distribution |
| Lybia australiensis (Ward, 1933) | None available. | Australia |
| Lybia caestifera (Alcock, 1898) | | China, Indian Ocean, Indonesia, Pacific Ocean, Papua New Guinea |
| Lybia denticulata Nobili, 1906 | | Red Sea |
| Lybia edmondsoni Takeda & Miyake, 1970 | | Hawaii (endemic) |
| Lybia leptochelis (Zehntner, 1894) | | Red Sea, Fiji, Indonesia, Indo-Pacific (broad oceanic range) |
| Lybia plumosa Barnard, 1947 | None available. | South Africa, Indo-Pacific (broad oceanic range) |
| Lybia pugil (Alcock, 1898) | | Australia, Indo-Pacific (broad oceanic range) |
| Lybia tessellata (Latreille in Milbert, 1812) | | East Africa, Red Sea, Indonesia, Australia, Indo-Pacific (broad oceanic range) |
Using mitochondrial genomics, it has been estimated that the anemone-holding habits of Lybia's subfamily Polydectinae likely evolved during the Eocene 43 million years ago. A fossilised carapace attributed to a species of Lybia has been retrieved from Megami Limestone, a locality from the Megami Formation, Japan, and dated to the early Middle Miocene.