Giant Sea Spiders

Giant sea spiders are also known as the pycnogonids or pantopoda. They are more commonly found in the shallow waters. However, they can be found at the depth of about 7,000 meters all over the world, especially in the Caribbean and Mediterranean Seas as well as in the Antarctic and Arctic Ocean. There are nearly 1300 known species of the sea spiders, which are divided into 8 families and 86 genera. They vary in size ranging from a few millimeters of leg-span to the giant-sized with a leg-span of about 75 centimeters.

The giant sea spiders walk along the bottom of the water, or swim using an umbrella pulsing motion. Most sea spiders are carnivorous and they feed on sponges, bryozoans, polychaetes and cnidarians. They feed by sucking the juice from the soft-bodied invertebrates through their long proboscis.

Morphology of Giant Sea Spiders

The giant sea spiders possess long legs with a small body. They have 8 walking legs (means four pairs). However, there are some species with five and six pairs of legs. Due to their small and slender body and long legs, they have no respiratory system. An exchange of gases is made through a process of direct diffusion. The body is composed of two sections, that is an abdomen and a cephalothorax. The abdomen is very small in size. Their digestive system consists of diverticula, extending into the legs. They suck the nutrients from the soft-bodied invertebrates through their proboscis.

A proboscis is a unique feature of the pycnogonids. The size and shape of the proboscis is variable from species to species. A degree of movement of the proboscis is also variable. However, it has a restricted range of the dorso-ventral (up and down) as well as lateral (sideways) movement. The internal structure of the proboscis and entire alimentary canal appears trifoliate in its cross-section.

The body is supported by a non-calcareous exoskeleton. It is made up of a thick hardened ectocuticle, a thin epicuticle and a soft, more flexible endocuticle. The joints are movable as they consist of flexible endocuticle. There is an eye turret, known as an ocular tubercle, on the dorsal side of the cephalothorax. It supports 4 simple eyes, which offers a good all-round light detection. The cuticle also consists of hair and pits, which act as the chemosensory and tactile sense organs.

The ovigerous legs are a peculiar feature of the Pycnogonida. In many species, they are present only in the males; while in some genera such as Pycnogonum, they are present in the females too. They consist of 5-10 segments along with a claw at the distal end. In females, they are used for grooming and cleaning the carapace. During the breeding season, the males use them for the courtship and then to carry the eggs until they hatch. The remaining appendages are the walking legs, also known as the ambulatory legs. They contain a femur, 3 coxae, a tarsus, a propodus, two tibias and a claw. The legs consist of the muscles and nerves, which allow them to move, as well as the extensions of the gut and reproductive organs.

The pycnogonids have a small, thin, long heart that beats at 90-180 beats per minute, creating the substantial blood pressure. The sea spiders have an open circulatory system and a nervous system including a brain. The brain is connected to the two ventral nerve cords that in turn are connected to the certain nerves.

The pycnogonids are very small in size and their tiny muscles contain only a single cell, surrounded by the connective tissue. The anterior end (head) of the body has two pairs of non-walking appendages and proboscis. These appendages are known as chelifores and there are palps behind them. In some species, the palps and chelifores are reduced or absent. In such species, the proboscis is well developed and more flexible and mobile. They are provided with a number of sensory bristles and strong rasping ridges around the mouth. The last segment contains the anus and tubercle that projects dorsally.

Reproduction and Development of Giant Sea Spiders

The pycnogonids are gonochoristic, i.e. they have two distinct sexes. However, there is an exception of one species known as Aschorhynchus corderoi, which is a hermaphrodite. Each spider is both male and female. The reproductive system of the giant sea spiders consists of the organs such as a pair of testis or a pair of ovaries, lying above the digestive tract. The reproductive organs extend into the leg cavities as far as the femur. The most species, the eggs ripen in the legs and the genital openings are on the legs, usually on the second coxae. The reproduction is by the means of external fertilization after a brief courtship. The males take care of the laid eggs and the young ones. The larva possesses a blind gut and its body consists of a head and three pairs of cephalic appendages such as ovigers, palps and chelifores. The abdomen and thorax with thoracic appendages develop afterwards. There are four types of larvae such as the typical protonymphon larva, the atypical protonymphon larva, the encysted larva and the attaching larva. The typical protonymphon larva is the most common. It is free-living and gradually develops into an adult.