And Along Came a Spider...

True spiders, or animals that would be placed in the group Arachnida within the huge phylum Arthropoda, are not found in coral reefs or other submerged marine environments, although some spiders are active predators in intertidal habitats around the world. There is, however, one group of spider-like animals that is found in most marine environments, and that is the sub-phylum or class Pycnogonida, or the so-called "sea spiders." These are some of the more peculiar beasts that we can get in our aquaria, and most of them are probably unwelcome guests.

Arthropods are animals with a jointed exoskeleton made mostly of protein, but with a bit of an odd structural material called chitin, and in the case of many marine representatives, more than a smidgen of calcium carbonate. This huge array of animals includes among its representatives the insects in all their buzzing, biting, and annoying glory. As insects make up the majority of scientifically-described animals, the phylum Arthropoda is considered to be the largest group of animals. Fortunately, there are very few true marine insects, and the marine diversity of the group is correspondingly less. Most marine arthropods are crustaceans, which are animals such as crabs, shrimps, and the ubiquitous "pods" found in marine aquaria. Crustaceans are characterized by having two pairs of antennae or "sensory whiskers" on their head, as well as jaws to mash their food.

A few other arthropods, having neither antennae nor jaws, are found in the marine realm. As they lack some of the definitive crustacean characteristics they, therefore, cannot be crustaceans. These animals are called chelicerates (pronounced "KEL - iss - er - ates (preferred) or ka-LISS-er-ates (acceptable)). Terrestrial chelicerates include the scorpions, spiders, mites and ticks, as well as some other rather odd animals. Marine chelicerates include horseshoe crabs, mites, and pycnogonids.

All chelicerates, terrestrial and marine, share a few common characteristics. First, they are arthropods, so they have a jointed, external covering that doubles as both a protective armor and as a skeleton. Internally, they have a long tubular heart, and a circulatory system that is largely without vessels. Unlike us, and other vertebrates, their central nervous system is ventral, running down the bottom midline of the animal, rather than dorsally along the back. Generally, their blood pigment is hemocyanin and based on copper rather the familiar hemoglobin of vertebrates. When their blood is oxygenated it is a robin's egg blue in color; when deoxygenated, it is colorless.

Follow this link for pictures of the blue blood of chelicerates: LINK

Chelicerates have some specific and particular characters. They lack both antennae, and jaws. The latter characteristic is pretty important, as it means they must eat prey that is either liquid or made of very small particles. They often have a pair of pincer-like appendages, called chelicerae, (singular = chelicera) which appear to be similar to the claws of crabs. However, these appendages arise from the head and are structurally and embryologically different than the claws of crabs, which are derived from thoracic appendages. In fact, the structure of the chelicerae is so different and distinctive that it is what gives the group its name.

All chelicerates have two major body regions, each of which is composed of several fused embryological segments or regions. In the adult, however, these components may be effectively impossible to distinguish. The front region is called the fore-body or "prosoma," and consists of, what in other arthropods, would be the segments of the head and thorax. It bears the legs and the other appendages, the chelicerae and pedipalps. The long, often leg-like, pedipalps are considered to be sensory, but in some chelicerates they are quite large and they seem to function as aids in locomotion. . Generally, the pedipalps are held up in front of the chelicerate as it moves, waving around and touching the substrate as needed to "taste the world" or "see which way the wind blows." The back region, called the abdomen, after-body or "opisthosoma," often lacks appendages (see Figure 1). If appendages are present on the opisthosoma, they are reduced in size and are often involved in respiratory functions. Most chelicerates such as horseshoe crabs, spiders, mites and scorpions have four pairs of walking legs; other appendages are one pair each of chelicerae and pedipalps.

There are a lot of marine chelicerates, but the majority of these are small mites. Although occasionally found in aquaria, they are generally overlooked. The largest living chelicerates are marine animals called Xiphosurans, or horseshoe crabs. There are four living species of this group, which is also called the Merostomata. Three species of horseshoe crabs are found in Southeast or Eastern Asian waters, and one species, Limulus polyphemus, is found off the eastern shore of North America and in the Caribbean. In some very ancient seas, a group of animals quite similar to horseshoe crabs and modern scorpions, called Eurypterids, were found. A few species reached lengths in excess of ten feet long, and were quite massive. Referred to as "water scorpions" they were the largest arthropods in the Earth's long history. Eurypterids were predatory and were likely the largest predators of their day.

The Pycnogonida or Sea Spiders

There are probably somewhere around 1000 scientifically described species of pycnogonids, or sea spiders. There are undoubtedly far more than that, and the majority of these are likely to be found on coral reefs eating corals, sea anemones, hydroids, soft corals and other soft-bodied animals.

Follow this link to sea a beautiful sea spider that eats various types of sea slugs:

Pycnogonids are found in all seas but, with few exceptions, are seldom common animals. They are typically small animals, from about 0.04 inches to few inches across the leg span. However, there are some giant forms, with a leg span of a couple of feet. The larger animals are generally from the deep sea or polar areas and, in fact, may often be quite abundant and found in large aggregations. If you have real or incipient arachnophobia, photos of thousands of these large "spiders" crawling across the ocean bottom can make your skin crawl.

To see some of the larger pycnogonids, follow these links: LINK 1, LINK 2.

The name "pycnogonid" is derived from the Greek "pyknos" meaning "strong or compact" and "gonia" meaning "a joint, knee, or angled," and refers to the fact that these animals look like a collection of legs and/or angles with no real body. The body is reduced to a thin tubular section in between what are, often, quite long legs. Because of their very odd shape, pycnogonids have often been placed in a group of their own and are considered to be quite distinct from the other chelicerates. Modern genetic and morphological analyses, however, places them firmly in the chelicerates. Because of this rather long history of separation, a jargon terminology specific to the group has been applied to them. As in all chelicerates, pycnogonids possess chelicerae, here called chelifores, and pedipalps, simply called palps. I have reverted to the more general terms of chelicera and pedipalps in the illustrations for this column. Either the palps or chelifores may be absent in various species. Unlike the situation found in most groups of arthropods, the number of body segments and legs can vary from species to species, although they are consistent within each species. However, there are pycnogonids with 4, 5, or 6 pairs of legs. Additionally, the males have an extra pair of smaller "legs" that they use to carry around the eggs laid by the females.

There are two distinct types of pycnogonids: those with short legs that are often predatory or ectoparasitic on animals such as hydroids and corals, and those with long legs which are often predatory on other animals. The short-legged animals are often quite tiny, while the long-legged animals may be quite large (see Figures 2, 3).

Figure 1. A diagram of a generalized pycnogonid showing some of the body features. Modified from Kozloff, 1990.

Figure 2. Even relatively large pycnogonids may be quite difficult to see, depending upon their position on the background. In A, above, a pycnogonid that is about 1.5 inches across the leg span fills much of the picture. In B, below, I have outlined the legs and body in purple. This ability to blend into a background, coupled with exceptionally slow movements, may make these animals essentially invisible to potential predators.

Pycnogonids have a poor fossil record, but a few that have been found are truly ancient, indicating they have been around for hundreds of millions of years, see here:

Follow this link to see a scanning electron micrograph of the body structures:

In many pycnogonids the body is exceedingly narrow, about the width of one of the legs, and the posterior body region, the opisthosoma, is reduced to a tiny nubbin. At the front end of the animal is a large suctorial proboscis; a tube used in piercing and sucking up food. The proboscis, in most species, is lined with ridges of hardened cuticle and is quite muscular. Such a structure is quite capable of masticating food, so even though they lack jaws they may chew their food. Downstream of the proboscis, the gut forms a short esophagus and then passes into the main part of the gut, which occupies much of the body. The body is so thin that much of the digestive and absorptive glandular tissues of the gut are found out in the legs, an arrangement seen in no other type of arthropod. Their digestive process may best be described by one word - STRANGE. The food slurry is shunted to glandular lobes of the midgut. These lobes extend out into the legs where they are bathed by blood in a body cavity called the hemocoel. The distribution of digested nutrients is very poorly known in pycnogonids. It has been stated that the cells lining these glandular lobes absorb food or actively ingest food, and then break free and "drift" off into blood flowing throughout the hemocoel. These "drifter" cells supposedly attach elsewhere in the body some distance from the gut. After they have done so, the cells now adjacent to the attached drifters, receive digested food from these "drifters." The blood is moved through the body by the action of a long tubular heart, and by generalized body movements. Although this may seem somewhat haphazard, it works well to distribute the "drifter" cells as well as other corpuscles. Nonetheless, this, to say the least, is a bizarre way to distribute nourishment, and it is by no means certain that this is actually how it is done. Digestion and food distribution in these beasties needs to be studied in much greater detail.

As befitting their thin nature, which creates a lot of potential surface area for gas exchange, they have no specialized organs for respiration. They also have no specialized organs for the production of nitrogenous wastes. Presumably both respiration and excretion take place across the body wall through the exoskeleton. If so, this is a decidedly unusual arrangement for any arthropod. On the other hand, these animals are poorly known, and specialized microscopic excretory structures may well be found with more rigorous and thorough examination.

As with other aspects of their biology, reproduction is relatively poorly known. The gonads extend out into the legs. Eggs or spermatozoa are released from the bases of some of the legs. Unlike most arthropods, fertilization appears to be external, and the males brood the eggs on thin, small, appendages called "ovigerous legs." After embryonic development, an odd parasitic larva is formed. This larva, called "the protonymphon larva," has a suctorial mouth, and two pairs of appendages. It makes its way to a juvenile host where it eats host tissues and fluids, and undergoes several molts. Sometimes these larvae even live internally in the host. Finally, they emerge from the juvenile host as the predatory adults. The juvenile host may or may not be the same as the adult host. For example, some pycnogonids whose adults are predatory on hydroids spend their larval period in the gills of clams.

For much information, great photos and descriptions of feeding and locomotion, including animations in pycnogonids, see this site:

Aquarium Occurrences:

Pycnogonids have been reported and sent to me for identification sporadically over the last few years. Small, but relatively long-legged, animals have been recorded from a couple of aquarists in California. These small animals, often less than one-quarter inch across, appear to be predatory or parasitic on various corals and are quite capable of killing the corals. A type of short-legged pycnogonid has been found that is, of all things, a predator on that bane of reef aquarists, Aiptasia. Unfortunately, attempts to culture this predator were unsuccessful (see Figure 3).

Figure 3. Two individuals of a short-legged form of pycnogonid found parasitizing a large clonal aggregation of Aiptasia. Unfortunately, they were not able to be cultured. The animals are covered in anemone mucus, and I could not clean it off them for a clear photograph.

Sea spider predation or parasitism is a potential threat to all corals, soft corals, and sea anemones that are kept in aquaria. If an aquarium system gets infected, just how easy it is to control the pest species will really depend on the type of pycnogonid that is present. The large forms are typically slow, and readily apparent, and may be easily removed from aquaria. The small forms, on the other hand, may be quite difficult to see, as they may be as small, or smaller, than the polyps of small-mouthed corals. Additionally, they may live under the coral mucus layer, and this would further obscure them. They do not appear to be easily removed by "dips," poisons, or potential predators. Frankly, if they are found on corals in any tank, the best option would likely be to remove the coral from the tank, and simply dispose of it.

If you have any questions about this article, please visit my author forum on Reef Central.

References of Interest:

Child, C. A. 1998. Nymphon torulum, new species and other Pycnogonida associated with the coral Oculina varicosa on the east coast of Florida. Bulletin of Marine Science. 63:595-604.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

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And Along Came a Spiderů