A careful hunt through and around the algae, silt and rocks in a rockpool may turn up a very strange creature indeed, including sea spiders. Looking a bit like a child’s crude attempt at modelling a spider from a bunch of pipe cleaners, this one is an odd animal discovery that doesn’t initially make much sense.
What are sea spiders?
It belongs to a group of animals referred to as sea spiders, but while it has eight legs and looks superficially like a spider, that is where the similarities end. It doesn’t move like a spider, feed like a spider or indeed live in the places where you might find a spider. Sea spiders’ place in the animal kingdom remains the subject of debate, but current thinking is that they are a group that split off early in the arthropod evolutionary line.
Sea spiders are not even arachnids, so they are more distantly related to spiders than are ticks, scorpions and mites. They do belong to a group (the chelicerates) that otherwise includes only arachnids and horseshoe crabs. However, even that might be misleading, because evidence
is rapidly mounting that pycnogonids are in fact the sole survivors of a truly ancient group that split off from all other arthropods hundreds of millions
of years ago.
What do sea spiders eat?
Sea spider prey consists of soft-bodied creatures such as hydroids, anemones and bryozoans, which are slowly grazed using the chelifores to snip up sections of tissue. These are then sucked up and filtered out via the sea spider’s bulbous proboscis, located in the middle of its head and terminating in a set of jaws. In many cases, sea spiders don’t kill their prey, rather just nibble at it, meaning their prey lives to see another day.
The well-named gangly lancer (Nymphon gracile) is one of about 27 species of sea spider found in the waters surrounding the UK. It is widespread and common among the algae on rocky shores all around our coastlines. With a body about 10mm long and four pairs of wiry legs, each three to four times its body length, it is one of the bigger sea species likely to be encountered.
What does the gangly lancer sea spider look like?
Having said this, the word ‘body’ might be a little misleading. The great 19th-century marine biologist Phillip H Gosse himself even went as far as describing the sea spider as “Mr Nobody”. When you scrutinise the gangly lancer up close, he’s got a point: they appear to be all legs. Even the alternative name for their phylum is Pantopoda, which translates as ‘all legs’, while the more often used name for their class, Pycnogonida, means ‘with many knees’ referring to the large number of leg joints. The legs of the sea spider clearly defines it.
The body, if it can be said to have one, comprises of minimal bulk; it is more of a collection of hinges and joints that allow limbs to flex and animate. There is so little space inside the main trunk that many of the sea spider’s internal organs – associated with reproduction, gaseous exchange and digestion – squeeze out and into its leg cavities. Inside the legs can be found the diverticula of the digestive tract, the testes of the males or the ovaries of females, and in gravid females the eggs can be seen through the thin cuticle at certain times of the year. Breathing is also carried out in the leg department.
Can sea spiders swim?
The eight fibre-thin legs of the sea spider are used for walking, but in this particular species they can also be used to actively paddle through open water – when it is called a ‘dancing lancer’. It is this swimming habit that makes it one of the species most likely to be encountered.
These marine arthropods also have three more pairs of appendages: sensory palps called chelifore that work and perform as antennae, ending in tiny pincers that are used to manipulate food and hang onto mates or substrate.
What are ‘ovigers’?
And then there are a pair of ‘ovigers’ – legs unique to sea spiders. The ovigers have comb-like bristles for cleaning. In males they are better developed because in springtime, after breeding in deeper waters, they carry their precious cargo of fertilised eggs on them, like a cheerleader carrying pom-poms, to the intertidal zone. Males guard, clean and aerate the developing eggs, and later the early-stage juveniles, known as protonymphons.
More by Nick Naker
- Porcelain crabs guide: what they are, where they live - and why they don't mind losing a limb or two...
- What is a snow flea? And just how does it survive winter?
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- Meet the purse-web spider, Britain’s tarantula
- Spitting spider guide: where they live, how they kill their prey – and why they’re called spitting spiders
Main image © Peter David Scott/ The Art Agency
