Rather than going to all that bothersome effort of having to catch their own dinner, researchers have found that some sea slugs are much sneakier. They’ll wait until the organisms they live on, known as hydroids, catch their food before gobbling them up in what is thought to be an entirely new form of predation called kleptopredation.
For researcher Trevor Willis it seemed odd that while the sea slugs known as nudibranchs live on hydroids – colonial organisms not unlike coral – they also eat them. “There would always be the risk that the food would run out before the nudibranchs could reproduce, which didn't strike me as a particularly intuitive strategy,” explained Willis, who led the research published in Biology Letters.
In exploring this relationship, however, Willis and his colleagues stumbled across something that would surprise even him. They carried out nitrogen stable isotope analysis of not only the nudibranchs and hydroids, but also zooplankton, which is what the hydroids prey on. To their confusion, they found that the concentration of nitrogen in the sea slugs was actually the same in the hydroids.
For most, this might not seem particularly odd, but for Willis and his team, it was bizarre. As creatures eat other creatures, the concentration of certain nutrients builds as you go up the food chain, in a process known as bioaccumulation. This means that while the hydroids have more nitrogen in them than their zooplankton prey, the sea slugs should contain more nitrogen than the hydroids.
This suggested that the hydroids were not as major a component of the nudibranchs' diet as assumed and that they were likely eating the same prey, even though the sea slugs simply can’t catch zooplankton. So the team set up an experiment where they tested the nudibranchs to see if they preferred hydroids that have just caught their own prey. They found that the sea slugs were indeed twice as likely to chomp on a newly fed hydroid than a hungry hydroid.
“People may have heard of kleptoparasitic behaviour – when one species takes food killed by another, like a pack of hyenas driving a lion from its kill for example. This is something else, where the predator consumes both its own prey and that which the prey has captured,” said Willis.
From this, the new feed strategy was named “kleptopredation”. It is unknown how common or widespread the behavior might be, but it highlights simply how little we know about the marine environment, even as large swathes of it are being decimated by climate change faster than we can record it.