biology zoology blog benno meyer saltwater spiders

Marine Spiders

Can there really be spiders in saltwater?

During my first “honey moon” many years ago, we spent a week on the Adriatic Sea island of Rab, which at that time was a part of Yugoslavia and now belongs to Croatia. While my new wife enjoyed getting a suntan lying on her bath towel on the pebble beach or refreshing herself in the water careful not to step on the thousands of sea urchins, I examined the porous stones in the ocean water down to a depth of my hips. What I then saw in the holes and crevices of some of these stones surprised and excited me: spiders, real spiders! I knew of only the freshwater spider Argyroneta aquatica that I had kept in one of my aquariums, which constructs its web and “diving bell” under water, but oceanic spiders? That was something new for me and not just me: it was the spider Desidiopsis racovitzai’s first record for Rab.

Spiders have no gills and need atmospheric air to survive. Yet the spiders I found even had egg sacs which they guarded in some of the stones’ holes that they resided in. Divers know that sometimes underwater caves are not totally filled with water, but contain air pockets under their ceilings and to the little spiders the holes and crevices must have been something like little air-filled caves to survive in. The air could have got into the blind-ending holes of the stones in two ways: either at very low tides the stone would fall dry (which never happened during the week we were there), or tiny air-filled bubbles in the surf, i.e. the breaking waves near the beach could release their contents and fill the blind-ending holes of the porous stones. Many years later in Papua Niugini, I came across marine spiders of a different habitat, but once again true spiders at the edge of an ocean in saltwater, but not associated with porous stones but coral debris.

These true spiders (which have nothing to do with the multilegged Pycnogonids, which are frequently referred to as “sea spiders”, but share no more than their mouthparts known as chelicerae with those of the true spiders that are known as Araneae) most commonly belong to the genera Desis and Desidiopsis. They occur in the intertidal zones that may be flooded for hours or days and they construct silk chambers in air-filled pockets between the corals, in empty shells and kelp holdfasts. They have large chelicerae (commonly termed “jaws”, which of course is not a good description), very hairy legs, and silvery brownish bodies of usually no more than 1 cm in length. Not having seen them feed, I can only assume they would wait until their habitat emerges temporarily above water and they can feed on small beachhoppers and other arthropods, including a few flies that will be around at that time. D. Mcqueen and C. Mclay in 1983 found out that even after a 19-day tide-induced submergence, the marine spider Desis marina, without any special respiratory adaptations, survived. It seems there is a sufficient amount of air (at least at 17.5℃) in and around the nest even during the most extreme period of submergence.

Although the spider’s ancestors known as Eurypterida evolved in the sea and contained huge scorpion-like arthropods (the Gigantostraca), spiders are terrestrial animals that colonized land nearly 400 million years ago. That currently so few species of spiders are known from the coastal marine and intertidal areas is a little surprising. But perhaps additional species can be discovered, if (like I did) people during their sea-side honey moon are willing to turn over submerged stones and examine them for spiders.

© Dr V.B. Meyer-Rochow and, 2020.
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