Sea spiders are crustaceans that, as the name implies, resemble spiders, with long legs extending from their segmented bodies, and they inhabit marine waters around much of the world.
However, little was known about how these animals reproduce and care for their offspring, until a trio of researchers from the University of Hawaiʻi at Mānoa decided to embark on an expedition to Antarctica to study the life cycle of a giant sea spider, the species Colossendeis megalonyx.
Photo: S. Rupp
Although most sea spiders, also known as pycnogonids, do not measure more than the width of a fingernail, the legs of this Antarctic species can reach more than 30 centimeters in length, something scientists say is a striking example of the so-called ‘polar gigantism’, a scientific principle that argues that in polar regions animals tend to be larger than their relatives in warmer areas.
Although it is known that, in the world of sea spiders, it is the males who care for the offspring, carrying them on their backs, literally, from one side to the other as they develop, little is known about how C. megalonyx cares for its young, despite the species being described in 1881.
The mystery began to be unveiled when, in October 2021, Amy Moran and two PhD students, Aaron Toh and Graham Lobert, were diving beneath the ice in Antarctica and came upon a group of sea spiders that appeared to be mating. They collected some specimens and placed them in observation tanks, and the results, published in the journal Ecology, reveal an aspect of the life of these marine invertebrates that had been unknown until now.
Photo: R. Robbins
Although the males of C. megalonyx care for their offspring, they do so in a simpler way than the males of other sea-spider species, and this may help to understand, according to Moran, the evolution of behaviors that give rise to paternal care by males that is much more intimate and continuous.
A few weeks after the male had left the eggs on the rocks, the researchers noticed they had become virtually invisible, hidden by a layer of algae, probably a protective measure against predators.
“We could barely see the eggs even though we knew they were there,” says Lobert in a statement. “It’s probably why researchers have never seen this before.”
