In nature, in order to adapt to their living environment, many organisms will change some of their own morphological characteristics, which are strange and become a strange flower in nature. Crabs are one of the most evolutionaryly successful animals in nature, with various crabs such as boxing crabs, hermit crabs, steamed crabs, spider crabs, teddy bear crabs, etc., which have evolved to adapt to their natural habitats.
Arrow crab (Stenorhynchus seticornis, English name arrow crab), is a member of the spider crab family, because of its special form, attracted a lot of attention. Compared with other spider crabs, arrow crabs also have 8 long legs, even up to 5 times the length of the body, but it is a small point, the body size is generally 4-6 cm, some can reach 10 cm, the female crab is usually smaller.
From a distance, the arrow crab resembles a dry branch. Its pointed head looks like a miniature version of the Eiffel Tower. This pointed head, combined with spherical eyes, and a pair of mini blue pliers (clawed feet) on the chest, is the origin of the name Arrow Crab.
Arrow crabs come in a variety of colours , with a body that may be golden , yellow , or milky white with brown, black, or brilliant blue stripes , the legs are peach or yellow , and the claws are usually blue or purple. The combination of long legs, a pointed head and protruding bulbous eyes, coupled with a cheeky face, makes this crab look funny and impressive.
In fact, the arrow crab is a highly adaptable crustacean, and every physical feature evolved to adapt to the marine environment in which it lives.
Figure 1 Arrow crab
Arrow crabs are found mainly in the western Atlantic Ocean, from the United States and Bermuda to Brazil and throughout the Caribbean. They are inactive during the day, usually like to hide in caves or crevices in coral reefs or rocks, wait until they come out at night to feed, mainly plankton, but also eat worms, algae and other animal carcasses, eat almost whatever they touch, and can become "cleaners" for a group of animals in coral reefs or aquariums. Arrow crabs are also known for their aggressiveness towards small fish and other crustaceans.
Arrow crabs, especially male crabs, are usually aggressive and territorially conscious. In addition, their aggressiveness is latent until they grow up. At this point, they may become aggressive and can attack other small prey, such as invertebrates or even small fish. Chasing and attacking creatures of similar size gives the arrow crab a higher chance of finding food in its area.
Fig. 2 Arrow crab hiding in a cave under the sea
Under good conditions, arrow crabs can breed all year round. After mating, the female arrow crabs carry their eggs under the abdomen until the juveniles hatch. Newborn juvenile crabs have transparent, rounded bodies and prefer to live in open water, swimming to the surface and feeding on small plankton. After morphing, they will return to the bottom of the sea again, go through a series of molting shells, and eventually become crabs.
Resources:
[1] Mariana Antunes,Fernando José Zara, Laura S. López Greco, et al. Morphological analysis of the female reproductive system of Stenorhynchusseticornis (Brachyura: Inachoididae) and comparisons with otherMajoidea[J]. Invertebrate Biology, 2016, 135(2).
[2] Barros AlvesSamara P., Alves Douglas F. R., Mariana Antunes, et al. Nutritionalvulnerability in zoeal stages of the yellowline arrow crab Stenorhynchusseticornis (Brachyura: Majoidea)[J]. Marine and Freshwater Research, 2018, 69(6).
[3] Cobo ValterJosé. Breeding period of the arrow crab Stenorhynchus seticornis fromCouves Island, south-eastern Brazilian coast[J]. Journal of the MarineBiological Association of the United Kingdom, 2002, 82(6).
[4] Diego V.Medeiros, José de Anchieta C.C. Nunes, José Amorim Reis-Filho, et al. Yellowline arrow crab Stenorhynchus seticornis (Brachyura: Majidae)acting as a cleaner of reef fish, eastern Brazil[J]. Marine BiodiversityRecords, 2011, 4.
[5] Okamori ClaudiaMelissa, Cobo Valter José. Fecundity of the arrow crab Stenorhynchusseticornis in the southern Brazilian coast[J]. Journal of the MarineBiological Association of the United Kingdom, 2003, 83(5).
Source: Aquatic Animal Health Assessment