"Have you heard of a pyrosome?" asks Carin Bondar.
My immediate answer was, no; no, I haven’t – but you know me, I’m always curious 🙂 Turns out that pyrosomes – which look almost other-worldly in the video below – are colonial tunicates: the same taxonomic group as the perhaps-more-familiar sea squirts. And that means that they’re in the same phylum as us, for both tunicates and mammals are chordates.
These are adult seasquirts (image from reefbuilders.com).
They’re filter-feeders, sucking water in through a siphon and extracting all the edible bits & pieces using a mesh-like filtering system (that in evolutionary terms is homologous to fish gills), before squirting the water back into the ocean. True, they don’t look anything like the more familiar chordates (e.g. fish, amphibians, reptiles, birds & mammals), but at the embryonic stage all these groups have some features in common: a hollow dorsal nerve chord; a stiff notochord – which gives our phylum its name – running the length of the body; a tail that extends beyond the anus; and a system of pharyngeal pouches. These last become the filter-feeding mechanism in tunicates and give rise to the gills – and the gill bars that support them – in fish. In mammals the pouches are homologous to the various glands found in the neck, while the same tissues that formed gill bars in fish are involved in trachea and larynx formation.
And this amazing creature is a pyrosome. It’s about 20m long and is a colony of tunicates, each one sucking water in through its ‘mouth’, filtering out its dinner, and expelling the water into the cavity running the length of the organism. You can see this happening later in the video. Ultimately the water leaves through a single opening at one end, & this means that the pyrosome is capable of moving under its own – albeit very slow – steam.
I do love finding out new stuff!