Holothurians (namako or “sea cucumbers”) are the largest and most abundant megafauna that typify abyssal plain ecosystems. The influence of holothurians in deep-sea environments is most often seen in their abundant and distinctive coiled faecal traces, which is evidence for their role as a key contributor to global carbon cycling (Williamson et al. 2021). Some deep-sea holothurians are adapted for swimming, including the charismatic and distinctive Enypniastes eximia (the “dreamer holothurian”, also known as yume-namako or “dream holothurian” in Japanese). This species is a moderately large (25 cm in length) animal with a reddish body that has an anterior velum and posterior webbed tube feet used for swimming “flights”. As Enypniastes is bentho-pelagic and will swim in response to disturbance, it is rarely observed on the seafloor. It forages on the sediment surface, and typically defecates immediately before “taking off” (Ohta 1985). Although not frequently seen, this species apparently follows typical pattern of large congregations known for other deep-sea holothurians, as evidenced by substantial accidental catches of hundreds of individuals (Solís-Marín et al. 2012). This large bodied holothurian is not only an iconic deep-sea oddity, but contributes substantially to bioturbation in the abyss.

The monotypic Enypniastes eximia is distributed worldwide, mainly in warm water regions (Solís-Marín et al. 2012; Supplementary Table 1; Supplementary Figure 1). Here, we report the northernmost seabed sighting of this species to the Aleutian Trench off Alaska in the northeast Pacific (Fig. 1; Supplementary Videos 1–2). This represents a significant northern range extension especially for the Pacific. During the recent “AleutBio” expedition on board RV SONNE (cruise SO293 July–September 2022), we deployed the ship’s “Ocean Floor Observation System” towed-camera system that captures full-HD video. We recorded sightings of two Enypniastes separated by 2.1 km on a single dive, between 5272 and 5320 m depth. This is in the deeper part of its known range; the deepest record is at 6900 m in the Java Trench. An early specimen from the Southern Ocean (Hérouard 1906) hinted its range extends to higher latitudes, recently substantiated by an adult sighting off East Antarctica (BBC News 2018). Here, we confirm the same for the northern hemisphere to show that it truly has a global distribution.

Fig. 1
figure 1

Still frames from video images of two individuals of Enypniastes eximia recorded on 8 August 2022 on the southern rim of the Aleutian Trench, off Alaska, USA. a One individual seen from above, anterior view, at 50° 38.4704′ N, 169° 49.1442′ W, depth 5320 m (for video, see Supplementary Video 1); b a second individual, with partial view of the posterior left fins, seen at 50° 39.5232′ N, 169° 48.5049′ W, depth 5272 m (for video, see Supplementary Video 2)

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It is reasonable to speculate whether the genus Enypniastes may represent multiple species across such a very broad distribution. Most modern records are limited to video observations, which provide no molecular nor detailed anatomical comparative data. Swimming pelagothuriid holothurians have completely reduced body wall ossicles and no calcareous rings, key morphological characters differentiating holothurian species (Gebruk 1989). So for older specimens not suitable for DNA analysis, even potential morphological distinctions are effectively impossible. In fact, Enypniastes eximia was once split into several species based on the number of papillae, the shape of tentacle discs, and the arrangement of tube feet, but now all regarded as variations within E. eximia (Gebruk 1989). Although widely distributed marine invertebrate species are often revealed to be species complexes with additional investigation, recent work combining molecular and morphological evidence showed that some benthic deep-sea holothurians can truly have a global distribution (Thomas et al. 2020). One study observed a closely related holopelagic species, Pelagothuria natatrix, over a wide range of depth and temperature within a geographically limited study area (Selig et al. 2019). Enypniastes may also be tolerant of a relatively wide thermal range, enabling it to occupy a wide latitudinal range. Swimming holothurians have direct development but can disperse as pelagic juveniles that already have the adult body plan (Gebruk 1989) — Enypniastes may have sufficient swimming capacity to add to this as even adults — indicating both benthic and bentho-pelagic holothurian species have sufficient mobility at different life stages to occupy extremely broad distribution ranges. This new observation of the “dreamer holothurian” serves as a confirmation of its range in high latitudes but also as a reminder to deep-sea exploration worldwide of the unseen abundance of this iconic species.