Put it in reverse! Captivating deep-sea footage reveals the mysterious movements of abyssal fish

Abstract

There are many common myths about fish, spanning from the 3-s memory of goldfish to shoals of hungry Amazonian piranha consuming people in a frenzy, both of which have been firmly debunked (Haddad & Sazima, 2003; Sibeaux et al., 2022). However, some misconceptions come from more reasonable observations, such as ‘fish cannot swim backwards’. For most (fusiform) fish that people encounter, moving backwards is not very common, given that it is clumsy and inefficient, relying heavily on the pectoral fins (Blake, 2004). However, although most fish do predominantly swim forwards, there are always exceptions to the rule, and nowhere do the rules seem to be broken more often than in the deep sea.

One of the main drivers of how fish move is body form. Backward swimming is commonly seen in elongated fish, as this body form facilitates such movement, as seen in fish such as lancelet, hagfishes, lampreys and eels. The latter lends its name to the term ‘anguilliform swimming’, which is movement through body undulation in elongated fish, that can be put in reverse with ease. What connects this with the deep sea is that in marine environments, the degree of body form elongation increases with depth, which is most apparent in Gadiformes (cod) and Osmeriformes (smelts and allies) (Neat & Campbell, 2013). As a result, many deep-sea fish are capable of swimming backwards, an ability that Priede and Jamieson (2025) show in this issue with captivating new footage (Video 1).

Using baited camera landers at depths of 4500–6300 m, Priede and Jamieson (2025) filmed the inhabitants of the Pacific Ocean floor, hungry for a mackerel supper. What followed did not disappoint, with a spectacular array of the weird and wonderful coming out for a bite to eat, with backward swimming using reverse undulation observed in four species: the cutthroat eel (Ilyophis robinsae), abyssal grenadier (Coryphaenoides yaquinae) and cusk-eels (Bassozetus sp. and Barathrites iris). In their article, Priede and Jamieson (2025) highlight the advantage of backward swimming, as it allows the fish to return safely along a recently travelled path, an important consideration when manoeuvring through an environment without any visual information. The footage also further supports recent studies highlighting the slow pace of life in deep-sea environments (Woodworth et al., 2025).

Getting a glimpse into the deep sea never fails to amaze, especially when it is in such high definition. Footage like that collected by Priede and Jamieson (2025) is also a stark reminder of how alien Earth's largest habitat appears, and how much more we still have to learn about this extensive frontier.