Human foodies "catch strong men", female coconut crabs can't find a husband

"Don't get too fat, or people will eat you" - this famous line from the Ghibli masterpiece "Spirited Away" not only made this movie scene an emoticon that instantly swept the screen, but also revealed the bitterness of today's protagonist, the "Coconut Crab".

Terrible but delicious coconut crab

Coconut crab Birgus latro, which can weigh more than 3 kilograms as an adult, is the world's largest terrestrial crustacean and was once widely distributed on islands from the Indian Ocean to the South Pacific. Whether it is the Chinese name or the English name Coconut crab, it is inevitable that we will think of a "giant crab that crushes coconuts with its powerful claws and feeds on coconut meat" in our minds.

Calendar Lady: In fact, the coconut crab is not a crab. It belongs to the same order as hermit crabs and pseudo-stone crabs. It has only six legs and, like hermit crabs, it burrows into a snail shell for defense.

However, coconut crabs are not vegetarians. National Geographic magazine once recorded a video of coconut crabs sharing a pig's body with many hermit crabs on the beach of Nikumaroro Island. In addition, Mark Laidre, a biologist at Dartmouth College in the United States, recorded that coconut crabs climbed up a tree in the Chagos Archipelago in the Indian Ocean, not to look for fruits, but to attack a seabird, breaking its wings and eating it as a midnight snack.

With a bite force comparable to that of a lion, coconut crabs can easily open coconuts and break bones and meat, which is why they occasionally play the role of "scavengers". There are even rumors that they "eat people" on some tropical islands.

Coconut crabs prefer moist and cool environments, and are often found in rock crevices, underground caves, and low vegetation in the forest. They are more active at night. Especially in areas with few people and many crabs, they will even appear near the coastline during the day to forage for food. Therefore, contrary to rumors, on tropical islands where food is scarce, the slow-moving coconut crabs are an easy-to-catch and delicious source of protein in the eyes of locals.

Coconut crab on a branch | Zhang Xiaofeng

The coconut crabs closest to us are distributed in Hainan Province, Taiwan Province, and parts of the Ryukyu Islands, which are now under the jurisdiction of Japan. Human economic activities have gradually eroded the natural environment on which they depend for survival, and the number of wild populations has continued to decline, gradually becoming a rare "dream creature". Although it is no longer an era of lack of food and clothing, some people still regard coconut crabs as rare delicacies, or even novel pets, resulting in the few coconut crabs being poached and sold, becoming an endangered species.

The life of a coconut crab

In Okinawa Prefecture, Japan, where I live, coconut crabs are mainly found on sparsely populated outlying islands and in the northern part of the main island, where a relatively large area of ​​pristine vegetation is preserved. Although coconut crabs have adapted to life on land, they still cannot completely leave the sea and hide in the mountains and forests, away from humans. They mainly live in vegetation-covered areas within 1 to 3 kilometers along the coastline, feeding on pandanus fruits and small bird and animal carcasses.

When the north wind blows in autumn and the temperature drops, the activity frequency of coconut crabs gradually decreases, and they hide in limestone caves in the woods to spend the winter, waiting for the weather to warm up. When the sea opens in April, coconut crabs also begin to increase the frequency of foraging for food to reserve nutrients for the summer breeding season.

Coconut crab in the northern part of Okinawa Main Island | Chino

Okinawa is geographically further north than Fujian and Taiwan, and is almost at the same latitude as Wenzhou, Zhejiang. Although it benefits from the humid and warm subtropical marine climate brought by the Japan Current, it is almost the northern limit of the coconut crab's life in the northern hemisphere. Because of this, the breeding season of coconut crabs here comes later and is shorter, from June to August. During this period, adult coconut crabs will look for the opposite sex and frequently hold each other for mating. The male crab looks up and presses his abdomen against the abdomen of the female crab covering him, in order to stick the white gel-like sperm bag on the male crab's abdomen near the reproductive opening of the female crab's abdomen. When the female crab lays eggs from the reproductive opening, it will use the sperm bag to fertilize the eggs outside the body, and attach the fertilized eggs to the hairy abdominal limbs to hold the eggs for about 1 month.

Spermatozoa on the abdomen of a male crab, and a female crab carrying eggs | Saikai Fisheries Research Institute, Ishigaki Branch

After that, the female coconut crab will leave the vegetation area, walk slowly to the seaside, and release her offspring into the sea. At this time, the coconut crab larvae are in the stage of zoea to glaucothoe. Like the larvae of other common hermit crabs on the seaside, they will spend 3 to 4 weeks in the sea as plankton, and then find a small conch shell and carry it ashore until they grow to several centimeters long. It is because of this process that the coconut crab population can spread with the ocean currents and not be trapped on an isolated island.

Larvae and postlarvae of the Kamchatka stone crab Paralithodes camtschaticus, which, like the coconut crab, belongs to the order Paralithodes | Anya Dunham et al. / Journal of the Marine Biological Association of the UK (2006)

After a "hermit crab-like" childhood of moving from house to house, the coconut crab gradually grows up and shows a different physique from other hermit crabs. It is difficult to find suitable conch houses, so it begins to build its own hard shell and slowly grows into the largest land crustacean, as majestic as its parents. How slowly does the coconut crab grow? Generally speaking, it takes 10 years for female coconut crabs to grow to sexual maturity (chest length of about 20 mm), 10 years for male crabs to reach the "edible size (chest length of about 40 mm)" sold on the market, and more than 25 years for female crabs. If you are lucky enough to see a giant coconut crab weighing four or five kilograms, it is likely to be over 50 years old.

Improving the sex ratio

The slow growth makes it difficult for coconut crabs to recover in the wild, and what makes matters worse is the human pursuit of "large coconut crabs". Male coconut crabs are about 1.5 times the size of female coconut crabs of the same age, so large male coconut crabs are more likely to become dishes and caged treasures.

Live coconut crabs for sale at the fish market on Okinawa Main Island | Chino

Under normal environmental conditions, more mature and stronger male coconut crabs are more likely to win the competition and get more mating opportunities. They have a larger sperm volume and are more likely to leave their own offspring. However, due to human capture, coconut crabs in the wild are in an abnormal situation where there are more females than males, and stronger females than males. The lack of sperm from weak males, the unbalanced sex ratio, the miniaturization of males and the decrease in the number of offspring may have become the biggest obstacle to the recovery of the coconut crab population.

In view of the current situation of the decline of the wild coconut crab population, Taiwan Province of China has included coconut crab in the list of Taiwan's protected species. In June 2014, Ishigaki Island in Okinawa Prefecture promulgated the "Ishigaki City Coconut Crab Protection Ordinance", which not only demarcated the coconut crab protection area, but also stipulated that December to August is a complete fishing ban period. Only male crabs with a chest length of 40 to 55 mm can be caught outside the protection area from September to November, and artificial breeding research on coconut crabs is carried out in the island's research institute.

Coconut crab on a branch | Zhang Xiaofeng

Although the protection efforts are still insufficient, there is still hope. In particular, Okinawa has gradually used "coconut crabs" as a natural tourism resource in recent years, and has launched nighttime outdoor adventure tours with the theme of finding coconut crabs on the main island and outlying islands. Perhaps when coconut crabs living freely in the wild can become a long-term and stable source of economic income, locals will be willing to give up catching and selling coconut crabs for temporary profits. If one day coconut crabs become a common species on tropical islands, perhaps those "exotic pet" players who are looking for novelty to satisfy their own showing-off mentality will also give up the idea? Instead of seeing them at the dinner table, it is better to look forward to meeting these interesting coconut cracking experts in the wild in the future!