While humans and most animals depend on a beating heart to keep blood and oxygen flowing through their bodies, the same does not hold true for sea spiders. A new study revealed that instead of a beating heart, these creepy sea creatures depend on pumping guts to circulate oxygen through their bodies. Results of the study were published in Current Biology.
Unlike humans, whose guts are centrally located within a single body cavity, sea spiders have a very unusual gut that branches out, multiple times and in multiple body cavities, very much like that of a human circulatory system. Sections of the sea spider’s gut tube even branch all the way down each of their legs.
During the study, H. Arthur Woods with the University of Montana, Missoula spent a lot of time observing the flow of blood and guts in the skinny sea spiders at the McMurdo Antarctic Station. He noticed that the spiders’ hearts beat weakly and were not circulating blood beyond the central portion of their bodies. However, their guts showed strong waves of activity through peristaltic contractions.
Peristalsis is the constriction and relaxation of muscles, which enables the contents within the guts to churn and move through the body. Humans use gut peristalsis to aid in digestion. However, the pumping of the gut observed in sea spiders was far more vigorous than needed for the process of digestion. This led Woods to hypothesize that the spiders were using their guts as a way to circulate oxygen, which they take in through their cuticles, throughout their bodies.
To test his hypothesis, Woods carried out a series of experiments on 12 sea spiders. Using video microscopy of tracers in the spiders’ guts and hemolymph, along with gut manipulation, Woods was able to confirm his theory.
While these findings clearly highlight evolutionary diversity, researchers are uncertain if the spiders first evolved this gut function purely for digestive reasons and then further evolved to include respiratory benefits or if the opposite scenario occurred.
Additional research in needed to fully trace the evolutionary origins of the sea spiders unusual use of their guts as a way to circulate oxygen throughout their bodies. Woods said future fossil finds could help. He also said it would be wise to study other living arthropods that also have complex guts to determine how they circulate oxygen through their bodies.
By Trixie Dillwood
Photo by Timothy R. Dwyer (PolarTREC 2016), Courtesy of ARCUS