Parasitic crustaceans can evolve into some pretty funky forms and they have been featured multiple times on this blog. These crustaceans don't so much flaunt, but completely toss out all your expectations of what a crustacean or even an arthropod is "supposed" to look like. And among the best examples of that is Dendrogaster.
|Left: Female Dendrogaster nike side view, with attached male (bracketed in the square), Top right: Female D. nike frontal view, Bottom right: Male D. nike. Photos from Fig. 2. of the paper
Dendrogaster is a genus of crustaceans that live as internal parasites of sea stars, nestled snugly within the body cavity of its host. So much so that their body shape seems to have evolved into somewhat resembling that of its host. In contrast to other crustaceans, Instead of having hard carapaces, segments, or jointed legs, Dendrogaster has multiple branching lobes, like some kind of fleshy, parasitic antler. They belong to a group of crustaceans called Ascothoracida - a sister group to the barnacles, who themselves are no strangers to the way that evolving towards parasitism can warp their body.
Dendrogaster rivals those parasitic barnacles in the "WTF Evolution?" department, and despite how bizarre they may look to us, they are not some rare oddity lurking in an obscure corner of the world. There are 35 known species of Dendrogaster and they have been found parasitising eighteen different families of sea stars all over the world, ranging from those dwelling in the shallows, to those inhabiting the deep sea over 2500 m below sea level. It seems that wherever sea stars went, Dendrogaster followed.
The paper featured in this blog post adds another species to this roster of evolutionary weirdos. This newly described species was found from sea stars living 1970 m below sea level, collected during a biodiversity survey at the An'ei Seamount, an offshore marine protected area off the eastern coast of Japan. The host was Asthenactis agni - a sea star which itself was newly discovered and described just late last year. This parasitic crustacean has multiple, wing-like branches protruding from its body, and it is this appearance which inspired its scientific name, Dendrogaster nike, named after Nike, the Greek winged goddess of victory.
But that's only how the female of the species looks like. The male is less than a quarter the size of its partner, and unlike the female Dendrogaster with its multiple protruding branches, the male is comparatively unremarkable, with a simple ovoid-shaped body and a pair of long thin testes dangling from it. It is usually found attached to its much larger and more flamboyant partner, floating inside the body cavity of a sea star.
Dendrogaster nike is just one of many new species of Dendrogaster that have been described over the last few years. In 2020, there were three other species of Dendrogaster which had been discovered from sea stars collected from the depths of the bathyal zone. It seems that sea stars from the deep sea are particularly favoured by this parasitic crustacean, and there are probably many other species of Dendrogaster yet to be discovered which are lurking in the abyss.
When scientists compared the DNA sequences of different Dendrogaster species, they found that the genus seems to be divided into two main sub-groups - those who stuck to the shallows, and those who ended up partying in the deep. While the evolutionary pathways of many parasites somewhat parallel that of their hosts, for Dendrogaster, it followed the hosts' habitats instead. This may provide some insight into the evolutionary origin of this bizarre, but widely found group of parasitic crustaceans.
When life hands you a sea star, sometimes it comes with a free Dendrogaster.
Jimi, N., Kobayashi, I., Moritaki, T., Woo, S. P., Tsuchida, S., & Fujiwara, Y. (2023). Insights into the diversification of deep-sea endoparasites: Phylogenetic relationships within Dendrogaster (Crustacea: Ascothoracida) and a new species description from a western Pacific seamount. Deep Sea Research Part I: Oceanographic Research Papers 196: 104025.