An introduction to limulus
Limulus, commonly referred to as the “horseshoe crab,” can be dated back to four billion years ago, which have been living in the ocean since the ancient days, much earlier than the mankind. Over the last decades, however, this species has seen a decline in its population and inhabitation zones, which is believed to be involved with the environmental destruction on a global scale.
There were a good number of horseshoe crabs in western Taiwan’s sandy beach, yet they seem all gone today. About thirty years ago, many horseshoe crabs could be found in Penghu, most of which disappeared too. Currently we can only see the number-dwindling horseshoe crabs in Kinmen, which indicates the rarity of this species and the unprecedented ecologic beauty of this county. Therefore it is critical that we find a way to promote the preservation of the limulus in the future.
As a spineless organism inhabiting in deep layers of the ocean, Limulus belongs to Order Xiphosurida, Class Merostomata, Phylum Arthropoda or Subphylum Chelicerata.
The species and distribution of horseshoe crabs:
Today the limulus is categorized into four kinds:
1. Limulus polyphemus Linnaeus (1758): The exterior of this species appears greenish brown to dark brown. Mostly the species is distributed from the coastal area of Maine (New England region) to Gulf of Mexico.
2. Atachypleus tridentatus Leach (1819): This strain commonly appears round Japan, the coastal area of the mainland China (including provinces of Zhejiang, Fujian, Guangdong, Guangxi, Hong Kong, Taiwan and Hainan Island) as well as Vietnam, Philippines, Borneo and the coastal regions of Indonesia.
3. Tachypleus gigas Muller (1785): This strain looks green, and is widely distributed round the seaside of India, Singapore, Borneo and Indonesia.
4. Carcinosvorpius rotundicauda Latreille (1802): This strain looks green to greenish brown, and has been commonly found in Bangladesh, Thailand, Indonesia and Borneo. They are prone to inhabit in less salty estuary and have an inclination to swim upward a river.
The attributes and habits of limulus:
The adult limulus lays eggs from May to September, and migrates to seawater regions (20-30m) for winter when northeast monsoon starts. Female limulus along with male limulus would swim up toward high tidal marks of the intertidal zone and lay eggs in early morning or late afternoon when tides rise. A batch of over 100 light-yellow eggs with a diameter of 0.3cm is usually laid in 5cm deep sand.
After incubation, the baby horseshoe crabs would float over to the muddy sand in the vicinity of high tidal marks, and transform into adult limulus, inhabiting in deep-sea regions. Adult limulus lives on tiny living things. As they reach 4-5 centimeters long, they move along with the current to sandy regions. When they grow to 10 centimeters, they would leave the intertidal regions and move into deeper seawaters. A couple of years afterward, when they are fully grown, male limulus begin to seek opportunities for mating, and will swim back to coastal areas to lay eggs in due season.
Limulus live in sandy areas round shallow waters, inclined to crawl above or below the muddy sand. With the increase of age, they swim outward into the ocean from sandy beach. In winter, limulus are used to inhabit in deeper seawaters for the reason of temperature. They would wait till next spring, when the temperature rises, to migrate to shallow waters for food and laying eggs. The baby limulus that is born in late summer can live through winter round the lower tidal mark. Baby limulus come out for food when the weather looks good.
How to distinguish male from female:
The upper part of a male limulus has a dent, with the second and third feet transforming into hook-like shape, whereas the upper part of a female limulus is round curved shape, with crab-shaped feet. In regard with the abdomen, a male has six pairs of spina marginalis, while a female has three shorter pairs of spina marginalis and three longer pairs, which is the result of evolution so as not to hinder the hug from males. Also the sex can be recognized by genital pore inside the operculum genitalis.
How they shuck off shells:
Horseshoe crabs, like other arthropods, begin to mature after shucking off shells. Once they shuck off a shell, they would grow 1.3-1.4 times than before, yet the potential of death lies with such shucking. Hence it may be said that the growth is at the price of life. Before the incubation, an egg has had shucking membranes for four times. Take a close look at the transformed egg and you’ll find a delicate membrane covering the shell, at which time the embryo has turned into the shape of body an adult limulus has. The first ever born baby limulus resembles the adult limulus without sword-like tail. The tail would grow when the egg is incubated for the second time. The baby limulus reaches 10mm when it is incubated at second time.
When shucking off shells, the process starts from the head-to-chest part. The new shell is so soft that it seems unbelievable that it will one day return to previous hardness. Due to a slower pace of growth, the horseshoe crab is an animal that shows very little change over a long duration of time. It is still uncertain as to the period of time the limulus grows into adults. It is generally believed that males would need 15 times of shucking off shells before becoming adults in the 13th year, whilst females would shuck off shells for 16 times and become adults in the 14th year.
Colloquial expressions about horseshoe crabs:
●Picking horseshoe crabs: This expression is used to describe someone who is very lucky and strikes a bargain.
●Catching horseshoe crabs: The couple of horseshoe crabs are called mandarin duck fish. Hence people refer to the act of adultery as “horseshoe crabs,” which means catching adulators on the spot.
●Catch a male limulus and you’ll be out of luck for three years; catch a female limulus, and you’ll be doomed for an entire lifetime: Adult female horseshoe crab shoulders the male one no matter they are swimming in waters or crawling in the sandy beach. Therefore fishermen can never pick one and let go of the other.
●Pick a horseshoe crab and you will be doomed for an entire lifetime: This expression means that horseshoe crabs are happy couples. Once you pick one, the other would be left lonely and unhappy, This is a metaphor aimed to warn people never to tear a couple apart; otherwise bad luck would follow the culprit all his life.
●You would kill a beautiful horseshoe crab like shit if you don’t have the knack:
Killing a horseshoe crab requires skills. Those who don’t have the knack would kill it in a mess. The metaphor is a good thing would turn miserable without necessary skills. |