7/22/2023 0 Comments Sea spider predatorsThe adults are free living, while the larvae and the juveniles are living on or inside temporary hosts such as polychaetes and clams. Not much is known about the development of the atypical protonymphon larva. The encysted larva is a parasite that hatches from the egg and finds a host in the shape of a polyp colony where it burrows into and turns into a cyst, and will not leave the host before it has turned into a young juvenile. ![]() The typical protonymphon larva is most common, is free living and gradually turns into an adult. One theory is that this reflects how a common ancestor of all arthropods evolved starting its life as a small animal with a pair of appendages used for feeding and two pairs used for locomotion, while new segments and segmental appendages were gradually added as it was growing.Īt least four types of larvae have been described: the typical protonymphon larva, the encysted larva, the atypical protonymphon larva, and the attaching larva. The abdomen and the thorax with its thoracic appendages develop later. The larva has a blind gut and the body consists of a head and its three pairs of cephalic appendages only: the chelifores, palps and ovigers. ![]() Reproduction involves external fertilisation after “a brief courtship”. Females possess a pair of ovaries, while males possess a pair of testes located dorsally in relation to the digestive tract. Reproduction and developmentĪll pycnogonid species have separate sexes except for one species that is hermaphroditic. These creatures possess an open circulatory system as well as a nervous system consisting of a brain which is connected to two ventral nerve cords, which in turn connect to specific nerves. The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a moult. The morphology of the sea spider creates an extremely well suited surface-area to volume ratio for any respiration to occur through direct diffusion. The organs of this chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone. The abdomen does not have any appendages, and in most species it is reduced and almost vestigial. A cephalothorax and much smaller abdomen make up the extremely reduced body of the pycnogonid, which has up to two pairs of dorsally located simple eyes on its non-calcareous exoskeleton, though sometimes the eyes can be missing, especially among species living in the deep oceans. In total, pycnogonids have four to six pairs of legs for walking as well as other appendages which often resemble legs. The last segment includes the anus and tubercle, which projects dorsally. In those species that lack chelifores and palps, the proboscis is well developed and more mobile and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. In some species, the chelifores, palps and ovigers can be reduced or missing in adults. The anterior region consists of the proboscis, which has fairly limited dorsoventral and lateral movement, and three to four appendages including the ovigers, which are used in caring for young and cleaning as well as courtship. Pycnogonids are so small that each of their tiny muscles consists of only one single cell, surrounded by connective tissue. A proboscis allows them to suck nutrients from soft-bodied invertebrates, and their digestive tract has diverticula extending into the legs. Because of their small size and thin body and legs, no respiratory system is necessary, with gases moving by diffusion. The number of walking legs is usually eight (four pairs), but species with five and six pairs exist. ![]() Sea spiders have long legs in contrast to a small body size.
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