Shoaling behaviour in the European cuttlefish Sepia officinalis

Short running title: Shoaling behaviour in Sepia officinalis

Christian Drerup1, 2, *, Gavan M. Cooke2, *

1Marine Behavioural Ecology Group, Department of Zoology, University of Cambridge, Downing St, Cambridge CB2 3EJ, United Kingdom
2The Cephalopod Citizen Science Project, Poole BH12 4AB, United Kingdom

*Corresponding author’s e-mail address: 	cd731@cam.ac.uk (CD)
						cephcitscience@gmail.com (GMC)

ACKNOWLEDGEMENTS
The authors would like to thank Alix Harvey, Andy Jackson, Anna Kennedy, Claire Johnson, Jackie Daley, Jane Jory, Jon Bailey, Jon Bunker, Lisa Frew, and Tony Reed for their valuable video and photo contributions, as well as Michael Kuba, Tamar Gutnick, and the two anonymous reviewers for their constructive comments on this manuscript. There is no financial support associated with this study.





ABSTRACT
Group living is a common behavioural feature in many different animal species. It offers a multitude of fitness benefits, such as increased opportunities to find resources, improved predator vigilance, and potentially even social learning. In cephalopods, social grouping behaviour has mainly been reported for squid species, with up to several thousand individuals displaying different forms of shoaling and even schooling behaviour. Despite being held in groups in captivity, cuttlefish (Cephalopoda: Sepiidae) have long been considered rather asocial animals. However, reports of breeding aggregations as well as one recent schooling observation from the wild started to bring this characterisation into question. Following this, we here present 10 observations of the European cuttlefish Sepia officinalis (Linnaeus 1758) forming groups of up to 30 individuals along the South coast of the UK. The majority of the observed cuttlefish appeared to be juveniles or subadults and showed different shoaling orientations, such as linear or spherical shaped formations. This indicated the grouping behaviour did not derive from coincidental accumulations. No mating or courtship behaviour could be identified in these groups, and as all observations were made in August or September and therefore outside their mating season (March to June), it is unlikely that reproductive behaviour motivates these aggregations. As S. officinalis is known to migrate to deeper overwintering grounds in autumn, we propose that cuttlefish may temporarily form groups in late summer/early autumn as part of their migration pattern, and that their shoaling behaviour likely offers similar fitness benefits as in other migrating shoaling species. 

Keywords: cephalopod, group living, migration, schooling, Sepiidae, sociality


INTRODUCTION
Forming groups and social structures is a widespread phenomenon in the animal kingdom that offers a range of ecological advantages, such as increasing the opportunity to find resources or conspecifics for mating, reducing the risk of predation, and potentially even social learning (Krause & Ruxton, 2002; Ward & Webster, 2016b). Social groups may be relatively short lived, such as lekking species that group for reproduction, whilst other species may live in tight social groups their entire lives, e.g. the naked mole rat Heterocephalus glaber (Reeve et al., 1990), with many animals fitting somewhere in the middle. Birds and fish often form loose groups, termed ‘flocks’ and ‘shoals’ respectively (Pitcher & Parrish, 1993), that may forage or migrate together. Furthermore, groups of fish that synchronise their swimming velocity and direction are considered as ‘schools’ (Krause et al., 2000; Pitcher & Parrish, 1993).
For cephalopods, shoaling or schooling behaviour has mainly been reported for squids (Teuthoidea), potentially offering the same advantages as reported for fish (Hanlon & Messenger, 2018). In the reef squid species Sepioteuthis lessoniana and Sepioteuthis sepioidea, linear to spherical-shaped shoals with both adult and juvenile animals of both sexes have been observed (Moynihan & Rodaniche, 1982; Sugimoto et al., 2013). Furthermore, shoaling and schooling groups of Dosidicus gigas (Benoit-Bird & Gilly, 2012), Doryteuthis opalescens (Hurley, 1978), Illex illecebrosus (Mather & O'Dor, 1984) and Loligo reynaudii (Sauer et al., 1992) have frequently been reported, with the last two species reaching group sizes into the thousands. 
In contrast, octopuses have long been considered strictly asocial apart from brief moments during reproduction, even then often carried out ‘at arm’s length’. However, two recent discoveries (Godfrey-Smith & Lawrence, 2012; Scheel et al., 2017) describe how the gloomy octopus (Octopus tetricus) has been found in groups of 10 to 15 individuals with the animals occupying dens only centimetres apart. 
Similar to octopuses, cuttlefish have not been characterised as social animals (Boal, 2006) despite being held in groups for aquaculture or research purposes for several decades (Hanlon & Messenger, 2018; Iglesias et al., 2014). In these captive environments, cuttlefish were observed to form dominance hierarchies (Boal, 1996; Zhang et al., 2021), potentially based on their body size (Boal, 1996), or displayed shoaling (Choe, 1966) or even schooling (Nabhitabhata, 1997) behaviour as juveniles. However, observations of social behaviour in wild cuttlefish are rare. Field research has provided evidence that Sepia apama form large breeding aggregations (Hall & Hanlon, 2002; Hanlon et al., 2005) but aside from this are considered solitary (Hall & Hanlon, 2002). Recently, Yasumuro et al. (2015) have documented schooling in the broadclub cuttlefish Sepia latimanus, which included groups of up to nine individuals displaying synchronized and polarized swimming formations. This example of schooling appears unrelated to reproduction and therefore widens the scope of social behaviours in cuttlefish. Following this observation, herein we report empirical descriptions of 10 groups of S. officinalis displaying distinct shoaling formations. 
MATERIALS AND METHODS
All observations presented in this paper were collected via The Cephalopod Citizen Science Project (https://www.cephalopodcitizenscience.com/), whether pro or retroactively. The videos presented were recorded from SCUBA divers between 2013 and 2020, and each observation occurred between mid-August and late September of the corresponding year. Although the Cephalopod Citizen Science Project works globally, all observations of S. officinalis grouping behaviour so far reported and used in this study derive from the English Channel, in particular from the British South coast (Table 1; Figure 1), with reports of grouping cuttlefish from Cornwall (Newlyn, 50°06'27.2"N 5°32'35.5"W; Porthkerris, 50°03'56.6"N 5°03'53.8"W; Rame Head, 50°18'51.1"N 4°12'22.2"W), Devon (Heybrook, 50°03'56.6"N 5°03'53.8"W), and Dorset (Branksome Chine, 50°42'15.4"N 1°54'24.6"W). As the groups of cuttlefish were already formed before the divers approached and continued beyond each recording, the total duration of these events cannot be stated. In all observations, maturity stages (juvenile, subadult, and adult; with subadults being morphologically similar to adults but smaller and not yet reproductively mature (Young & Harman, 1988)) were estimated. This was achieved by assessing their mantle length (there is no proposed guideline for these stages, however, mature individuals (= adults) commonly have a mantle length of at least 12 - 15 cm in the English channel, see Dunn et al., 1999; Gras et al., 2016) and analysing their behaviour and body patterns for distinct features commonly expressed in either juvenile/immature or adult/mature cuttlefish, e.g. reproductive traits for suspected adults (following e.g. Hanlon & Messenger, 1988, 2018; Tinbergen, 1939). 
Due to the nature and sources of the videos obtained, we were unable to perform analytical tests to determine the nearest neighbour distances and angles, as conducted by Yasumuro et al. (2015), or to determine the exact duration of the observed shoaling formations. Instead, this study aims to investigate the occurrence of social grouping behaviour in S. officinalis and puts it into an ontogenetic context of this species’ life cycle and history.   
RESULTS
The first observation of grouping behaviour in S. officinalis was filmed off Newlyn Beach in early September 2013 and comprises six presumably subadult S. officinalis (approx. mantle lengths 10 - 15 cm) in roughly 10 m depth. For the entirety of the recording the cuttlefish moved slowly around a submerged small cove (approximately 20 m x 30 m) comprising a mixture of sand/gravel substrate and large outcrops of mature kelp (Laminaria sp.) on the East and West side (Figure 2a-b; Figure S1; Video S1). All individuals displayed either mild zebra patterns or, when closer to the substrate, a mottled colouration. Occasional false eye spots were displayed to either the diver or, more rarely, to conspecifics. However, little to no behavioural interactions or activity were observed, and we believe the animals were resting, possibly even sleeping at times (Cooke et al., unpublished data). Throughout the recording, the cuttlefish remained in close vicinity to each other (Figure S1) and organised themselves occasionally in a linear formation, with one individual protruding and facing in the opposite direction (Figure 2a; Video S1a), or in a spherical cluster with most individuals facing outwards (Figure 2b; Video S1b). These formations became looser over time, leading to periods in which the cuttlefish did not seem to follow any organisational patterns anymore (Figure S1). However, the observed individuals then returned to either linear or spherical formations again, but we were unable to identify any reasons that evoked this behaviour. On two occasions, a single individual distanced itself from the group for several minutes (Figure S1f,j) and seemed to explored its surrounding, but no specific behaviours (e.g. hunting attempts) could be observed for these cuttlefish.   
The second observation of grouping behaviour in S. officinalis was recorded off Rame Head in September 2017, lasting around 4 min and beginning with a group of approximately 10 cuttlefish swimming along the bottom (Figure 3a). To the left of this group, nine additional cuttlefish then appeared, first in a loose formation (Figure 3b) which then rearranged into a line (Figure 3c), floating approximately 2 – 3 m off the sea floor. The group of nine then moved and merged with the other group, as well as other individuals not previously recorded. The roughly 30 individuals appeared to be moving in one direction, consistently and as a group for the rest of the video (Figure 3d-f). The whole sequence can be observed in Video S2. Most cuttlefish in this observation displayed mild zebra patterns and occasionally false eye spots when in close vicinity to conspecifics. However, little to no behavioural interactions were observed throughout the recording. This group contains the largest cuttlefish of all our observations, with estimated average mantle lengths ranging between 15 - 20 cm. These individuals are not juveniles, and some of the visible cuttlefish could have reached maturity already. However, no reproductive behaviour (e.g. courtship behaviour, mating attempts, strong agonistic displays, copulation, mate guarding, or egg laying) could be observed. 
The third and fourth observations are from Heybrook Bay and Porthkerris, both recorded on August 14th, 2020. The group at Heybrook Bay consisted of 12 individuals, with some possibly resting on the sediment next to a crop of seaweed at a depth of 4 m. The second group from Porthkerris consisted of 10 individuals which were observed to surround a school of fish and, according to the observer, appeared to hunt together. An additional observation from Porthkerris (Observation 5) from August 29th, 2020, reported a group of 5 individuals of S. officinalis sheltering in vegetation. No photo or video material was provided for these observations, however, the observers reported an estimated mantle length of 5-10 cm, classifying them as juveniles. 
Observation 6 to 10 were all reported from Branksome Chine Bay in September 2020, with groups of 5 to 9 juvenile individuals (mantle lengths around 10 cm). Whether some observations show the same group on different days, or whether each observation displays a different group could not be determined. In each observation, the group of cuttlefish exhibited rather mottled or uniform displays and swam above or along rocky reefs, covered with algae or short vegetation, or sparsely vegetated sandy patches. The cuttlefish in these observations frequently established spherical formations with most individuals facing outwards (Figure 4a; Video S3a-c), linear formations with one individual facing the other way (Figure 4b,d; Video S3d-g), or moved along the reef structure in a moderately synchronous formation with all individual facing in the direction of travel (Figure 4c). These formations were relatively stable (bearing in mind the shorter durations of these recordings), however, the distances between the individuals varied to some degree, especially in the linear formations or when the groups were moving. Cuttlefish formations seemed to break for short moments when a diver became too close (although not captured in the video recordings) but the cuttlefish reorganised themselves seconds later. Some individuals occasionally left their positions or even the group, e.g. when somehow startled, whereas other individuals seemed to purposely head to a certain location, potentially to approach prey, but returned to the group shortly after. Indeed, some cuttlefish in these observations clearly devoured food items while shoaling, and potential kleptoparasitic behaviour (Brockmann & Barnard, 1979), as reported from captive individuals (Alix Harvey, pers. comm.), was observed.
DISCUSSION
The observations presented in this study display the first documented social grouping behaviour in the European cuttlefish S. officinalis in the wild. Cuttlefish have long been considered solitary (Boal, 2006). However, recent evidence of breeding aggregations (Hall & Hanlon, 2002) and shoaling/schooling behaviour (Yasumuro et al., 2015; this study) suggests that on a ‘sociality continuum scale’, reaching from the almost entirely asocial octopus to some lifelong social squid species, cuttlefish probably reside somewhere in between and, considering their occasional social interactions, rather be classified as semi-solitary.  
Although our observations of grouping behaviour in S. officinalis further support the slowly shifting consensus on sociality in cuttlefish, they also raise the questions of why these cuttlefish form groups, and which benefits they draw from it. One might argue that each group is just a coincidental accumulation of S. officinalis, potentially even caused by the presence of the divers. In each finding, however, the reported groups of cuttlefish were observed by the divers before their approach and remained together after the divers distanced themselves. Additionally, several forms of shoaling behaviour could frequently be observed in our findings, lasting up to several minutes. Across all observations, S. officinalis formed linear shaped formations (Figures 2a;3c;4b,d), with most individuals facing in one direction while one individual potentially acted as a sentinel by facing the other way, or spherical clusters with most, if not all individuals facing outwards (Figures 2b,4a). These observations strongly resemble the formations reported for the broadclub cuttlefish S. latimanus (Yasumuro et al., 2015) and reef squid of the genus Sepioteuthis (Adamo & Weichelt, 1999; Moynihan & Rodaniche, 1982; Sugimoto et al., 2013) and provide good evidence that the cuttlefish in our observations socialise on purpose. While Yasumuro et al. (2015) considered their observations as schooling behaviour, we are more reluctant about this terminology for our observations as we were unable to perform analytical tests, and therefore consider our observations as shoaling behaviour only. 
Another potential explanation for our observations could be reproductive behaviour. While the giant cuttlefish S. apama is known to form large breeding aggregation with thousands of individuals (Hall & Hanlon, 2002), the European cuttlefish S. officinalis occasionally forms smaller aggregations consisting of only a few individuals during mating season (Tinbergen, 1939). However, reproduction of S. officinalis along the South coast of the UK, form where all our observations derive (Figure 1), takes place from March to June (Bloor et al., 2013; Boucaud-Camou et al., 1991; Dunn, 1999). As all our observations were made in August and September, no reproductive behaviour was observed, and in most observations the cuttlefish appear to be juveniles or subadults, we cannot identify any likely correlation between the observed cuttlefish groups and reproductive behaviour. 
We propose that our reported social groups of S. officinalis are linked to the offshore migration pattern of this species. In autumn, S. officinalis individuals leave their inshore nursery grounds and migrate to deeper overwintering grounds in the Western part of the English Channel and the Northern part of the French Atlantic Coast (Bloor et al., 2013; Boucaud-Camou & Boismery, 1991; Dunn, 1999, see fig. 1 in Gras et al., 2016, and fig. 1.2 in Bloor et al. 2013 for life cycle illustrations). Following Wang et al. (2003), S. officinalis first aggregates in the Western part of the English Channel in autumn, followed by migrating to deeper waters from January onwards (Figure 1). We believe that juvenile European cuttlefish form groups while migrating westwards along the Southern coast of the UK, presumably due to fitness benefits of grouping behaviour. As the juveniles of S. officinalis migrating to deeper waters have not yet mated, reaching the overwintering grounds to subsequently move back to the inshore breeding grounds in spring for mating and spawning can be considered as their key intention. Therefore, temporarily forming groups while migrating might increase their survival chances by reducing the risk of predation (Ward & Webster, 2016a). The described shoaling formations, such as linear shaped organisations with one individual potentially acting as a sentinel, or spherical shapes with most individuals facing outwards, further support this hypothesis, as these formations increase the spotting of predators from each direction (Lima, 1995).
Another benefit of grouping behaviour in S. officinalis in autumn as part of their migrating behaviour could be the “many wrongs principle”, stating that a group of animals can increase their navigation accuracy by pooling individual, less accurate estimates of the “correct” navigation (Berdahl et al., 2018; Simons, 2004). As environmental cues used for navigation can be masked by different types of acoustic, visual, or chemical noise (Hawkins & Popper, 2017; Shannon et al., 2016), sensory channels of a single individual are inherently prone to error. By migrating as a group, individuals can further utilise and integrate information acquired by conspecifics, which can increase navigational accuracy in migrating species (Berdahl et al., 2018; Couzin, 2018). 
Lastly, grouping behaviour also offers the opportunity for social learning (Laland & Williams, 1997; Thornton & Clutton-Brock, 2011). Here, individuals learn through the experience of conspecifics, thereby minimizing the need for self-induced trial and error processes (Galef & Laland, 2005). In cuttlefish, however, previous studies on juvenile/adult cuttlefish reported no improved hunting behaviour (Boal et al., 2000) or danger avoidance (Huang & Chiao, 2013) through observational learning. Contrarily, a recent study from Sampaio et al. (2020) exhibited that cuttlefish hatchlings can use social information, obtained through observing conspecifics, to improve their predatory behaviour. As cuttlefish hatch in close proximity to their conspecifics, it can be hypothesised that the (inevitable) close vicinity to other hatchlings might facilitate social learning, whereas this feature diminishes as cuttlefish grow and exhibit (at least at certain times) a more solitary lifestyle. However, whether the grouping behaviour reported in this study facilitates social learning will need to be addressed in future work.  
Our observations show several groups of S. officinalis consisting of approximately 5 to 10 animals, whereas in one observation a group of more than 30 individuals was encountered. It can be hypothesised that groups undergoing the autumnal westward migration along the UK South coast consist of smaller numbers (~ 5 – 10 individuals), but then merge in the Western aggregation locations (Wang et al., 2003) to higher numbers of 30 or potentially even more individuals. Alternatively, cephalopods are known to go through boom-and-bust population cycles (e.g. Perretti, 2014) and so very large groups may form periodically simply through stochastic processes.
In summary, our observations show the first reported grouping behaviour of the European cuttlefish S. officinalis. The observed groups of cuttlefish exhibited different shoaling formations, similar to those previously reported in other cuttlefish and especially squid species. We propose the grouping behaviour may be linked to the species’ migration pattern by reducing the risk of predation while increasing the migration accuracy. 
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Figure captions

FIGURE 1 Map of the South coast of England with the location of the presented observations, numbered in chronological order. 1 Newlyn (50°06'27.2"N 5°32'35.5"W), 2 Rame Head (50°18'51.1"N 4°12'22.2"W), 3 Heybrook (50°18'58.3"N 4°07'06.4"W), 4-5 Porthkerris (50°03'56.6"N 5°03'53.8"W), and 6-10 Branksome Chine (50°42'15.4"N 1°54'24.6"W). The solid arrow depicts the autumnal migration route of S. officinalis along the Southern coast of the UK to the aggregation hot spots in the Western part of the English Channel (September to December), whereas the dashed arrow indicates the following migration route to the deeper overwintering grounds in the North-East Atlantic (January onwards) following Wang et al. (2003). 


FIGURE 2 Screenshots of the first observation filmed off Newlyn Beach in early September 2013. (a) Linear shaped shoaling formation with six individuals facing towards the camera (white arrows), and one individual facing the opposite direction (red arrow), potentially acting as a sentinel. (b) Spherical shoaling formation with all but one individual (blue arrow) facing outwards. 


FIGURE 3 Screenshots of the second observation filmed off Rame Head (UK) in September 2017. (a) Accumulation of approximately 10 individuals at the beginning of the video footage. (b-c) A second group of 9 individuals appears roughly 10 meters next to the previously mentioned group, (b) first in a looser orientation which (c) then rearranges into a linear formation. (d-f) Both previously described groups merge with further individuals into a group of approximately 30 individuals.


FIGURE 4 Screenshots of the observations filmed off Branksome Chine Beach (UK) on several days in September 2020 (see Table 1). (a) Spherical shoaling formation of 5 individuals (white arrows) facing outwards. (b) Linear shaped shoaling formation with six individuals facing towards the camera (white arrows), and one individual facing the opposite direction (red arrow), potentially acting as a sentinel. (c) A group of 5 individuals (white arrows) moving moderately synchronously along the reef, all facing in the same direction. (d) Linear shaped shoaling formation with four individuals facing towards the camera (white arrows), two individuals protruding into the direction of the camera (blue arrows), and one individual facing the opposite direction (red arrow), potentially acting as a sentinel.