Pom Pom Crab - The Crustacean with Venomous Boxing Gloves

Lybia edmondsoni, commonly known as the pom-pom crab or boxer crab, is a small crab species endemic to the Hawaiian Islands. Its distinctive feature lies in its peculiar habit of carrying stinging sea anemones in each of its claws, resembling pom-poms or boxing gloves. This behavior serves as both a defensive mechanism against predators and a potential means to stun prey. In ancient times, this intriguing creature was even utilized by individuals claiming to possess sorcerous abilities.

Hawaiin Boxer Crab

( Lybia edmondsoni )

Lybia edmondsoni 

Scientific classification
Kingdom	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Infraorder:	Brachyura
Family:	Xanthidae
Genus:	Lybia
Species:	L. edmondsoni

Lybia edmondsoni, commonly known as the pom-pom crab or boxer crab, is a small crab species endemic to the Hawaiian Islands. Its distinctive feature lies in its peculiar habit of carrying stinging sea anemones in each of its claws, resembling pom-poms or boxing gloves. This behavior serves as both a defensive mechanism against predators and a potential means to stun prey. In ancient times, this intriguing creature was even utilized by individuals claiming to possess sorcerous abilities.

The carapace of L. edmondsoni measures up to 0.5 inches (13 mm) in width, exhibiting unique polygonal patterns in varying hues of pink, brown, or yellow. Unlike most crab species, its chelipeds are delicate, with a highly flexible propodal-carpal joint and backward-pointing spines. Its walking legs are adorned with dark purple bands, rendering it nearly invisible on its preferred substrates like thin sand or rubble.

Distribution and Habitat

Range map of Lybia edmonsoni from iNaturalist.org, CC BY-NC 4.0.

L. edmondsoni is endemic to Hawaii and shares striking similarities in size and behavior with its sister species, L. tessellata, which enjoys a more extensive range across the tropical Indo-Pacific. It thrives in shallow waters, typically found at depths of up to 20 meters (66 feet), seeking refuge under rocks, amidst coral debris, or camouflaging seamlessly on sandy or gravelly flats. Furthermore, it clings to live corals using its slender, elongated legs.

Biology

Boxer Crab carrying eggs by  prilfish from Vienna via Wikimedia commons

Lybia edmondsoni displays a unique adaptation. Unlike many crabs, it possesses relatively unarmored chelae, making it less adept at both defense and feeding. However, this vulnerability is offset by its ingenious use of sea anemones. Delicately grasping the anemones with its chelae, the crab employs small spines to secure them. These anemones serve a dual purpose: acting as a defensive shield against predators and actively gathering food particles with their tentacles. This collaboration exemplifies nature's capacity for intricate symbiosis.

Furthermore, the crabs demonstrate noteworthy molting behavior. Predominantly occurring at night, they adeptly secure their anemones by morning. This process showcases the crab's ability to navigate its environment and interact with its symbiotic partners even during phases of vulnerability.

The sea anemone, Triactis producta, also plays a vital role in this symbiotic relationship. Capable of asexual reproduction through fission,

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Fission

Fission, in biology, is the process of dividing a singular entity into two or more segments and subsequently regenerating those segments into distinct entities resembling the original. The entity undergoing fission is typically a cell, but the term may also encompass how organisms, bodies, populations, or species separate into discrete segments. The fission process can take the form of binary fission, where a single organism generates two segments, or multiple fission, where a single entity produces multiple segments. Any division of a unified population of individuals into discrete segments can be regarded as fission.

These can include evidence such as burrows, borings (resulting from bioerosion), urolites (formed by liquid waste evacuation), footprints, feeding marks, and root cavities.

it exhibits a remarkable ability to regenerate and adapt. Observations of crabs missing one anemone illustrated their resourcefulness in tearing the remaining one in two, providing one for each claw. These torn anemones subsequently grow to the size of the original, underscoring the robust nature of this mutualistic partnership.

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Mutualism

Mutualism, characterizes the ecological interplay between two or more species, wherein each species derives a net benefit. This form of interaction is widespread in ecological contexts. Notable instances encompass the symbiotic relationships between most vascular plants and mycorrhizae, the pollination of flowering plants by animals, the dispersal of vascular plants by animals, as well as the association of corals with zooxanthellae, among various others.

The term 'mutualism' was coined by Pierre-Joseph van Beneden in his 1876 work 'Animal Parasites and Messmates,' signifying 'mutual aid among species'.

This behavior highlights the adaptability and resilience of both the crab and its associated sea anemones in the face of challenges.

Behavior & Symbiosis

Closeup of the Triactis producta anemone via floridamuseum.ufl.edu

In 1997, I. Karplus and colleagues conducted a study examining how these assertive crabs interacted with others of the same species nearby, and whether their anemones were employed as defensive tools in these encounters. Out of the 50 crabs observed, 48 initially had two anemones each, while the remaining two had only one. The findings indicated that when evenly matched crabs encountered each other in a neutral space, most of the aggressive activity occurred from a distance, involving the threatening extension of a leg bearing an anemone towards the other crab. Some crabs withdrew from the confrontation at this stage. However, if the encounter progressed to physical contact, the struggle involved the hindmost walking legs, and the anemones were kept well away from the contact area.

Alongside aggressive gestures, certain maintenance behaviors were observed when the crabs were in close proximity. These included a crab rubbing its legs together or positioning its mouthparts at the base of an anemone. However, these actions occasionally occurred in the absence of other crabs, leading to their exclusion as hostile behaviors. Three hypotheses were proposed to explain the non-use of anemones during encounters: T. producta might be highly toxic to the other crab, making the anemone risky to use; T. producta might not be toxic to the other crab, rendering the use of the anemone pointless; T. producta could be such a valuable resource for the crab that it couldn't be risked in such encounters. Despite efforts to verify these hypotheses, the results remained inconclusive.

The aggressive behavior among the crabs is predominantly ritualistic, involving significant displays of weaponry but minimal physical contact. The anemone evidently holds great significance for the crab, yet the advantages for the anemone in this relationship are less evident. Benefits for the anemone may encompass increased exposure to food and oxygen compared to if it were stationary, as well as the avoidance of the risk of being buried in shifting sediment.

In 1905, Duerden observed that boxer crabs, lacking a single anemone, would split the other in two to have one for each claw. A study in 2017 documented this behavior, along with instances of theft of anemone fragments from fellow boxer crabs. The torn anemones swiftly regenerate to the size of the original, which elucidates why nearly all wild boxer crabs, even those in their juvenile stage, bear two anemones.

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References

• Ng, P. K. L., Guinot, D., & Davie, P. J. F. (2008). Systema Brachyurorum: Part I. An annotated checklist of extant brachyuran crabs of the world (PDF) . The Raffles Bulletin of Zoology, 17, 1–286. Archived on September 1, 2023, from
  https://web.archive.org/web/20110606061453/http://rmbr.nus.edu.sg/rbz/biblio/s17/s17rbz.pdf.
   

• Karplus, Ilan & Fiedler, G. Curt & Ramcharan, P. (1998). "The Intraspecific Fighting Behavior of the Hawaiian Boxer Crab, Lybia edmondsoni -Fighting with Dangerous Weapons?" (PDF) . Symbiosis. 24. Retrived on September 1, 2023, from
https://www.researchgate.net/profile/G_Curt_Fiedler/publication/265679472_The_Intraspecific_Fighting_Behavior_of_the_...eyJjb250ZXh0Ijp7ImZpcnN0UGFnZSI6InB1YmxpY2F0aW9uIiwicGFnZSI6InB1YmxpY2F0aW9uIn19.

   

• Duerden, J. E. (1905). On the habits and reactions of crabs bearing actinians in their claws . Proceedings of the Zoological Society of London, 2, 494-511. [Digitized version available at
https://dp.la/item/42869adf5d7344b4cb6626a3180cb864]

   

• Schnytzer, Y., Giman, Y., Karplus, I., & Achituv, Y. (2017). "Boxer crabs induce asexual reproduction of their associated sea anemones by splitting and intraspecific theft". PeerJ, 5, e2954.doi:10.7717/peerj.2954. Retrived on August 6, 2023, from
https://peerj.com/articles/2954.pdf .