Sea spiders are probably one of the most enigmatic aquatic creatures and have remained nearly unchanged for more than 450 million years. They are so strange that for long, it was debated whether or not sea spiders were related to land-dwelling spiders as both groups share similar morphologies🕷️. We know now that the two groups belong to distinct classes of arthropods - this large group that include insects, crustaceans, millipedes and centipedes (the Myriapoda) and of course spiders and relatives (the Chelicerata) - and with their unique body functions and circulatory system setting sea spiders apart from other marine invertebrate animals. If over 1,300 sea spider species have been described, only a few phylogenetic studies attempting to uncover the relationships between those species were able to cover the whole family due to the inaccessibility of diminutive individuals. As a result, even nowadays, the mystery of its species-level phylogeny continues to fascinate scientists🧐.
Sea spiders belong to a class of organisms called Pycnogonida (Latreille, 1810), literally meaning “all legs” in Greek🦵. This refers directly to their spider-like exoskeletons which includes four pairs of jointed legs varying from just 2 to 700 mm in leg span! Most of the family members are tiny creatures living in shallow water, except for those living in the polar regions who have experienced polar gigantism💪. This is not just a cool term, but actually a trend observed in aquatic species becoming giant in frigid environments as metabolism rate decreases and dissolved oxygen level increases when the temperature is extremely low. One famous family of those gigantic organisms is the Colossendeidae (Jarzynsky, 1870), which grows up to 550 mm in leg span, swimming off the bottom of the southern Pacific and Antarctic Oceans.
More modest though despite its name is 𝘕𝘺𝘮𝘱𝘩𝘰𝘯 𝘨𝘪𝘨𝘢𝘯𝘵𝘦𝘶𝘮 (Goodsir, 1845). This sea spider has a noticeable unbalanced, almost comical, body proportion of long appendages to a fused mini body. Other than an extended part of a tubular mouth, the trunk body segments look like LEGO T-tubes used for leg attachments🧩. Ubiquitous pores - small openings- can be observed on the surface of sea spider legs under a microscope. These porous structures are part of the space-filling gut system. Surprisingly, the organs of a sea spider are housed within its leg attachments including some playing key roles in bodily functions. For instance, the gut peristaltic waves, which are wave-like muscle contractions associated with food digestion, is also driving the blood circulation and gas exchange throughout the body. Even though sea spiders do have a heart organ, it is beating weakly and does not seem to show importance in keeping them alive.
Further research on the adaptability of sea spiders to global warming is conducted to investigate their survival stories but as those creatures have lived since the Ordovician period or about 450 Mya, there is no doubt that they have already experienced major changes on Earth surface, or rather deep under water. Nonetheless, some species may be at a crossroads and scientists predict that elevation in water temperature will put enormous pressure on these giant organisms when their metabolic demand increases with temperature🌡️. Yet, it is observed that polar sea spiders will grow even larger in size under a hotter environment. The expansion in size allows the sea spider to generate more and larger porous for guts peristaltic motions, and therefore increase the efficiency of gut fluids and hemolymph (the watery structure carries oxygen and other molecules) circulations. Hence, more oxygen is absorbed to be used for metabolic reactions😯.
Interested in witnessing the differences between Pycnogonida and other arthropods? Then, sign up for your guided tour in HKBM with your friend or family now!
Text: Millian Hui
Comments