Pycnogonida

(Sea spiders)

Phylum Arthropoda

Class Pycnogonida

Number of families 8

Thumbnail description
Spiderlike marine creatures that both live on and eat seaweed and some marine invertebrates

Evolution and systematics

Pycnogonids have almost no fossil record, though three genera have been found in the Devonian in the Hunsruck Slate in Germany. Despite the lack of fossil evidence, scientists have deduced from morphological and embryonic studies that sea spiders are an old lineage of animals.

Pycnogonids are an unusual group that has been difficult to place relative to other arthropod groups. Pycnogonids are thought to represent an early divergence from the evolutionary line leading to other Chelicerates. Both pycnogonids and chelicerates have claws on the first appendages and a tubercle with simple eyes, and both lack antennae. Pycnogonids, however, differ by possessing features such as the proboscis, reduced abdomen, and ovigers.

About eight families and 1,000 species of sea spiders are known.

Physical characteristics

Pycnogonids are seldom seen because they are pale or cryptically colored. Most pycnogonids are about 0.39 in (1 cm) or less in size, but some deep-sea forms reach up to 27.5 in (70 cm) across between leg tips. They have extremely reduced bodies in which the abdomen has almost disappeared, while the legs are long and clawed. The head has a long proboscis with a terminal mouth and a single four-part eye on a central stalked tubercle.

The surface area of the thin body and legs allows diffusion of gases and wastes, since pycnogonids lack specialized respiratory or excretory structures. Digestion occurs in the gut, which sends branches into the long legs. Most species have four pairs of walking legs, but some have five or six pairs, with the reproductive organs located in the joints of the legs. The males, and often the females, have ovigerous legs in addition to the walking legs. These legs are used for holding and carrying eggs during the breeding season and, in females, for cleaning and grooming outside of the breeding season. The body is contained in and supported by a non-calcareous exoskeleton.

Distribution

All pycnogonids are found in the marine environment. They are distributed worldwide, from tropical waters to the poles.

Habitat

Pycnogonids are benthic organisms with a habitat ranging from intertidal zones to depths over 19,865 ft (6,000 m). While most live on seaweed, corals, and sponges, one species has been discovered in the hydrothermal vent communities at the great depths of the Galápagos Rift.

Behavior

Adult sea spiders are solitary but often live in close association with invertebrate food hosts or seaweeds. Species with smaller body size move slowly around the substrate, while the larger deep-sea pycnogonids tend to be more active; some even swim, using leg motions similar to walking.

Feeding ecology and diet

Sea spiders feed on seaweeds and soft-bodied invertebrates, including hydroids, soft corals, bryozoans, anemones, and sponges. Certain species also prey on opisthobranchs, small polychaetes, and other mobile soft-bodied invertebrates. Some pycnogonids pierce the skin of their hosts with teeth at the tip of the proboscis and suck the juices out of their prey. Others tear their prey apart and pass the pieces into a proboscis for feeding.

Reproductive biology

Male sea spiders often mate with more than one female. During courtship, male pycnogonids use their ovigerous legs to induce the females to release eggs. Once the female begins to lay eggs, the male fertilizes them as the female holds them on her ovigerous legs. In some genera, the eggs are relatively large and the female only releases 4–5. In other genera, the eggs are small and the female releases more than 100 eggs. Depending on the leg type, after fertilization, the male will either gather the eggs one by one onto his legs or hooks his ovigerous legs into the egg mass and gathers most of the eggs into a single mass onto his legs.

Male sea spiders carry cemented egg clutches gathered from females until they hatch; the actual amount of time the male carries the clutches depends on species and geographic location. Upon hatching, larvae have only three pairs of legs and develop an additional pair (or pairs) upon subsequent moltings. The most common larval type is a free-swimming naulpius-like larva. However, in some species, the males continue to provide care of the larvae until after several moltings. In this case, the females produce eggs with a large amount of yolk and the young continue to live off of the yolk while attached to the male's ovigerous legs.

Conservation status

No species are listed by the IUCN.

Significance to humans

Sea spiders have no known significance to humans. They are usually not even seen, or are ignored, by humans.

Species accounts

List of Species

No common name

Colossendeis megalonyx

order

No order designation

family

Colossendeidae

taxonomy

Colossendeis megalonyx Hoek, 1881.

other common names

None known.

physical characteristics

Body is approximately 0.78 in (20 mm) long and proboscis is 1.2–1.6 times the length of the body, with a broad rounded tip. As members of the genus with the longest leg span, it has legs spanning up to 27.5 in (70 cm), with each leg bearing a long slender claw.

distribution

From depths of 10–16,400 ft (3–5,000 m) throughout circumpolar Antarctica with northern extension into the south Atlantic Ocean, south Indian Ocean, and South Pacific Ocean, including the Antipodes Islands off of New Zealand.

habitat

Nothing is known.

behavior

Nothing is known.

feeding ecology and diet

Eats soft corals and small hydroids attached to sponges.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Anoplodactylus evansi

order

Pantopoda

family

Phoxichilidiidae

taxonomy

Anoplodactylus evansi Clark, 1963.

other common names

None known.

physical characteristics

Leg span up to 1.2 in (30 mm) across, with four pairs of legs.

distribution

Found along the New South Wales coast of Australia and as far south as Tasmania.

habitat

Nothing is known.

behavior

Nocturnally active, it seeks shelter in algae during the day. It hunts opisthobranchs on benthic algae and immobilizes them with immoveable claws on the front four legs while the back four legs remain secured to the substrate. It then consumes all of the soft tissue of the prey. While solitary in the wild, it will feed in groups on the same item when captured in a holding tank. It broods its young in seaweed. This behavior limits dispersal and can lead to resident populations at particular locations.

feeding ecology and diet

A generalist predator of small opisthobranchs, including nudibranchs and other soft-bodied invertebrates. Attacks prey up to 5–6 times its body weight. Some of the prey species secrete toxins, which it can tolerate at low levels, including compounds found in sponges, cnidarians, bryozoans, and opisthobranchs. One of its main food sources, juvenile sea hares, secretes ink that contains toxins when attacked. If the sea spider comes into contact with the toxic deterrent, it will vigorously wave the affected limb.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Child, C. A. Marine Fauna of New Zealand: Pycnogonida (Sea Spiders). Wellington: National Institute of Water and Atmospheric Research, 1998.

Hedgpeth, J. W. "Pycnogonida." In Treatise on Invertebrate Paleontology. Part P, Arthropoda 2: Chelicerata, edited by R. C. Moore. Lawrence, KS: Geological Society of America and University of Kansas Press, 1960.

Periodicals

Arnaud, Francoise, and Margo L. Branch. "The Pycnogonida of Subantarctic Marion and Prince Edward Islands: Illustrated Keys to the Species." South African Journal of Antarctic Research 21, no. 1 (1991): 65–71.

Rogers, C. N., R. de Nys, and P. D. Steinberg. "Predation on Juvenile Aplysia parvula and Other Small Anaspidean, Ascoglossan, and Nudibranch Gastropods by Pycnogonids." Veliger 43, no. 4 (2000): 330–337.

Other

Underwater Field Guide to Ross Island and McMurdo Sound, Antarctica. Scripps Institution of Oceanography, University of California, San Diego. [29 July 2003]. <http://scilib.ucsd.edu/sio/nsf/fguide/>.

Elizabeth Mills, MS