The underwater benthic environment of McMurdo Sound, where the giant sea spiders are found. (NSF/USAP, Steve Clabuesch)

Giant spiders are often considered to simply be a fictional creation, such as the huge spiders in Harry Potter or Shelob in Lord of the Rings, but giant spiders do actually exist as divers in Antarctica have discovered.

Sea spiders with a leg-span of 70cm have been seen in the Ross Sea, while 30cm individuals have been observed crawling along the seabed in McMurdo Sound. This phenomena is known as polar gigantism. Gigantism is when an organism is unusually or abnormally large, and has been observed in both polar and deep sea habitats. Scientists are still trying to understand what allows organisms, such as these sea spiders,  to grow to such large sizes.

Sea Spiders?

Sea Spiders can be found in UK waters – this 3mm individual (Nymphon brevirostre) was found in a local benthic trawl (Photo, Rhian Taylor)

While these marine organisms resemble spiders, they belong to a different group of arthropods and more than 1000 marine species have been described, varying in size from 1-10mm to 70cm. Sea spiders are a class of marine arthropods called Pycnogonida, and while they are found in seas globally, 40% of 80 genera are found in Antarctica. This dominance has led to the theory that sea spiders originated from Antarctic waters.

With their eight legs, small body and elongated proboscis, it is easy to see how they ended up with the name ‘sea spiders’. Not all of their legs function as walking appendages, but have other uses, such as sensing, cleaning, defence and reproducing. A NIWA survey of the Ross Sea in 2004 found 18 species from 10 genera (and 5 of the 8 known families). Three of the observed species had more than 4 pairs of legs.

The Pycnogonida have a variety of diets, with some eating algae, some scavenging and others acting as predators, and a couple of species live parasitically on molluscs. Most species are carnivores, feeding on corals, sponges and worms, sucking nutrients by inserting its proboscis into the prey and consuming body fluids and tissues fragments (similar to how a mosquito feeds).

Don’t hold your breath!

Sea spiders don’t have a heart to beat blood and oxygen around their body, moving these around their body by pumping their gut. Their weak heart functions only to move blood around their central body. As they don’t have gills or lungs for respiration, sea spiders absorb oxygen through their cuticle. The gut branches throughout the body of sea spiders, and essentially fill space.

This unusual breathing mechanism was first discovered by H. Arthur Woods at the McMurdo base in Antarctica after spending “a lot of time just watching blood and gut flows in sea spiders.” He then conducted several experiments using video microscopy, tracing the haemolymph movements. Other experiments have also shown that contractions of the gut (which is present in the legs) moves oxygen around the body. The sea spiders gut has a peristaltic wave (also found in humans at a lower intensity), which is more vigorous than would be needed solely for digestion.

The unusual way that sea spiders respire, National Geographic

The Beast Below

A number of scientists attempted to create guidelines to categorise gigantism; an organism is at least twice as large as the mean body size for the genus, or an organism with a body length in the top 5% for its taxon. In an attempt to understand how polar gigantism occurs, scientists are first looking into how common the phenomenon is. Polar gigantism has been observed in a number of different marine taxa, including copepods, isopods and sponges. Gigantism has also been observed in the fossil record.

The Antarctic sea spider, Colossendeis megalonyx, observed near McMurdo Base. Photo by B. Miller

An early explanation (2002) for gigantism was that size was dictated by oxygen availability, while later studies (2009) observed 12 species (over 3 orders of magnitude) were able to perform under hypoxic conditions. Low temperature and metabolism are both common abiotic factors in habitats where gigantism has been observed (gigantism is also seen in deep sea habitats).

One study came up with eight different hypotheses to try to explain polar gigantism. These included carbonate and silica levels in the Southern Ocean, starvation resistance in response to the seasonal availability of food, interspecific interactions (such as the absence of sharks, skates and decapod crustaceans from Antarctic waters) and the temperature-size rule (where organisms living at lower temperatures grow more slowly, but grow to a larger final body size).

Despite the fact that there have been increasing research efforts into understanding more about these giant Antarctic sea spiders, the explanation for how and why they grow to such giant sizes is is still under speculation.

print

(Visited 38 times, 1 visits today)