For the next stop on our macro voyage, I sure hope there are no arachnophobes reading this as we look at another bizarre critter, the Giant Sea Spider!

Giant Sea Spiders, also known as pycnogonids or pantopoda, are part of a huge family of 1300 known species, with it being widely believed that there are hundreds of undescribed species in the deeper oceans. Despite being a large family found in most waters, little is known about these often-elusive critters. During this report, we will be looking at the Giant Sea Spider but also at Sea Spiders in general to see what makes them tick.

Giant Sea Spider

What’s in a name?

Giant Sea Spiders get their name from their size, as one of the largest Sea Spiders that you can see. This also refers to the scientific name ‘Colossendeis’, which comes from the word colossus (although I couldn’t find whether the root is Greek or Latin).

The other name they are known by, pycnogonids, is made of 2 parts ‘pynco’ meaning closely packed and ‘gonids’ which means a large grouping of asexual reproducing cells.

There is one very important thing to bear in mind during this article and that is that Sea Spiders are not true spiders. While they are placed in the Chelicerata subphylum (a word from the Greek for horn and claw) alongside true arachnids and horseshoe crabs, there are many competing theories as to where they evolved from, with some calling for them to be placed in their own subphylum. If this happens, I will be sure to update this article so watch this space!

Habitat and distribution.

Sea Spiders can be found in almost every ocean, from shallow depths down to over 7000m. They are most commonly seen by divers in shallower waters, hiding under rocks and in and amongst the algae on rocky shorelines.

While the Giant Sea Spiders main habitat is the deeper areas of the Southern and Pacific Oceans and the Antarctic region, as a diver you are most likely to spot Giant Sea Spiders amongst the rocks in the Galapagos Islands. The only problem here is that there is so much else to look at there that you might not want to go looking around the seaweeds for Sea Spiders when you are surrounded by Hammerhead and Whale sharks!

Looks that kill.

Much like land spiders, Sea Spiders have four pairs of walking legs with multiple joints (usually made up of nine segments) allowing them to move around, giving them a similar look and gait to their land-based counterparts. Unlike land spiders, which are limited to four pairs, Sea spiders can also have five or even six pairs of legs. Alongside their long, thin legs, Sea Spiders have three small segments that the legs come out from; a head, a thorax and an abdomen. Due to the size of these segments, the organs of the Sea Spider are not only located within them but also inside the leg segments, otherwise they would not fit in the body. They also have no gills, internally or externally, as they exchange oxygen directly over their body surface.

Attached to their heads are a pair of feeding appendages called chelifores, each of which has a pincer on the end. They use these pincers to grab onto prey, such as sponges, which they feed on using a proboscis contained within the middle structure. Also attached to the head section are a pair of limbs used to carry eggs called ovigers. We’ll talk more about their breeding habits but both males and females have these egg carrying limbs.

One of the most impressive things about Sea Spiders is the huge variety of colours they come in. The Giant Sea Spider is a light orange as its main body colour with bright red stripes along the tops of its body and legs. You can see that some have more red on the top than others, which may be linked to their age, but I cannot find anything to link to this. Much like nudibranchs, there are as many different colours and patterns as you can imagine. Vivid purples, blues and reds. Pale yellows, pinks and greens. Some have stripes and lines, some don’t. It may be to camouflage themselves amongst the colourful corals or maybe the vivid colours help them hide in plain sight as the different coloured wavelengths of light are filtered out with depth.

Lifestyle choices.

Like land spiders, Sea Spiders come in two genders, male and female. There is, of course, one species that is hermaphroditic but I could not find anything more interesting to mention about it! They are very difficult to tell apart underwater as the external parts are similar for both sexes and the only other differences are internal.

As far as raising young is concerned however, much like seahorses this job is taken on by the male. After a brief display to attract the female, she lays her eggs and the male fertilizes them externally. This is the end of the females’ involvement; the eggs are looked after by the male, removing algae and other encrusting growths as well as ensuring enough oxygenated water can get to them. His care extends to the larval stage for a short time as well, depending on what sort of larva the egg becomes.

Sea Spider larvae fall into four different categories and each species will use one of the forms. The first one is a free-swimming planktonic form, similar to many other marine larval forms, which is called the typical protonymphon form. The second is an attaching form, similar to that of a barnacle. The third is a parasitic form, where the free-swimming larva attaches to a suitable host, such as a sponge or soft coral, burrowing inside to form a cyst-like structure until grown enough to burst out. The fourth and final form is called an atypical protonymphon larva, which means that it attaches to the legs of its father, similar to the attaching form until it has at least two pairs of walking legs.

In all four species the larvae start life as incredibly simple forms; literally no more than a gut, mouth and mandibles. Their legs, thorax and abdomen will develop later, with legs growing in pairs allowing for more and more complex movement. It is believed that this way of growing is how all early species of arthropods and insects started, before evolving into the more complex forms we see today.

Unfortunately, little is known about the reproductive cycle of the Giant Sea Spider as more research is needed however as the most common form is the free-swimming planktonic form, my best-educated guess would be that this is the way it grows.

Life on the reef.

Most Sea Spiders, when they aren’t hiding, can be seen either slowly walking around the reef using their legs or swimming through the water column. Walking around they look like slow-moving land spiders or like something out of the War of the Worlds, hunting around for something to eat. When swimming, they propel themselves around using a motion called umbrella pulsing, similar to how a feather star or crinoid swims in the water.

All species of Sea Spider are carnivorous and they feed on a variety of things from sponges and sea squirts to soft coral and anemones. They eat using a long thin proboscis, much like a mosquito, which they stab into the soft bodies of their prey and suck out the juices from within. Due to the small, thin nature of the Sea Spiders, their prey most often survives this attack with little ill effect.

Giant Sea Spiders grow to a maximum leg span of 70cm, much larger than the average reef Sea Spider species, which tend to be between 5cm and 10cm. I cannot find any information about the lifespan of the Sea Spider but my guess is that deeper water and larger species would live longer than the smaller, shallow water ones.

Personal experiences.

Most Sea Spiders that you will encounter are going to be small so you will need to keep your eyes peeled if you want to find them! Taking your time to slowly scour the reef will help you find them so don’t be disheartened if you don’t find them right away. It took me hundreds of dives before I saw my first one and I haven’t seen many since then. A macro lens is a must to get the best shots of these critters alongside good lighting and a good set of diopters in some cases. While Sea Spiders have very small, simple eyes, it is still worth considering their sight when you are taking pictures with flash. Take the time, get the shot right and then pull the flash rather than shoot hundreds and potentially damage their eyes. This is also important for other fish around you, I’ve mentioned this in previous articles but fish can’t blink and neither can many other critters.

Both of our in-house photo pros, Mario Vitalini and Martyn Guess, have written some awesome articles about shooting small things and can give all sorts of great advice about macro photography, have a look and see if their tips can come in handy next time you go spider hunting.

The bottom line.

I am always super happy to find new, brightly coloured things to take pictures of on a dive and Sea Spiders are always high on my list! They are not easy to find so like everything else in diving, it is worth taking your time and looking in and amongst the coral. There are so many different varieties and colours of these incredible critters and while you might not be able to tell which species you have seen, they are a great addition to any photo portfolio.

If you fancy going looking for the Giant Sea Spider and can tear your eyes away from all the Hammerhead sharks, sea lions and marine iguanas, the Galapagos Sky is a great way to experience the Galapagos Islands and everything they have to offer!

Or if you want to chalk up as many different colours and patterns as you can, the Philippines is a great place to go. Malapascua Exotic is great for macro diving alongside the amazing shark encounters and Atlantis Peurto Galera is another macro haven. If any of these locations take your fancy, give us a shout and we will get you out there!

Don’t let their creepy appearance fool you, Sea Spiders will spin a line directly into your favourites list!