Abundance and morphology of Paleodictyon nodosum, observed at the Clarion-Clipperton Zone

The seabed in the APEI-6 consisted of fine-grained sediment with polymetallic nodules (mean dimensions 16 mm x 20 mm, determined from box cores) visible on the surface at a density of 338 nodules m⁻² (standard error 53 nodules m⁻², n = 17). A total of 841 specimens of Paleodictyon nodosum were observed in the photographs, equivalent to 0.33 individuals m⁻². The density of Paleodictyon nodosum at APEI-6 in the CCZ was two orders of magnitude smaller than the maximum densities (45 individuals m⁻²) found at the MAR (Rona et al. 2009), and the distribution of these traces at the MAR was patchy on the scale of kilometres. Paleodictyon was most abundant at shallower depths (3430-3575 m) on margins of a relict hydrothermal zone on the MAR, with sparse occurrences to the end of this field (up to 4 km distance). It was absent around an active high-temperature sulphide mound, 2 km to the west of the high-abundance area. The density in the CCZ was similar to that on the shallower eastern Australian margin (0.2 individuals image⁻¹), where the sediment was a sandy mud of carbonate detritus.

Modern Paleodictyon nodosum is thought to occur at sites with low sedimentation rates (<5 mm ka ⁻¹), a characteristic of the CCZ (Khripounoff et al. 2006). It is found at the MAR, where sedimentation is estimated to be 18 mm ka⁻¹ (Scott 1978 in Rona et al. 2009), but not on the Porcupine Abyssal Plain (J. Durden, pers. comm.), which has a sedimentation rate of 1.4 mm ka ⁻¹ (Carvalho et al. 2011). It is also notably absent from other abyssal sites, including Station M in the eastern Pacific Ocean (4000 m water depth; Jacobsen Stout et al. 2015) and sites on the western margin of Australia (1500-4400 m water depth; Przeslawski et al. 2012). However, the association of P. nodosum with areas of low sedimentation may be related to their longevity. If individuals are not covered by sediment (as in the case of a turbidite) or detritus, and bioturbation is low owing to low organic matter input, then the rate of trace erasure may be low (Wheatcroft et al. 1989), and the individuals may remain visible on the seabed long after they have been abandoned (in the case of a burrow) or after the death of the organism.

In the APEI-6, the mean size (maximum dimension) of the Paleodictyon nodosum patterns was 45 mm ± 16 mm SD (n = 841). Complete forms included both regular (59%, n = 496) and irregular (41%) arrays of holes (Fig. 2). Regular forms had equal numbers of rows along the three axes (6 × 6 × 6 in 77% and 8 x 8 x 8 in 23% of complete “regular” specimens), and irregular forms generally had one axis with one row of holes fewer or more than the other two axes (e.g. 6 × 6 × 7; 7 × 7 × 8, etc.). Paleodictyon traces very similar to those observed in APEI-6 were identified during the ABYSSLLINE project in images from the UK-1 exploration claim area and the “EPIRB” area situated 250 km to the east of UK-1 (C.R. Smith, D. Amon and A. Ziegler personal communication; site locations in Amon et al. 2016). Rona and Merrill (1978) reported a similar size (∼50 mm) for Paleodictyon at the MAR, although the number of holes in the arrays was greater (10 × 10 × 10 to 13 × 13 × 12). The more recent investigation of Rona et al. (2009) found diameters at the MAR ranging from 24 to 75 mm in diameter (mean 50 mm), with 20 to 40 rows of holes on each axis. They also observed traces with unequal numbers of rows along the three axes, though these constituted only 1% of those analysed. The numbers of rows in Paleodictyon nodosum on the Australian margin was similar to that in our CCZ images (6 × 6 × 6, 7 × 7 × 7 and 8 × 8 × 8), but the overall size of the patterns found in Australia was only ∼30 mm (Dundas and Przeslawski 2009). All those illustrated in Dundas and Przeslawski (2009) contained equal numbers of rows along the three axes.

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The morphology of P. nodosum in the APEI-6 appears to be more variable than at the MAR or on the Australian margin. Of the 120 specimens inspected in detail, only 22 (18%) were complete. The remainder were either formed around surface nodule(s), with the nodule(s) interrupting the pattern, or were missing holes in a central portion of the pattern resulting in “blank” sections in the pattern (Fig. 2). Although some nodules were found immediately below the sediment surface (<20 mm) in the box cores, covered in a thin layer of sediment, the “blank” portions of the pattern are unlikely to be attributed to the presence of such nodules, since the nodules were generally much larger than the area of missing pattern. Some specimens had very round holes, while others had irregular-shaped holes (Fig. 2). The roundness of holes could reflect how recently the structure was constructed, as the agglutination agent binding the sediment (Rona et al. 2009) may degrade over time. “Degraded” specimens were often observed to have missing portions of the pattern. At the MAR, “degraded forms” were covered in millimetres of sediment (Rona et al. 2009). In any case, it cannot be assumed that all Paleodictyon traces in the APEI-6 were “alive” or occupied by the maker. None of those analysed by Rona et al. (2009) yielded any sign of the organism responsible for their creation.

Paleodictyon nodosum has now been observed in abyssal locations in the North and South Atlantic (Rona and Merrill 1978; Ekdale 1980; Rona et al. 2009), the SW Pacific (Przeslawski et al. 2012) and the eastern equatorial Pacific (this study). The wide distribution and abundance of these traces suggests that they influence the local sedimentary structure. However, our new observations cast no light on the maker of these enigmatic traces and their function remains difficult to explain. Future imaging studies could employ time-lapse photography or video to observe the development and degradation of Paleodictyon and any epibenthic or bioturbation activity in the vicinity of a specimen, stereo images to examine its in situ structure, or assess co-occurrence of Paleodictyon to fauna or lebensspuren (life traces) in the seabed. Such studies could provide insight into the origin and ecological function of Paleodictyon in the deep-sea.