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From: Rick Webber Te Papa
_: 5
Date: 29-07-10
Time: 17:36
Spiders of the sea
courtesy SEAFOOD MAGAZINE July issue
Sea spiders, or Pycnogonids (Class Pycnogonida, pronounced pick-no-gonida) are part of almost every fishers’ workplace. They range in size from a few millimetres leg span, to over 700 mm and are found worldwide, from tropical to polar seas and in depths ranging from rock pools to over 7,000 metres. But there is still a lot we don’t know about this pervasive creature. Te Papa technician Giles Speeden with Curator of Crustacea Rick Webber look closer.
If you went to sea to escape spiders, think again! Wherever you are, on land or sea, they are never far away
Let’s play a quick game; “I usually have four, five or six pairs of legs. My mouth is at the end of a tube which is occasionally longer than my body. I have three lips. My organs are spread through my body and down my legs. What kind of creature am I?”
If you answered ‘Sea Spider’ then give yourself a pat on the back!
I’m talking about pycnogonids rather than the feather starfish sometimes affectionately called sea spider.
Pycnogonids entered my life almost a year ago, or to be technically correct, I noticed Pycnogonids about a year ago. In reality, they had been there with me for some time before then. I was working away in a lab and had observed a large jar with what I assumed (for the best part of six months) to be some kind of plant (Fig 1). Lo and behold, how naive of me!
As it turns out, I had been quietly sharing my space with a rather neat specimen of a large pycnogonid. My interest sparked, I did some research and found that pycnogonids are actually very common. You yourself may have seen the odd one or two and not realized just what you were looking at. So, with the help of Rick Webber, Curator of sea spiders and other marine creatures with jointed legs, I’d like to open up your world to these strange animals.
Who are they?
There are over 90 species of Pycnogonida in New Zealand seas; about 1,300 world wide. There is much that is unknown about the biology of these animals, despite them being common.
Peculiar, slow moving and harmless as they are, sea spiders have obviously adapted well because fossils indicate they’ve been around for over 400 million years.
What are they? Pycnogonids are different, to say the least. It’s their unlikely shape and unique structure that make them interesting, so to reassure you that they are indeed real, here’s a bit more on their anatomy.
An alternative name for sea spiders, Pantopoda, means ‘all legs’ and it’s easy enough to see why they got this name. In most species, specially the long-legged ones, the body itself looks like little more than a piece of lego to join the legs to.
The body is, of course, a little more complicated than that (Fig 1), with a head part that carries a huge feeding tube (proboscis) and some additional limbs; a turret behind the head with simple eyes - usually four to give 360° vision (Fig 2); and an abdomen at the rear, which is the largest part of an ordinary spider but reduced to a little tail-like thing in pycnogonids. Given the fact sea spiders have a proboscis which is often longer than the body, plus a miniscule abdomen, you can imagine why people sometimes think these creatures walk backwards.
But back to the legs. Pycnogonid legs consist of nine segments and leg sizes range between species, from no longer than the body, to as much as seven times its length. Two extremes represented in local waters are species of Colossendius with long legs (Figs 1) and little Pycnogonum species with far shorter ones (Fig 3).
Most species have eight legs like regular spiders; rarely there may be fewer, but some can have 10 or even 12 legs. This ‘polymerous’ characteristic of having a variable number of legs, is unique to sea spiders among all the arthropods (spiders, crustaceans, insects and everything else with jointed exoskeletons).
Another remarkable aspect of sea spider legs is their tendency to contain the animal’s internal organs. Pycnogonids hardly have a body worthy of the name so the intestines and reproductive organs extend into the upper halves of all the legs. As a result, they have as much internal gut surface to absorb nutrients in their food as other animals with one long gut. But to be different once again, sea spiders don’t have just one genital opening but have a genital pore on each leg in both sexes, corresponding to the reproductive organs that extend into them – eight times the fun, at least!
The feeding tube (proboscis) on the head part of Pycnogonids is worthy of further note. It is often as long as the body (see Fig 4) and can even be more than twice as long. As mentioned earlier it usually has three lips at its tip which can have teeth or spines on them, used to pierce the bodies of prey when feeding.
The proboscis can be moved around to some extent. It varies in shape between species and in some it is specialised for eating a certain diet. For instance, it may be shaped to extract the tiny animals from colonies of bryozoans (moss animals); in other species it is fashioned for penetrating and eating sea anemones. In the Antarctic species Austropallene cornigera (Fig 5) the rostrum is pointed, but its diet is not yet known.
The head appendages (Figs 1 & 5) are not for walking, but when they are present, the palps are used for cleaning and sensing, the ovigers for reproduction and grooming and the chelifores (pinchers) for feeding and defence (Fig 6).
Where are they? You can hardly be blamed for not seeing them; as part of their slow-moving, inoffensive lifestyles many pycnogonids are specialists in camouflage. Some sand dwelling species stick sand to themselves while others are transparent and display the colour of what they have eaten in their gut (Fig 6); and since they are most often found on the seaweed or invertebrate animals they eat, this makes them very hard to see.
Shallow water pycnogonids tend to be small with shorter legs, are slow-moving, and move little. They live intertidally and below low tide where they are found clinging to seaweeds, under rocks, with tube worms, with sea anemones or with a variety of different colonial invertebrates including bryozoans, sponges and hydroids (small branching colonies related to corals). Where ever they are found these habitats are usually what the pycnogonids feed on.
On the other hand, long legged species live in deeper waters usually on more open ground, and are more active than their shallower water, short-legged relatives. Photographs of sedimentary bottoms of the continental shelf find them walking about in the open.
Pycnogonids are not necessarily inactive. Short legged ones such as Pycnogonum (Fig 3) are relatively sedentary but long legged ones can be quite mobile and walk about on tip-toe. As a way of relocating, some species swim up into the water to be carried about by currents. To descend they fold up like an umbrella frame and drop back down to the bottom.
Some of the largest may also use deep currents to tumble across the sea floor like Spinifex on the beach.
What about eating? Pycnogonids have specific tastes or preferences for particular types of prey. This is something they develop depending on what they were fed as juveniles, or to put it another way, they like to eat what tasted good when they were youngsters. Some specialise in eating bryozoans, some feed on anemones which they seldom kill due to size. Some parasitise jellyfish so they presumably swim up into the water column,to catch onto them, and the larval stage of sea spiders (protonymphon) is sometimes found with its proboscis piercing little jellyfish.
Pycnogonids feed in a variety of ways which conform to the proboscis. Most are carnivorous, a few eat seaweed and some scavenge, and it is the carnivores that usually have 3 lips with cuticular teeth. Some species insert their proboscis into soft prey such as sea anemones and quietly suck out their internal juices. Others secrete digestive juices out the end of the proboscis into the prey and then suck up the partially digested matter. Yet another species uses its small chelifores (pinchers) to break off pieces off the host and pass them to the proboscis. Typically, there is such a large difference between size of predator and prey that prey usually survive their encounter with the Pycnogonid.
How do they replace themselves? The sexes are separate but the reproductive habits of Pycnogonids are not well documented. And nothing is known of reproduction in most deep sea forms although their small eggs suggest they hatch larvae which parasitise other deep sea invertebrate animals. However, it is known that reproduction generally involves a brief courtship before the male attaches himself to the underside of the female (back to front, just to be awkward). From this position the eggs and sperm are released into the water via the genital openings in the legs.
Some species of sea spider have a set of small leg-like appendages on the head, (called ovigers) which are typically used by the males to collect the fertilized eggs which are then ‘glued’ together into a ball, and then to the ovigers themselves. In species without ovigers the egg mass is glued directly to the underside of the body (e.g. Pycnogonum rhinoceros Fig 3). The males brood the eggs like this until larvae, called a protonymphons, hatch.
Protonymphons become free living or sometimes parasitise jellyfish, mollusc shells, echinoderms (starfish, sea slugs sea eggs etc) or other invertebrates. The little tick-shaped protonymphon has cheliphores and six legs rather than eight. It goes through several growth stages adding segments and appendages until it becomes a fully fledged juvenile pycnogonid.
What are their chances? There is much to be learned about these odd creatures, and their future looks bright. We do hope our information leads to more specimens being collected and more research being carried out into the part pycnogonids play in the marine ecosystem.
Who helped with this? The following people kindly helped us to track down photos and gave permission for their use; Kareen Schnabel, Anne-Nina Loez, Dave Allen and Michele Hollis of NIWA; Karen Gowlett-Holmes; Iain MacDonald, University of Auckland; Stefano Schiaparelli of Italy; and Johanna Nielsen, University of Edinburgh.
Caption
A long-legged, deepwater species of Colossendius from the Ross Sea. Size approximately 400 mm leg-span. The colour would blend well with its surroundings. Photo Rick Webber, Te Papa.
Last changed: 23-Jul-2011