Sea Spiders
Etymology, behavior, and development
Sea spiders, or pycnogonids, can be found in all oceans, often in littoral zones, but have been found everywhere from intertidal to abyssal vent communities. Because they are small, cryptic animals usually only found in low densities, they are often overlooked by studies (Arnaud & Bamber 1987). Scientific nomenclature of this group of organisms has Greek roots, pyknos means crowded and gony means knee, and the common name is sea spider because this group is often placed within arachnida and is still thought to be closely related (Cobb 2010).Sea spider movement is either active, walking or in some species swimming, or passively carried on currents while latched onto a food source. Whichever form of locomotion sea spiders choose, they are very slow moving, so much so that a variety of other organisms will attach themselves to the legs of sea spiders. Sponges, cnidarians, and bryozoans are the main food source for sea spiders (Arnaud & Bamber 1987). Reproductively, sea spiders have a behavior that is different from all other invertebrates, which is that the males carry the eggs until they hatch and the young crawl off the adult male bodies (Dunlop & Arango 2005).
According to Arnaud and Bamber (1987), reproductive organs are paired and located dorsally to the gut, surrounding the heart and extending into the walking legs. The arrangement is the same for both the testes of males and ovaries of females with the exception of ovaries protruding further into the legs than testes. Eggs develop inside female walking legs and are excreted through gonopores. The eggs are fertilized externally before the male balls them up and carries them on ovigers until the sea spiders are developed enough to crawl away. More than one ball of eggs can be carried on a single oviger and the balls of eggs can be from more than one female, in different stages of development, or just from different legs of a single female.
Figures 1-2. Left: Sea spider oriented ventral side up with the anterior end facing to the left. Ovigerous legs are holding protonymphs. Right: Sea spider oriented ventral side up witht the anterior end facing to the right. Ovigerous leg is holding one ball of eggs that are earlier in development than the juveniles in the photo to the left.
External body arrangement
Sea spiders are usually characterized by their external traits, including: the cephalon, which bears a proboscis, ocular tubercle, paired palps, chelifores, and ovigers; the trunk, which bears walking legs; and the abdomen bearing walking legs and anus. Feeding is accomplished with the proboscis, which deviates in size, shape, flexibility, and orientation between species. The proboscis articulates with extrinsic muscles and the mouth at the end of the appendage is able to rasp and suck food with intrinsic musculature. Rotational range of the proboscis depends on the width of the arthrodial membrane, which is a soft collar at the point of attachment to the cephalon. Chelifores are claws used to assist in feeding and are lacking in sea spiders with a highly developed proboscis. The chelifore is made up of two ‘fingers’, one of which can move and the other is immobile, and the ‘fingers’ also have ‘teeth’ to improve grip. The palps are often reduced, but can be up to 20 segments in length and covered in setae, especially in distal segments. Palps assist food gathering and are hypothesized to be sensory in some species. Ovigers are only present on the ventral side of males and exist to carry eggs, with the exception of the Colossendeidae family. Walking (or swimming in some species) legs are always nine segments long. Moving distally, the first three segments of a sea spider leg are coxae, next is a femur, two tibiae, a tarsus, a propodus, and terminates with a claw. Pycnogonids are able to regenerate walking legs if they are broken off and are more effective at regeneration if the loss of the leg happens earlier in life (Arnaud & Bamber 1987).
Figure 3. Dorsal view of Nymphon australe with anatomical labels. Anterior end is at the top of the photo.
Cuticle
As described by Arnaud and Bamber (1987), above the muscle tissue, there are three layers; the exocuticle, which is thin, hardened, and present on the outside of the animal with the exception of joints, the endocuticle, which is thicker and the outermost layer at joints, and the innermost layer is the hypodermis, which surrounds the muscle tissue. The cuticle is shed out of necessity as sea spiders grow larger in size with age. During molting, each leg is shed in a single piece, each chelifore is shed, and the rest of the body is split dorsal-ventrally and splits into two pieces. A sea spider cuticle also contains many ducts present throughout all segments of the body. The ducts originate from glands located in the hypodermal tissue and some end in spines protruding from the exocuticle with sundry functions. Many glands serve an unknown function, but some known functions include: excretion from chelifores of larval stages that allows for attachment to other larvae, the male parent, or a food source and some males of cement glands in their legs to keep eggs in place on their ovigers.
Digestion and excretion
When a sea spider feeds, the proboscis is used to collect the food, often with assistance from other appendages. The food is then sucked into the muscular pharynx, which is located in the posterior section of the proboscis and this is where mastication occurs. The digestive process was also detailed by Arnaud and Bamber (1987), starting with the movement of macerated food into the esophagus through continued suction and is now in the first segment of the body. Posterior to the esophagus is the midgut and the two are separated by a valve that is held closed while food is digested or secreted. So when the midgut is empty, there is no pressure holding the valve closed and food can proceed from the esophagus into the midgut. The midgut is dispersed throughout trunk and into the chelifores and walking legs as caeca. Food in the midgut is moved in a peristaltic manner, while food is digested intracellularly. Remaining gut contents are moved through another valve and into the hindgut, which is located in the abdominal segment of the trunk. Waste is moved through the hindgut and anal muscles are stimulated when the hindgut is filled with waste and contract to open for the excretion of that waste.
Circulatory system
Pycnogonids have an open circulatory system similar to many arthropods in that the circulated substance is colorless plasma called hemolymph, which is analogous to blood. Blood and hemolymph differ in that hemolymph does not contain hemoglobins and the body itself contains the hemolymph, allowing it to move freely in the hemocoel. The hemocoel, also common to arthropoda, is comprised of interconnected sinuses surrounding organs. A muscular dorsal vessel that runs the length of the trunk, a pumping heart, contractions of digestive caeca, and leg movement powers circulation of hemolymph throughout the hemocoel. Hemolymph circulates away from the heart in the ventral sinuses and towards the heart in dorsal sinuses. Possible purposes of hemolymph are: transfer of nutrients, hormones, gases, and pressure throughout the body. An ability to control pressure throughout the hemocoel is useful during molting (Arnaud & Bamber 1987).
Nervous system
Sea spiders have a brain consisting of a ganglionic mass located dorsally to the esophagus. The ganglionic mass is made up of three cerebral segments; moving anterior to posterior the segments are the protocerebrum, deuterocerebrum, and tritocerebrum (Maxmen et al. 2005). Arrangement of head appendages and neural connections is one way to discern evolutionary relationships within arthropoda (Maxmen et al. 2005). As described in Arnaud and Bamber (1987), the innervation throughout the body moving along the brain anterior to posterior: optical nerves, cheliforal nerves, the rostral nerve, palp nerves, oviger nerves, and a dorsal and a ventral nerve for each leg. The rostral nerve leads to stomodeal nerves, which extend through the length of the proboscis and a possible cluster of sensory cells at the end of the proboscis.
Systematics
Phylogeny of the class pycnogonida remains largely unresolved and has long been subject to debate. Currently, pycnogonida is within the subphylum Chelicerata of Arthropoda, along with horseshoe crabs and arachnids. All extant sea spiders are categorized into the Pantopoda order, while extinct species are placed within Paleopantopoda. Pantopoda families are listed below and there are more than 1,300 recognized species of sea spiders.
Ammotheidae
Austrodecidae
Callipallenidae
Colossendeidae
Endeididae
Nymphonidae
Pallenopsidae
Phoxichilidiidae
Pycnogonidae
Rhynchothoracidae
References
Arnaud, F., and R.N. Bamber. 1987. The biology of pycnogonids. Adv. Marine Biol. 24, 1–95.
Cobb, M. 2010. Pycnogonids. Current Biology 20(14).
Dunlop J.A. and C.P. Arango. 2005. Pycnogonid affinities: a review. Journal of Zoological Systematics 43(1): 8-21.
Maxmen, A., W.E. Browne, M.Q. Martindale, and G. Giribet. 2005. Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment. Nature 437 (7062): 1144–1148.
