The “Seasonal Seas” episode of Blue Planet utilizes a trope commonly used in nature documentaries to raise tension: an unwitting prey being chased down by an unceasing predator. In this case, it is a large, smooth Navanax sea slug trailing behind a Janolus sea slug like the monster in a horror movie. However, this is not intentional on the part of Navanax: as Attenborough describes it, it is following the mucus trail left on the sea floor by the Janolus it is pursuing.
Navanax inermis is found in the bays and rocky shorelines along the west coast of North America, which indicates the species of Janolus shown is Janolus barbarensis . Navanax has no eyes, and so relies on being able to detect the chemicals in its environment to locate its prey [1,2]. This is known as chemoreception. Chemoreception and trail-following are not only present in Navanax, and in fact can be found in other predators. Research has shown that snails and sea slugs are capable of chemically differentiating between the trails of different species. . This behavior evolved along with that of their prey; as these mucus trails also allow sea snails and slugs to find mates and reduce their energy use by following in previously-made trails, the trails provide benefits to their producers . This makes for an evolutionary trade-off, meaning the trait is not selected against by excessive predation, allowing it to continue on in most snail species.
Despite the predatory adaptation of the Navanax, its success in capturing the Janolus is not guaranteed. For Janolus has adapted as well, to avoid predation: when its pursuer catches up to it, it rapidly curls into a ball that the Navanax cannot swallow whole . It has adapted to avoid the predator, just as its predator has adapted to find it as prey.
by Leah Scott
Blue Planet, “Seasonal Seas” (Season 1, Episode 5), starting at 24:35
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- Susswein AJ, Cappell MS & ML Bennett. 1982. Distance chemoreception in Navanax inermis. Marine Behaviour and Physiology 8(3): 231 – 241.
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- Ng TPT, Saltin SH, Davies MS, Johanneson K, Stafford R & GA Williams. 2013. Snails and their trails: the multiple functions of trail-following in gastropods. Biological Reviews 88(3): 683 – 700.