While beachcombing in the lagoons of Zanzibar, we found a White-spotted hermit crab (Dardanus megistos) housing itself in a Fusinus sp. shell. These crabs are exceptionally beautiful, and I therefore want to present them in this article.
Description of White-spotted hermit crabs
Giant spotted hermit crabs are easily recognizable by their coloration. They have bright red and orange bodies and legs with small black-ringed white spots. The body and legs are covered in light brown bristles. They have two sets of antennae: one short pair in the middle with sensory organs on it and the other a white pair that extends outside.
Typically, they spend their lives in empty gastropod shells; only their legs and head protrude. The first set of legs has claws, and the left is considerably larger than the other. These are usually used to seal themselves into their shell when threatened. The second and third legs are used for walking.
The rear of the body is soft and adapted to fold into the shell. As they grow, they need larger shells, often in short supply. They actively fight over shells, with larger crabs pulling a weaker crab out of its shell using their claws.
Concerning diet, they are carnivorous and are mainly scavengers, but they will break open and eat any mollusk they can open. Generally speaking, these crustaceans are pretty self-confident and often fearless of beachcombers and divers.
Distribution and habitats
White-spotted hermit crabs are found on hard substrates of the shallow sublittoral, including coral heads in lagoons across the Indian Ocean and Pacific. Due to their size and colorfulness, Solomon Island, Tonga, Seychelles, and Australia produced stamps with the motives of this crab species.
Why are White-spotted hermit crabs so unique?
According to (Ohashi & Kamezaki, 2022), besides of their striking appearance, White-spotted hermit crabs have a unique molt behavior in which they hide their outer, shed shells in the sand. After molting, the crabs fold the molted exoskeleton with both forceps and bury the shell with their walking legs until it is no longer visible. This ensures the exoskeleton does not signal predators that a crab with soft, freshly shed skin is around this area.
Uses of White-spotted hermit crabs
These crabs are rarely used for human food, as extracting them from their shells is difficult. Other edible organisms from lagoons and intertidal waters are easier to collect.
However, fishermen use these crabs as bait to catch predatory fish in the lagoons, such as snappers, breams, and other species.
Further abroad from Zanzibar, at the Lakshadweep Islands off Kerala (India), the laxative effect of raw, crushed White-spotted hermit crabs is utilized as a measure against theft. (Meghwal, 2021) presents a concise report about ‘Neera,’ which is the sweet sap of the inflorescences of 12 different palm species. This sap is collected in cans fixed at the harvested palms’ crowns. Sometimes, thieves steal these collecting vessels. When continuous theft occurs, the palm tree owners will add crushed White-spotted hermit crabs into the Neera cans. After the thieves drink the stolen juice laced with crushed hermit crabs, they will get diarrhea.
Lessons learned about White-spotted hermit crabs in Zanzibar:
- White-spotted hermit crabs occur throughout the Indian Ocean and Pacific.
- They are beautifully colored and usually not shy of people.
- These hermit crabs have a unique way of covering their molted exoskeleton.
- Raw crabs are also used as fishing bait and anti-theft measures due to their laxative effect.
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