Imagine relaxing on an Indo-Pacific tropical island beach under a palm tree with a coconut drink in your hand. You hear a mysterious tapping noise above you, so you look up and see a meter-wide crab at the top of the palm tree, tearing a red-footed booby apart with its big, meaty claws. Sorry for the nightmares, but this is the coconut crab (Birgus latro), the largest terrestrial crustacean in the world, which became famous a few years ago after a photo of one climbing a trashcan went viral. These giant, omnivorous hermit crabs are known for a variety of odd behaviours, including producing eerie clicking sounds. However, nobody knew how or why they produce sounds, until a recent study published in Zoology by a team of scientists led by Shin-ichiro Oka of the Okinawa Churashima Foundation in Japan. The researchers figured out the coconut crab's means of sound production, along with providing some explanations for how they use these sounds.
As crustaceans, coconut crabs are covered in hard segments, which show up well in X-rays. Therefore, Oka and his team had a straightforward way of determining how the crabs produce sound: filming the crabs with X-ray videography while recording the sounds they make. By looking at which body parts were moving only when noises were audible, Oka was able to determine the parts used for sound production. Unlike many crustaceans that use their claws and appendages to produce sound, coconut crabs produce sound by beating hard parts of their mouth structures – known as the scaphognathites, which draw water and air over the crab's gills – against hard panels in the gill channels. The work by Oka and his team suggests that rather than developing a new organ, coconut crabs use a behavioural modification of existing organs to produce sound. However, what are these sounds for?
When in the water, crustaceans produce sounds mainly to deter predators, court mates and ward off competitors for resources. The purpose of the sounds made by the crustaceans when on land was largely mysterious, but we now have some possible explanations, thanks to Oka and his team. They recorded sounds from male and female crabs during mating and at other times. Oka found that both sexes produce sounds regardless of whether they are trying to attract the most desirable mate with their devastatingly attractive clicks, suggesting that that they use sound for more than just casual sex. Additionally, both sexes produced a variety of sounds by adjusting the pitch and sound intervals between their clicks, demonstrating a potential ‘multi-word vocabulary’. Some of these ‘words’ likely serve a courtship role to woo mates, as the crabs’ sounds changed throughout the mating process. However, the rest of the coconut crab language is still a mystery.
Many crustaceans communicate using odours underwater, but these giant crabs spend most of their lives on land, so some of their aquatically adapted senses may not function so well in the air. While they have evolved a strong sense of aerial smell, odours are carried differently through the atmosphere. Therefore, the coconut crab's diverse acoustic vocabulary may complement their odour repertoire to communicate a variety of messages to other crabs on land. The work by Oka and his team highlights just how little we know about some of the biggest, most conspicuous animals on our planet and why we need to develop a ‘Rosetta shell’ to understand these overgrown hermit crabs.
