Biologists have discovered why sea spiders bury themselves in the ocean floor with their legs facing upwards.

Sea spiders, or pycnogonids, are quite remarkable creatures and distant relatives of ticks and spiders. They are fascinating because they breathe through their entire surface and reproduce as well as digest food using their large legs. Recently, zoologists discovered that Antarctic pycnogonids, specifically Nymphon australe, have an unusual behavior: they frequently bury themselves in the ocean floor, leaving their limbs exposed in the water. It appears that this is a method for them to extract bacteria or decomposed organic matter from the sediment.

2025-01-20 10:01:23https://naked-science.ru/article/biology/pycnogonida-undercover

Among the most peculiar inhabitants of the seas are sea spiders (Pycnogonida). These arthropods belong to the chelicerate group, which also includes arachnids, horseshoe crabs, and extinct eurypterids (sea scorpions). Over a thousand species of sea spiders are known to inhabit all parts of the World Ocean at various depths.

Pycnogonids are remarkable for many of their features and behaviors. They possess a unique segmentation with arrangement of limbs (usually seven pairs), a special mouth apparatus, the absence of respiratory organs, and, of course, hypertrophied legs. These legs make up a large portion of the sea spider's body and can reach lengths of several centimeters. The long "stilts" are traversed by the intestine, which also houses the animal's gonads, where the reproductive cells mature.

A new study has added to the list of known peculiarities of sea spiders. It focuses on the species Nymphon australe, which resides on the seabed of the Southern Ocean near Antarctica. These pycnogonids form dense clusters: more than a thousand individuals, each nearly four centimeters long, can be found in a square meter.

Biologists have described the unique behavior of N. australe while observing their lives in an aquarium. Moving in an absurdly slow manner, the animal gradually buries its tiny (in comparison to its legs) body into the substrate on the ocean floor. This can take ten minutes or more, during which the sea spider's limbs remain sticking out and fluttering in the current. It then emerges to the surface and buries itself again, moving to another spot and repeating its maneuver over and over.

It is commonly believed that pycnogonids are predators, parasites, or something in between. However, we still know little about their dietary preferences, as well as who or what preys upon them. Thus, interpreting the unusual behavior of N. australe is not straightforward. There is a hypothesis that burying helps the sea spider breathe through its legs, or that it is searching for animals—its prey.

However, the authors of the study lean towards a different hypothesis. They sequenced the DNA of microorganisms from the intestines of sea spiders and compared it with that found in the seafloor. The samples turned out to be very similar. Apparently, the animals are more likely searching for microbes or decaying organic matter in the substrate to consume. This finding does not align well with what is currently known about the ecology of these unusual creatures.