Benthic foraminiferal species sometimes produce a covering made of sediment and detrital material around their tests (shells). These sedimentary envelopes, termed ‘cysts’, have been observed in a number of species, from organic-walled and agglutinated to calcareous (e.g., Linke and Lutze 1993; Cedhagen 1996; Gross 2000, 2002; Gooday and Hughes 2002; Heinz et al. 2005). However, almost all published records of this phenomenon originate from coastal or bathyal settings, and there are very few examples from abyssal depths, i.e. deeper than 3500 m.

During the analysis of Megacorer samples (25.5 cm2 surface area, formalin-buffered, 0–1 cm sediment horizon, >150 μm fraction) collected in the area of the Porcupine Abyssal Plain Sustained Observatory (PAP-SO) in the northeast Atlantic (49°N 16.5°W, 4850 m water depth), we observed benthic foraminifera that had created partial or complete muddy coatings. Most belonged to Sphaeroidina bulloides d’Orbigny, 1826 (Fig. 1), and a few to Melonis barleeanus (Williamson, 1858). The S. bulloides cysts occasionally incorporated juvenile planktonic foraminiferal tests (<50 μm), and always included one or more flexible agglutinated tubes (20–35 μm wide, 140–400 μm long) that extended out of the main structure (Fig. 1a–e). There was no evidence of the presence within the cysts of microscopic organisms, comparable to the ciliates and nematodes observed by Linke and Lutze (1993) inside the cysts of Elphidium incertum.

Fig. 1
figure 1

Sphaeroidina bulloides forming mud cysts. Reflected light images (a–e); scanning electron microscopy image (f); transmission light image (g). Unattached, complete cyst incorporating juvenile planktonic foraminiferal tests (<50 μm) and including long (approx. 400 μm) tubes (a). Partial mud cyst (indicated by an arrow) sitting on top of large (>300 μm) planktonic foraminiferal test (b). Partial mud cyst attached to planktonic foraminiferal tests (c). Specimen with partial muddy coating and short tube (approx. 200 μm) (d–e). Specimens of S. bulloides after the removal of the cyst (f–g). Note that the interior is filled with green protoplasm (g)

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Although most of the S. bulloides cysts were unattached, some were sessile on large (>300 μm) planktonic foraminiferal tests (e.g., Fig. 1b) that dominated the sand fraction of the PAP-SO sediments. All of the specimens found forming cysts were ‘live’ (i.e. stained with Rose Bengal) and filled with green-coloured cytoplasm (Fig. 1g), which indicates that they were feeding on freshly deposited phytodetritus that formed patchy deposits on the seafloor at the time of sampling (Durden et al. 2015).

Encystment by S. bulloides was observed by Linke and Lutze (1993) from the Guinea Basin, off Ivory Coast (~700 m water depth). However, ours is the first record of this behavioural trait in S. bulloides at a much deeper abyssal site, despite the fact that this species has been recorded in other abyssal locations (Murray 2013; Table 16 in Supplementary Material). The only other example of which we are aware of an abyssal foraminiferal species creating a cyst is that of Quinqueloculina sp., also from the Porcupine Abyssal Plain (Gooday et al. 2010). Our analysis of ‘live’ benthic foraminifera in 16 Megacorer samples from the PAP-SO area revealed that S. bulloides had a density of 0–7 individuals per sample (i.e. up to 3.1 individuals per 10 cm2), or 0–6 % of the ‘live’ assemblage. These values are consistent with those recorded in other studies (Murray 2013, Table 2 therein). Overall, one of every four ‘live’ specimens of S. bulloides encountered in our samples had created a cyst.

The benefit of agglutinated cysts to the foraminifera is unclear, although it has been speculated that they serve various functions related to feeding, reproduction, growth and protection (Gross 2002; Heinz et al. 2005). As sediments on the Porcupine Abyssal Plain are well oxygenated and situated above the carbonate compensation depth, encystment of S. bulloides in this area is unlikely to serve the purpose of protection against corrosion, as has been proposed by Murray (1991). The tubes arising from the cysts of this species may function as a guide or anchor for its pseudopodia, as suggested for Cibicides refulgens from an Antarctic coastal habitat (Alexander and DeLaca 1987) and C. wuellerstorfi from the bathyal Mediterranean (Heinz et al. 2005). Individuals of Miliolinella subrotunda collected at depths to 1419 m in the Atlantic Ocean constructed a sediment cyst extending into a tubular structure (up to 6 mm long) that elevated the test above the sediment surface, thus providing access to high-quality suspended food particles advected by lateral currents (Altenbach et al. 1993). However, this structure clearly serves a different function from that of the flimsy, sometimes branched tubes described here.