# Seafoam avoidance is driven by both chemosensory and contact cues and points towards the presence of soft-surface thigmotaxis in the intertidal gastropod Littorina littorea

Solène Cazalis-Henry, Nicolas Spilmont, Elsa Breton, Florence Elias, Laurent Seuront

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Solène Cazalis-Henry, Nicolas Spilmont, Elsa Breton, Florence Elias, Laurent Seuront. Seafoam avoidance is driven by both chemosensory and contact cues and points towards the presence of soft-surface thigmotaxis in the intertidal gastropod Littorina littorea. Journal of Experimental Marine Biology and Ecology, 2025, 583, pp.152085. 10.1016/j.jembe.2025.152085. hal-05193540

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# Seafoam avoidance is driven by both chemosensory and contact cues and points towards the presence of soft-surface thigmotaxis in the intertidal gastropod Littorina littorea

**Article in Journal of Experimental Marine Biology and Ecology · February 2025**

DOI: 10.1016/j.jembe.2025.152085

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University of the Littoral Opal Coast

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# Seafoam avoidance is driven by both chemosensory and contact cues and points towards the presence of soft-surface thigmotaxis in the intertidal gastropod *Littorina littorea*

Solène Cazalis-Henry<sup>a, *,</sup> Nicolas Spilmont<sup>a</sup> Elsa Breton<sup>b</sup> Florence Elias<sup>c</sup> Laurent Seuront<sup>a,d,e</sup>

## ARTICLE INFO

### Keywords:

- Seafoam accumulation

- Rocky intertidal

- Behavioural avoidance

- Soft-surface thigmotaxis

- Chemical and contact cues

## ABSTRACT

Seafoam accumulation in coastal waters has long been acknowledged as ecologically, socially and economically relevant. However, its putative effects on the intertidal biota have been alarmingly under-researched despite the ubiquitous nature of onshore foam formation. This study examines the behavioural responses of the common periwinkle, *Littorina littorea*, exposed to two types of seafoam: a fresh seafoam (freshly formed) and an aged seafoam (collected after three hours of exposure in the field), using experimental assays carried out under controlled laboratory conditions. *Littorina littorea* consistently avoid seafoam, and exhibit a stronger aversion for fresh seafoam. They avoid crossing both seafoam, suggesting that contact cues play a crucial role in this avoidance behaviour. In this context, we introduce the concept of soft-surface thigmotaxis, by which *Littorina littorea* is able to follow soft physical discontinuities. Snails exhibited more tortuous movements and reduced speed before actually getting into physical contact with the fresh seafoam, suggesting that chemosensory influence foam avoidance, probably due to repellent volatile compounds requiring further identification. Observations suggest that snails show differential responses potentially influenced by variations in foam properties, such as phytoplankton content and the physical properties of the seafoam. Seafoam avoidance by *Littorina littorea* could have wider ecological implications by disrupting the snails' ability to find refuges, increasing their vulnerability to other environmental stresses. Investigating surfactants and pollutants in seafoam is crucial due to their toxicity to aquatic life. The study highlights the need for interdisciplinary research to fully understand and mitigate seafoam's effects on intertidal organism.

## 1. Introduction

In an era of global changes, extreme events such as heat waves, floods and storms are expected to increase in intensity, frequency and duration (IPCC, 2023). In this context, the nearly ubiquitous formation of white caps and seafoam (respectively transient and persistent bubble aggregates; Monahan, 2001) are likely to increase (Jenkinson et al., 2021). In particular, seafoam accumulation in coastal waters—typically produced by breaking waves in the presence of a mixture of natural and/or anthropogenic surfactants (i.e. compounds which modify the surface tension of the air-seawater interface) in bulk-phase seawater (Wegner and Hamburger, 2002; Schilling and Zessner, 2011)—has long been

acknowledged as ecologically, socially and economically relevant (Lancelot, 1995; Peperzak, 2002; Blauw et al., 2010; Peperzak and Van Wezel, 2023). These issues are particularly significant in intertidal environments where their putative effects on the biota have been alarmingly under-researched despite the ubiquitous nature of onshore foam formation (Seuront et al., 2024a, 2024b).

Marine foams (and more generally any aqueous foam) are formed by air bubbles interspersed in a water phase, made of interconnected liquid channels called Plateau borders (Cantat et al., 2013; Haffner et al., 2015; Louvet et al., 2010). They are inherently unstable and gradually disappear with time through three interconnected destabilizing processes: drainage, coalescence and coarsening; see Cohen-Addad et al. (2013) for

<footnote><sup>a</sup> Univ. Lille, CNRS, Univ. Littoral Côte d'Opale, IRD, UMR 8187 LOG, Station Marine de Wimereux, F-59000 Lille, France</footnote>

<footnote><sup>b</sup> Univ. Littoral Côte d'Opale, Univ. Lille, CNRS, IRD, UMR 8187 LOG, Wimereux, F-59000 Lille, France</footnote>

<footnote><sup>c</sup> Laboratoire de Physique et Mécanique des Milieux Hétérogènes, ESPCI-PSL-Sorbonne Université-Université Paris Cité, 75005 Paris, France</footnote>

<footnote><sup>d</sup> Department of Marine Resources and Energy, Tokyo University of Marine Science and Technology, Tokyo, Japan</footnote>

<footnote><sup>e</sup> Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: solene.henry@univ-lille.fr (S. Cazalis-Henry).</footnote>
a review. Drainage is the gravity-driven process by which the liquid phase of the foam flows downward. During drainage, the films separating bubbles thin down and eventually break, leading the bubbles to merge (i.e. to coalesce). This coarsening involves the transport of gas between bubbles of different sizes, where the gas diffuses from the smaller to the larger bubbles due to differences in Laplace pressure (i.e. the smaller the bubble the higher the pressure). The rate of these events consistently decreases as the foam ages and dries out (Durian et al., 1991; Cohen-Addad and Höhler, 2001). In addition, when the liquid phase is loaded with spherical particles of diameter *dp* (at a volume fraction of less than about 40 %), their dynamics depend on the characteristic size *dc* of the Plateau border cross-section, which decreases continuously with time as the liquid flows out of the foam under the effect of gravity: the particles present in the foam channels are freely advected by the liquid flow as long as $dc > dp$, while they are blocked in the channel as soon as $dc < dp$ (Haffner et al., 2015; Louvet et al., 2010). Consequently, in situ, the so-called stable seafoams persist over time and become loaded with sediment and phytoplankton as a result of destabilisation processes.

The formation of foam is intrinsically linked to the presence of natural or anthropogenic surfactants that enhance the intensity and stability of the foam produced (Callaghan et al., 2013, 2017) and the presence of proteins and particles can also enhance the formation of a stable liquid foam (Rio et al., 2014). However, the identification of the chemical compounds and the mechanisms leading to the formation of the stable foam observed along the world's coasts in general (e.g., Lancelot, 1995; Porri et al., 2021; Shetye et al., 2021) and along the French coasts of the eastern English Channel in particular (Seuront et al., 2006, 2007; Kesaulya et al., 2008; Spilmont et al., 2009; Seuront et al., 2024a, 2024b) remains unclear, especially when seawater is involved (Murray, 2007; Fameau and Salonen, 2014; Rio et al., 2014). These persistent foam formations have a noticeably relatively long history (Southward, 1953) and they were early suggested as an ecological habitat (Maynard, 1968), which was subsequently supported by the trapping of fungal spores, microalgae, bacteria and protozoa (Pascoal et al., 2005; Seuront et al., 2006; Spilmont et al., 2009; Roveillo et al., 2020) and even metazoan larvae (Castilla et al., 2007). The formation of seafoam has been associated with exudates produced by kelp beds (Velimirov, 1980, 1982), mass spawning events (Castilla et al., 2007) and phytoplankton blooms (Lewitus et al., 2012), in particular the haptophyte *Phaeocystis globosa* (Bätje and Michaelis, 1986; Blauw et al., 2010; Lancelot, 1995; Peperzak, 2002; Seuront et al., 2006). Seafoam has been shown to affect the structure and function of intertidal soft sediment ecosystems through increasing the rate of nitrification, community respiration and primary production (Spilmont et al., 2005, 2009). Aged seafoam deposits also induce anoxia which leads to the upward migration of benthic invertebrates, and eventually their death, thus shortening the local food web (Spilmont et al., 2009). More recently, seafoam has been identified as obstructing light penetration and reducing oxygen concentrations in salt marsh tidal ponds (Koop-Jakobsen and Gutbrod, 2019).

In the eastern English Channel, the recurrent formation and accumulation of seafoam in spring appears to be intrinsically linked to blooms of *P. globosa*, with the formation of large, gel-like colonies (Verity and Medlin, 2003; Schoemann et al., 2005) and the related release of exopolymeric materials that are the acknowledged precursor of foam formations occurring in the turbulent surf zone (Seuront et al., 2007; Seuront et al., 2006; Kesaulya et al., 2008; Spilmont et al., 2009). Note, however, that foam formation has also been recurrently observed both in the absence of *P. globosa* (Breton, personal observation) and when this species occurred in relatively low concentration (Seuront et al., 2024a, 2024b). *P. globosa* is also known for its considerable production of dimethylsulphoniopropionate (DMSP; Schoemann et al., 2005), a compound commonly accumulating in white caps and seafoam (Jenkinson et al., 2021) that is generally catabolized into the volatile compound dimethylsulphide (DMS; Curson et al., 2011; Zhang et al.,

2019). Consequently, both seafoam and white caps — known to play a major role in the exchange of gases between air and water and in the production of sea salt aerosols (Monahan, 1986; Monahan, 1971; Monahan et al., 1982)— are likely to release volatile compounds such as DMS, which has recently been suggested to be potentially toxic (Demchuk et al., 2018; Song et al., 2022). More generally, aqueous foams have been shown to accumulate a wide range of particulate and dissolved compounds such as heavy metals and radionuclides, DDT and PCBs (Fisenko, 2004). As such, they can interfere indirectly with intertidal species relying on chemical airborne-cues or directly through contact-cues to move, feed or reproduce; see e.g. Zimmer et al. (1999), Hay (2009), Seuront and Spilmont (2015), Suárez-Rodríguez et al. (2023). Despite the documented ecological significance of seafoam deposits on both sandflats and mudflats, there is still a notable lack of information on their effects on rocky intertidal ecosystems in general, and on the behavioural ecology of intertidal invertebrates in particular. To the best of our knowledge, this issue has only recently been addressed for the high-shore intertidal gastropod *Littorina saxatilis* which were found to actively avoid areas covered in foam, increase their activity in the presence of foam and increase their metabolic rate following a foam exposure (Seuront et al., 2024a, 2024b). Filling this knowledge gap is critical considering the key role of these systems in coastal habitats through their high taxonomic and functional richness (Satyam and Thiruchitrambalam, 2018) and the provision of habitats for major commercial value, ecological or sentinel species such as mussels or littorinids like *Littorina littorea* (Noventa et al., 2011).

Rocky intertidal ecosystems are fundamentally topographically complex ecosystems, where most gastropods, and littorinids in particular, use a range of directional cues, including chemotaxis towards both water-borne and air-born infochemicals from food sources and conspecifics (Shearer and Atkinson, 2005; Fink et al., 2006), phototaxis (i.e. the orientation and movement towards a source of light), scototaxis (i.e. a natural preference for dark areas; Newell, 1958; Petraitis, 1982; Thain et al., 1985), rheotaxis (i.e. a movement either towards or away from the stimulus induced by a flow; Duch, 1971; Gendron, 1977; Rochette et al., 1997), thigmotaxis (i.e. a movement towards or away from the stimulus induced by a physical contact), geotaxis (Evans, 1951; Petraitis, 1982; Thain et al., 1985; Seuront et al., 2018), which has been shown to be a by-product of thigmotaxis (Moisez and Seuront, 2020) and aphrotaxis (coined from the ancient Greek word ἀφρός, i.e. *aphrós*, 'foam'), recently identified in *L. saxatilis* (Seuront et al., 2024a, 2024b). However, it is still unclear how seafoam deposits can affect the ability of intertidal gastropod in general, and *Littorina littorea* in particular. In this context, our study aims to better understand the ability of gastropods to detect seafoam either indirectly through air-borne cues or directly through contact-cues and their potential subsequent behavioural response. More specifically, we investigated under laboratory-controlled conditions whether the accumulations of seafoam at different stages of ageing, i.e. foams that have respectively just been formed or gone through the destabilizing processes of drainage, coalescence and coarsening, can affect the behaviour of *L. littorea*, with a specific focus on the effects of direct and indirect foam-borne cues from fresh (smooth and wet) and aged (firmed and rough, enriched with sediment by winds that could not be reproduced in the laboratory) naturally occurring seafoam on their behavioural traits. This is important as, beyond the fundamental differences in the physical structure of fresh and aged foam driven drainage, coalescence and coarsening, these foams also consistently differ in situ in their macroscopic appearance with fresh and aged foam being respectively smooth and rough, the latter often encrusted with sediment and organic material. In this context, given the ability of foam to trap phytoplankton cells (Seuront et al., 2006; Seuront and Vincent, 2008; Roveillo et al., 2020), we also considered the phytoplankton species present in fresh and aged seafoam as potential qualitative and quantitative differences could influence the chemical cues (Seuront et al., 2024a, 2024b).
## 2. Materials and methods

### 2.1. Snail and seafoam sampling

Littorina littorea (14.72 ± 1.02 mm shell length; mean ± SD) individuals (n = 88) were collected on April 12 and 13, 2023 at low tide from the mid-shore level at the Pointe de la Crèche (50°45'09.9" N, 1°35'40.4" E), a rocky intertidal reef located along the French shore of the eastern English Channel where the species density can be higher than 300 ind m⁻² (Moisez et al., 2020). To avoid potential biases related to a potential previous exposure to seafoam, all the individuals used in the present work were specifically collected on rocks that did not show any evidence of foam accumulation. *L. littorea* individuals were brought back to the laboratory within 45 min and acclimated for 24 h in transparent PMMA (i.e. polymethyl methacrylate) cylinders (50 cm high and 20 cm in inner diameter, riddled with holes 5 mm in diameter) held in 120 L (90 × 50 × 30 cm) glass aquaria filled with flowing natural seawater at a temperature representative of in situ conditions (T = 10 °C). These 'acclimation towers' (Seuront and Spilmont, 2015) allow both continuous renewal of seawater and free movement of captive snails in and out of the water without being able to escape. Parasitism (in particular the trematod *Cryptocotyle lingua*) has been acknowledged to modify the behaviour of *Littorina littorea*; see e.g. Davies and Knowles (2001) and Seaman and Briffa (2015). However, the consistent absence of the orange staining of the foot caused by pigment release from damaged tissue (Huxham et al., 1993), the paucity of seabirds in our study area (in particular the herring gull, *Larus argentatus*, that convey *C. lingua* eggs in their faeces) and the demonstrated ability of *L. littorea* to avoid contaminated guano (Davies and Knowles, 2001) are consistent with previous evidence of a lack of patent or nonpatent parasitic infections in *L. littorea* along the French shore of the eastern English Channel (Seuront and Spilmont, 2015) and the local low infection risk and low prevalence of *C. lingua* in *L. littorea*; see e.g. Byers et al. (2015). It is also worth noting that as observed in previous work (Davies and Knowles, 2001), naturally deposited *L. argentatus* faeces at mid-tide level were consistently removed by one tidal inundation (Seuront, unpublished data), a period considered too brief to allow for ingestion of eggs in guano by *L. littorea*. All individuals were returned to their original location after the behavioural experiments.

To explicitly account for changes in the physical structure of seafoam during the ageing process (i.e. a decrease in both water content due to gravitational drainage and the number of bubbles, and an increase in their size due to coalescence), we specifically sampled three batches of each type of seafoam during two successive days throughout a spring foam event. Notably, no significant differences in phytoplankton community composition were observed in seawater (NMDS: *p* = 0.147 *D*<sub>f</sub> = 2 *F* = 2.056; Breton, unpublished data). On the first day, we collected "aged" seafoam (i.e. in the mid- to high intertidal zone about 3 h after the tide receded), while on the second day, we sampled "fresh" seafoam (i.e., collected on the seawater upon its formation in the wave-breaking zone at ebb tide). These two types of seafoam visually mostly differ by their colour: fresh foam appears white and translucent, while aged foam appears darker and opaque, likely due to the accumulation of sediment and organic materials. Seafoam was carefully sampled using 10 L glass beakers and transported back to the laboratory within 45 min. The beakers were always kept upright until the experiments were carried out to allow the foam to drain naturally. These foams, which are stable and typically persist from 3 to 6 h up to several days in the eastern English Channel (Seuront et al., 2006; Kesaulya et al., 2008; Spilmont et al., 2009), were used for experiments within 2 h after the sampling took place.

### 2.2. Phytoplankton species determination

We assessed the species abundance, composition and habitat of the phytoplankton and protozooplankton community present in the two

different types of seafoam. This investigation aimed to detect any potentially toxic species, due to the ability of seafoam to trap phytoplankton cells from the water column (Roveillo et al., 2020; Seuront et al., 2006; Seuront and Vincent, 2008) and the potential presence of toxic phytoplankton species in seawater, which recurrently occur in coastal waters of the eastern English Channel such as *Phaeocystis globosa* (Demchuk et al., 2018; Song et al., 2022). When present, the diatom *Pseudo-nitzschia* sp. may also be a source of domoic acid (Klein et al., 2010; Delegrange et al., 2018), a kainic-acid type neurotoxin which transforms some molluscs such as mussels (*Mytilus* spp.) and razor clams (*Siliqua patula*) into vectors of this biotoxin (Bejarano et al., 2008), which cause amnesic shellfish poisoning (ASP) in humans (Todd, 1993; Schoemann et al., 2005).

For this taxonomic identification, seafoam samples were fixed with a solution of Lugol-glutaraldehyde (1 % v/v; a fixative that does not disturb *Phaeocystis globosa* colonies; Breton et al., 2006) and then examined using an inverted microscope (Nikon Eclipse TE2000-S) at 100–400× magnification after sedimentation in 10 mL Hydrobios chambers for 12 h. Species identifications were conducted whenever feasible, relying on pertinent taxonomic literature for phytoplankton (Halse and Syvertsen, 1996; Hoppenrath et al., 2009), heterotrophic dinoflagellates (Schiller, 1931–1937; Maar et al., 2002; Gómez and Souissi, 2007; Gómez and Artigas, 2014; Hoppenrath et al., 2023) and ciliates (Kofoid and Campbell, 1929). Biovolumes were estimated using standard geometric forms according to Hillebrand et al. (1999). Carbon biomass (µgC L⁻¹) of phytoplankton and heterotrophic dinoflagellates was calculated using the carbon conversion factors of Menden-Deuer and Lessard (2000). For flagellated and solitary colonial cells, the carbon conversion factors of 10.8 and 14.15 pg C/cell were applied, respectively (Schoemann et al., 2005). The biomass of ciliates was calculated using a conversion factor of 0.19 pg µm⁻³ (Putt and Stoecker, 1989).

### 2.3. Experimental design and behavioural assay

The ability of *L. littorea* to detect foam-borne cues and the subsequent potential impact of foam-born cues on their movement behaviour was investigated under laboratory-controlled conditions. Specifically, the behavioural response of *L. littorea* was assessed in the presence of fresh and aged seafoam by running a series of single-individual trials over two consecutive days. Controls and treatments were conducted concurrently during each trial, with each individual used only once. Following seafoam sampling, the behavioural assays were conducted with aged seafoam (n = 30) and without seafoam (control 1; n = 24) on the first day, and with fresh seafoam (n = 18) and without seafoam (control 2; n = 16) on the second day. All assays were conducted in 22 cm inner diameter circular Pyrex dishes (referred to as arenas hereafter) where one individual *L. littorea* was consistently placed at the centre of the circular arena (Fig. 1) and exposed to either a fresh or aged concentric seafoam ring (diameter = 6.32 ± 0.67 cm and width = 2.03 ± 0.21 cm; mean ± SD; Fig. 1). Snail movements were recorded through time lapse photography (1 picture every 5 s for 30 min) using a GoPro camera (GoPro® Hero 8, GoPro Inc., San Mateo, California, USA) positioned 50 cm above the experimental arenas.

### 2.4. Behavioural analyses

The objectives of the above-mentioned experimental design were two-fold. First, we assessed the indirect behavioural responses of *L. littorea* to foam-borne cues (i.e. in the absence of physical contact with seafoam). We subsequently assessed the direct behavioural responses of *L. littorea*, i.e. when the snails actually came in physical contact with seafoam. These behavioural responses were quantitatively assessed using several metrics.

To assess indirect behavioural responses, we first evaluated the proportion of snails that (i) extended their tentacles, (ii) moved and (iii) reached a physical barrier, i.e. the wall of the arena in the control or the
<figure><img src="image_8.png" /><figcaption>**Fig. 1.** Experimental design followed for the control, and the fresh and aged seafoam experiments. One individual *Littorina littorea* was placed at the centre of the experimental arena in the absence or in the presence of a seafoam ring. The dashed circle drawn in the experimental arena for the control experiment provides a benchmark of the inner side of the foam rings used to compare velocity and NGDR (i.e. the ratio between the net distance (ND) and the gross distance travelled (GD) between its initial and final position) between treatments and the control over a standardized distance.</figcaption></figure>

ring foam in the treatments. We also recorded the time required to start moving (i.e. startle time). To allow direct comparisons between the control and the experiments conducted with fresh and aged seafoam, we defined a virtual ring of the same diameter as the seafoam rings (Fig. 1). We extracted the $x_t$ and $y_t$ coordinates of the gastropod during the experiments using Fiji software (Schneider et al., 2012). These coordinates were subsequently used to assess (i) the tortuosity through the net-to-gross distance ratio (NGDR), i.e. the ratio between the net distance (ND) and the gross distance (GD) travelled between its initial (centre of the arena) and final position (inner limit of the actual or virtual seafoam ring; Dicke and Burrough, 1988) and (ii) the velocity estimated over the gross distance.

L. littorea direct behavioural responses to seafoam was evaluated using the proportion of time spent (%) in contact with seafoam throughout the experimental test. This contact was subsequently classified into 2 categories, namely shell contact (i.e. when the snail moved with shell in contact with the seafoam) and tentacle contact (i.e. when the snail repeatedly touches the surface of the seafoam using its tentacles). Finally, we evaluated both the proportion of snails leaving the arena and the corresponding time, and the proportion of snails becoming motionless before the end of the experiment and the associated stop time in the control and seafoam experiments.

## 2.5. Statistical analyses

As none of the behavioural measures satisfied the normality assumption (Kolmogorov-Smirnov test, *p* < 0.05), non-parametric statistics were used throughout this work. As no significant differences (Wilcoxon-Mann-Whitney test, *p* > 0.05) were found between any of the behavioural metrics obtained for the control experiments conducted on two distinct days, the corresponding data were pooled to compare the control with the two treatments (i.e. fresh and aged seafoam). Startle time, NGDR and velocity were compared using a Kruskal-Wallis's test (KW), and a subsequent multiple comparison procedure based on the Dunn test was used to identify homogeneous groups. The proportions of snails exhibiting a specific behaviour (i.e., shell contact, tentacles contact or no contact with the foam) were compared using a Pearson's Chi-square test ($\chi^2$). Stop times were only compared between seafoam treatments using a Wilcoxon-Mann-Whitney test (WMW), since only 1 individual stopped in the control treatment.

# 3. Results

## 3.1. Phytoplankton community composition

The composition of the phyto- and protozooplankton communities

trapped within seafoam differed between our seafoam samples. More specifically, species diversity present was 2.5 times higher in the aged seafoam than in the fresh seafoam (Table 1). Diatom species —comprising psammic, epipsammic, epiphytic, and freshwater species— accounted for 89 % of the total diatom abundance in the aged seafoam, compared to only 14 % in the fresh seafoam (Table 1). Notably, *Attheya armata*, a surf diatom which dynamics are largely influenced by sediment movement (Louvet et al., 2018), was found at concentrations over 1000 times higher in aged foam compared to fresh foam. Additionally, psammic and epipsammic genera such as *Navicula* sp. and *Hantzschia* sp., the epiphytic species *Licmophora abbreviata*, and the freshwater species *Synedra ulna* were present in aged foam but absent in fresh foam. Interestingly, the carbon biomass of *Pseudo-nitzschia pungens* —a species capable of producing domoic acid (Bates et al., 2005; Trainer et al., 2002)— was also ca. 100 times more higher in aged than in fresh foam. However, its contribution to the total phytoplankton carbon biomass remained low, at just 1.6 % in aged foam and 2.2 % in fresh foam (Table 1). We notably did not find any *Phaeocystis globosa* cells in our seafoam samples, although they were present in seawater during our sampling (up to 5 mgC L⁻¹, Breton, unpublished data).

## 3.2. *Littorina littorea* behavioural response to foam-borne cues

The proportion of snails with tentacles deployed ranged between 83.3 % and 94.4 % and did not significantly differ between treatments and the control (92.3 %; $\chi^2$ test: *p* = 0.469 $\chi^2$ = 1.514). Noticeably, the snails that extended their tentacles subsequently started to move. Similarly, the startle time was not significantly different between treatments and control (KW test: *p* = 0.073 $\chi^2$ = 5.2, Fig. 2A). Fig. 2 provides archetypical examples of the paths followed by *Littorina littorea* in the absence of seafoam (Fig. 2A), and in the presence of fresh (Fig. 2B) and aged (Fig. 2C) seafoam. The nature of the trajectories of the snails clearly differed in the control experiments (Fig. 2A) and when they were exposed to seafoam (Fig. 2B, C).

In the absence of foam, virtually all individuals reached the wall of the arena (97.2 %). Most of those individuals (74.3 %) performed a thigmotactic behaviour along the wall, an archetypical behaviour of L. littorea in the presence of a topographical discontinuity (Moisez et al., 2020). The majority (65.4 %) of these individuals subsequently performed a negative geotactic behaviour, referred to as thigmotactically-driven negative geotaxis (Moisez and Seuront, 2020), while the others (34.6 %) continued to follow the wall. The individuals that did not exhibit a thigmotactic behaviour (25.7 %) directly climbed the wall.

These results are in sharp contrast with the observations conducted in the presence of seafoam. First, *L. littorea* never crossed the seafoam ring, irrespective of the nature (i.e. fresh or aged) of the foam (Fig. 2B,
<caption>**Table 1**</caption>

Phyto- (diatom) and protozooplankton (dinoflagellates and ciliates) abundance (10³N L⁻¹) and biomass (µgC L⁻¹) of both fresh and aged seafoam samples collected at Audresselles and their natural habitat: B: benthic, P: pelagic, S: surf zone, F: freshwater. The potential toxic species are in bold.

<table><thead><tr><th rowspan="2"></th><th rowspan="2">Habitat</th><th colspan="2">Fresh Seafoam</th><th colspan="2">Aged Seafoam</th></tr><tr><th>Abundance</th><th>Biomass</th><th>Abundance</th><th>Biomass</th></tr></thead><tbody><tr><td><strong>Diatoms</strong></td><td></td><td><strong>10.53</strong></td><td><strong>2.69</strong></td><td><strong>5000.7</strong></td><td><strong>472.67</strong></td></tr><tr><td>Achnanthes spp.</td><td>B</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.02</td></tr><tr><td>Attheya armata</td><td>S</td><td>0</td><td>0</td><td>4440.56</td><td>222.03</td></tr><tr><td>Brockmanniella<br>brockmannii</td><td>B</td><td>1.25</td><td>0.03</td><td>0</td><td>0.00</td></tr><tr><td>Cerataulina<br>pelagica</td><td>P</td><td>0.26</td><td>0.05</td><td>5.01</td><td>1.00</td></tr><tr><td>Cyclotella spp.</td><td>P</td><td>0</td><td>0</td><td>0.4</td><td>0.01</td></tr><tr><td>Dactyliosolen<br>fragilissimus</td><td>P</td><td>0</td><td>0</td><td>1.6</td><td>0.33</td></tr><tr><td>Ditylum brightwellii</td><td>P</td><td>0.01</td><td>0.02</td><td>0</td><td>0.00</td></tr><tr><td>Druridgea<br>compressa</td><td>B</td><td>0</td><td>0.00</td><td>2.58</td><td>0.49</td></tr><tr><td>Guinardia<br>delicatula</td><td>P</td><td>6.69</td><td>1.82</td><td>275.5</td><td>75.16</td></tr><tr><td>Guinardia flaccida</td><td>P</td><td>0.06</td><td>0.09</td><td>22.54</td><td>33.63</td></tr><tr><td>Guinardia striata</td><td>P</td><td>0</td><td>0</td><td>0.51</td><td>0.13</td></tr><tr><td>Hantzschia virgata</td><td>B</td><td>0</td><td>0</td><td>1.03</td><td>0.82</td></tr><tr><td>Lauderia annulata</td><td>P</td><td>0</td><td>0</td><td>0.01</td><td>0.91</td></tr><tr><td>Leptocyclindrus<br>danicus</td><td>P</td><td>0.26</td><td>0.03</td><td>5.01</td><td>0.5</td></tr><tr><td>Licmophora<br>abbreviata</td><td>B</td><td>0</td><td>0</td><td>0.74</td><td>0.39</td></tr><tr><td>Minidiscus spp.</td><td>B</td><td>0</td><td>0</td><td>0.48</td><td>0.01</td></tr><tr><td>Navicula spp.</td><td>B</td><td>0.05</td><td>0</td><td>0.02</td><td>2.23</td></tr><tr><td>Nitzschia<br>closterium</td><td>B</td><td>0.05</td><td>0</td><td>0.04</td><td>0.01</td></tr><tr><td>Odontella<br>mobiliensis</td><td>S</td><td>0</td><td>0</td><td>2.5</td><td>13.45</td></tr><tr><td>Odontella regia</td><td>S</td><td>0</td><td>0</td><td>5.01</td><td>65.91</td></tr><tr><td>Odontella rhombus</td><td>S</td><td>0.2</td><td>0.45</td><td>0</td><td>0.00</td></tr><tr><td>Paralia sulcata</td><td>B</td><td>0</td><td>0</td><td>5.01</td><td>0.31</td></tr><tr><td>Pleurosigma<br>angulatum</td><td>B</td><td>0.01</td><td>0.02</td><td>25.05</td><td>34.24</td></tr><tr><td><strong>Pseudo-nitzschia<br>pungens</strong></td><td>B</td><td>1.44</td><td>0.06</td><td>177.82</td><td>7.47</td></tr><tr><td>Raphoneis<br>amphiceros</td><td>B</td><td>0</td><td>0.00</td><td>2.5</td><td>0.61</td></tr><tr><td>Rhizosolenia<br>imbricata var.<br>shrubsolei</td><td>P</td><td>0.22</td><td>0.11</td><td>26.3</td><td>12.71</td></tr><tr><td>Rhizosolenia<br>setigera f.<br>pungens</td><td>P</td><td>0</td><td>0</td><td>0.01</td><td>0.02</td></tr><tr><td>Ulnaria ulna</td><td>F</td><td>0</td><td>0</td><td>0.2</td><td>0.08</td></tr><tr><td>Synedra<br>nitzschoides</td><td>S</td><td>0</td><td>0</td><td>0.25</td><td>0.2</td></tr><tr><td><strong>Dinoflagellates</strong></td><td></td><td><strong>0</strong></td><td><strong>0</strong></td><td><strong>0.36</strong></td><td><strong>5.5</strong></td></tr><tr><td>Protoperidinium<br>globulum</td><td></td><td>0</td><td>0</td><td>0.36</td><td>5.5</td></tr><tr><td><strong>Ciliates</strong></td><td></td><td><strong>0</strong></td><td><strong>0</strong></td><td><strong>0.01</strong></td><td><strong>0.07</strong></td></tr><tr><td>Acineta compressa</td><td></td><td>0</td><td>0</td><td>0.01</td><td>0.07</td></tr></tbody></table>

C). Respectively 100 % and 88.2 % of the individuals came in contact with the fresh and aged foam ring. In addition, they spent equal time following both fresh and aged seafoam in contact with their shell (i.e., with physical contact) as with their cephalic tentacles (i.e., with sensory contact; $\chi^2$ test: $p = 0.743 \chi_1^2 = 0.107$, Fig. 3), with an alternation between these behaviours. However, they stayed proportionally farther from the foam when it was fresh compared to when it was aged during their activity time ($\chi^2$ test: $p < 0.001 \chi_1^2 = 19.219$; Fig. 3). In addition, the number of physical contacts significantly differs between seafoam and hard wall in control (KW test: $p < 0.001 \chi_2^2 = 22.07$) with Contact$_{Aged} > Contact_{Fresh} > Contact_{Hard\_wall}$ (Dunn test: all $p < 0.05$); i.e. respectively, $3.6 \pm 1.4$; $2.5 \pm 1.5$; $1.2 \pm 0.5$; mean $\pm$ SD.

The tortuosity significantly differs between treatments (KW test: $p = 0.006 \chi_2^2 = 10.2$), with $NGDR_{Fresh} < NGDR_{Aged} = NGDR_{Control}$ (Dunn test:

<figure><img src="image_9.png" /><figcaption>**Fig. 2.** Archetypical example of the movement pathways of *Littorina littorea* in the control experiments conducted in the absence of seafoam (A), and in the presence of fresh (B) and aged (C) seafoam in the experimental arena during 30 min. The dashed circle drawn in the experimental arena for the control experiment provides a benchmark of the inner side of the foam rings. The scale bars represent 2 cm.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 3.** Proportion of time spent (%) by *Littorina littorea* in contact with fresh or aged seafoam, through a shell (in black) or tentacles (in grey) contact or without contact (in white).</figcaption></figure>

$NGDR_{Control-Fresh} = 0.001$, $NGDR_{Control-Aged} = 0.192$, $NGDR_{Fresh-Aged} = 0.049$; Fig. 4A). Significant differences were also observed in *L. littorea* velocity between treatments (KW test: $p = 0.002 \chi_2^2 = 12.58$). Movements were significantly faster (ca. 1.4 times faster) in the control
<figure><img src="image_11.png" /><figcaption>**Fig. 4.** Startle time (min) mean ± SE (A), net-to-gross distance ratio mean ± SE (B) and velocity (cm min⁻¹) mean ± SE (C) between their start of movement and the distance travelled corresponding to the radius of the circle of the foam depending of seafoam exposure. Letters depict significant differences (*p* < 0.05; Dunn's test) among treatments.</figcaption></figure>

experiments than in the presence of either fresh or aged seafoam (Fig. 4B). In addition, in the fresh seafoam treatment, 32.7 % of the individuals changed direction before actually touching the foam, which noticeably never happened in the presence of aged seafoam. Finally, the proportion of snails that stopped moving before the end of the experiment was significantly higher in the presence of fresh and aged seafoam (80 and 72 % respectively; Fig. 5) than in the control (1 %; $\chi^2$ test: *p* < 0.001 $\chi^2 = 42.021$). The stop time is also significantly higher (WMW test: *p* = 0.016 *z* = 2.413) in the presence of fresh seafoam (15.07 ± 4.90 min; mean ± SD) than in the presence of aged one (22.24 ± 3.91 min; mean ± SD), while only one individual stopped in the control after 24.08 min.

# 4. Discussion

## 4.1. Seafoam as a toxin reservoir potentially enhanced by ageing

Beyond the modification in the physical structure of seafoam related to the ageing process, fresh and aged seafoam drastically differ in their phytoplankton content. This observation is consistent with previous observations showing that phytoplankton cells are trapped by and accumulate in foam (Seuront et al., 2006; Seuront and Vincent, 2008;

<figure><img src="image_12.png" /><figcaption>**Fig. 5.** Proportion of *Littorina littorea* individuals (%) being motionless (in white) before the end of the behavioural experiments and still in motion (in black) at the end of behavioural experiments, in the control and in the presence of fresh and aged seafoam.</figcaption></figure>

Roveillo et al., 2020). More specifically, aged seafoam is characterized by the appearance of species typical of the surf zone such as *Attheya armata*, which is by far the dominant species observed in aged seafoam (88.8 %) and two species of the genus Odontella (i.e. *O. mobiliensis* and *O. regia*). The presence of these species, as well as the freshwater diatom *Ulnaria ulna*, further suggests that during the ageing process species occurring in the surf zone seafoam either naturally or through riverine inputs are increasingly retained and accumulated in the foam. Similar conclusions can be drawn from the presence of the dinoflagellates *Protoferidinium globulum* and the ciliates *Acineta compressa* in aged seafoam. During the ageing process when deposited on sand, seafoam also got richer in diatoms of different origins but also in *Pseudo-nitzschia pungens*, a potential producer of domoic acid. Consequently, seafoam could be a toxin reservoir that is increasing with the ageing process in situ.

## 4.2. Seafoam avoidance in *Littorina littorea*

In the absence of seafoam, *Littorina littorea* performed the typical behaviours expected following an encounter with a hard vertical substrate, i.e. either thigmotaxis followed by negative geotaxis or direct negative geotaxis (Moisez and Seuront, 2020; Seuront et al., 2024a, 2024b). In contrast, snails consistently avoided to cross both fresh and aged seafoam (Fig.2B, C). These results points towards an active avoidance of seafoam observed in *Littorina littorea*. This is consistent with recent observations conducted on the congeneric *L. saxatilis* (Seuront et al., 2024a, 2024b). Avoidance behaviour —as the result of numerous complex developmental and physiological functions— is mainly used as a defence mechanism to cope with stress in intertidal organisms (Gamperl and Kurt, 2022) and has been identified as a powerful contaminant marker (Zala and Penn, 2004). Therefore, the observation of foam avoidance suggests that snails are reacting to an inhibitory, repulsive or deterrent stimulus. In addition, the proportion of individuals that stopped moving before the end of the experiment is drastically higher in the presence of seafoam. This is notably not a consequence of the relative confinement of the snails in our experimental foam rings as observations conducted in a more physically confined neutral environment (i.e. a 12 cm diameter Petri dish) *L. littorina* did not stop significantly more than in our control conducted in a 22 cm glass areana (respectively, 12.5 % and 3 % respectively, $\chi^2$ test: *p* = 0.170 $\chi^2_1$ = 1.883; Cazalis-Henry and Seuront, unpublished data). These observations confirms that the observed seafoam avoidance and related decreased motility/activity is directly linked to the presence of seafoam, and not to a putative physical confinement.
### 4.2.1. Seafoam avoidance is driven by air-borne cues

In the presence of fresh seafoam, snails changed direction before coming into contact with the foam. They move also more slowly and their movements became more tortuous in the presence of fresh seafoam. Given the acknowledged very high sensitivity of *L. littorea* to air-borne and water-borne chemical cues (Davies and Knowles, 2001; Seuront and Spilmont, 2015) and the evidence that the congeneric *L. striata* and *L. saxatilis* have been shown to respectively avoid algal chemical defences (Granado and Caballero, 2001) and foam-born cues (Seuront et al., 2024a, 2024b), our observations strongly suggest that *L. littorea* detects foam-borne volatile cues. This hypothesis is consistent with previous evidence that seafoam accumulates and releases potentially harmful or repellent chemical volatiles compounds (Monahan, 1971, 1986; Monahan et al., 1982) and suggests that the negative aphrotaxis recently identified in *L. saxatilis* (Seuront et al., 2024a, 2024b) is also present in *Littorina littorea*. Further work is nevertheless needed to assess the presence of aphrotaxis in other intertidal gastropods as well as subtidal species to assess the generality of this putative new taxis. The presence of *Phaeocystis globosa* in the Eastern Channel during our study period (Breton, unpublished data), also suggests a possible presence of volatile Dimethyl sulfide (DMS; Schoemann et al., 2005; Zhang et al., 2019) in seafoam. Given its volatility, it is possible that DMS was present in a gaseous phase in aged foam, which would explain why the airborne avoidance of aged seafoam was weaker, though chemical analyses need to be conducted to confirm this hypothesis. In addition, the activity time was related to the age of seafoam, i.e. individuals in the presence of fresh sea foam were proportionally less active in the presence of aged seafoam. Taken together, our results are consistent with remote sensing of foam-borne cues, which further suggests that the chemical cues released by fresh seafoam is more deterrent than those released by aged seafoam. This hypothesis is noticeably consistent with the physics of foam ageing. As foam ages, the concentration of volatile substances in the bubbles decreases, owing to the gas permeability of the thin liquid membranes between bubbles, which tends to balance the osmotic pressure of these volatile components between the bubbles and the outside of the foam (Cohen-Addad et al., 2013). This could explain the stronger repulsive effect of fresh foam when compared to aged foam. Therefore, the increase in the tortuosity of the trajectory and a reduction in activity time of *L. littorea* individuals in the presence of fresh seafoam suggest that seafoam accumulation could lead to an inability of gastropods to find refuges to cope with the various stresses of the intertidal zone such as thermal stress or predation (Yamada and Boulding, 1996; Behrens Yamada et al., 1998; Jones and Boulding, 1999; Ansell, 2001; Moisez et al., 2020).

### 4.2.2. Seafoam avoidance is driven by visual and contact cues

Given that *Littorina littorea* movement did not become more tortuous in the presence of aged seafoam, its reduced speed (Fig. 4C) may be influenced by factors other than volatile chemical cues. Indeed, the green colour of aged seafoam may be perceived as a distant obstacle, since a previous study showed that the main reason for olivine avoidance in *L. littorea* was aversion to the light green colour of pure olivine (i. e. positive scototaxis; Flipkens et al., 2024).

Moreover, the abundance of *P. pungens* contained in the aged seafoam was more than two orders of magnitude higher than in the fresh seafoam (i.e. 7.46 µgC L⁻¹ versus 0.06 µgC L⁻¹; Table 1) could also explain the active avoidance of *L. littorea* after coming into contact with aged seafoam. These observations are consistent with previous observations of *L. littorea* systematically avoiding Ficoll gels seeded with environmentally realistic concentration of domoic acid (i.e. in the 10 to 100,000 ng L⁻¹; (Trainer and Bill, 2004; Trainer et al., 2007; Barbaro et al., 2016; McKibben et al., 2017) after probing by the tentacles (Seuront, unpublished data). Further investigations such as the presence of domoic acid as a function of seafoam type are nevertheless required to confirm this hypothesis. The observed foam avoidance may also result from the inability of gastropods to enter the foam due to the presence of

air bubbles, which could potentially obstruct their mantle cavity and disrupt gas exchange with their respiratory cells (Reid, 1996). Finally, factors influencing active avoidance responses may vary or overlap between the two foam types. Taken together, our results highlight the need for deeper insights into seafoam and its potential effects across different stages. However, due to the design of the study—in particular the use of foam collected on two consecutive days—the observed differences cannot be attributed unequivocally to the duration of exposure in the field alone. They could also reflect the influence of other uncontrolled variables, such as temporal variability and environmental conditions, which warrants the need for further investigations to unambiguously decipher the role of ageing in the observed differences in *L. littorea* aversion for fresh and aged seafoam.

### 4.3. The responses of Littorina littorea to seafoam as a soft-surface thigmotactic behaviour

Thigmotaxis is the tendency of animals to move towards (positive thigmotaxis) or away (negative thigmotaxis) from the stimulus induced by a physical contact such as a topographic discontinuity. This behaviour is typically characterized by a displacement along a hard physical discontinuity in a range of intertidal organisms such as the Asian shore crab, *Hemigrapsus sanguineus* (Delaeter et al., 2023), the green crab, *Carcinus maenas* (Burrows et al., 1999; Balci et al., 2014; Chabenat et al., 2021), as well as in cerithiids gastropods (Jethva et al., 2022) and littorinids such as *Littorina littorea* (Moisez and Seuront, 2020) and *L. saxatilis* (Seuront et al., 2024a, 2024b). In the present work, the so-called soft-surface thigmotaxis, i.e. the observed tendency of *L. littorea* to follow the foam surface either through a shell- or a tentacle contact, was twice as frequent in the presence of aged seafoam compared to fresh one (Fig. 3). They spent a similar amount of time following the foam in contact with their shell (i.e., with physical contact) as with their cephalic tentacles (i.e., with sensory contact), with an alternation between these behaviours. However, in the presence of seafoam, snails tend to move away from it and interrupt their thigmotactic behaviour, since we quantified a higher number of contacts with the seafoam than with the hard wall. In this context, our results suggest that gastropods may actually be able to follow soft physical discontinuities such as seafoam, although the presence of a repellent compound in the foam could impede this behaviour. Though speculative, the observed putative soft-surface thigmotaxis being more frequent with aged seafoam (which contains a greater quantity of phytoplankton, i.e. 474-fold higher than fresh seafoam), the resistance of the foam could be strong enough to be perceived by the animal's body. From a purely physical perspective, a foam can be considered as a solid when the applied stress is smaller than the yield stress (Marze and Saint-Jalmes, 2009). Because, the yield stress typically increases when the liquid fraction decreases, it is expected that aged foams might be considered as more solid than fresh ones, hence induce more resistance towards snail movement. In practice, the foam is solid provided that the applied stress is smaller than the yield stress (Marze and Saint-Jalmes, 2009). For a foam with a surface tension ~35 mN/m, a bubble radius ~ 1 mm and a geometrical factor k ~ 0.6, the yield stress is between 1 and 2 Pa depending on the liquid fraction epsilon (taken between 2 and 10 %). Style et al. (2014) showed that the applied stress exerted by a gastropod to move is between 1 and 100 Pa. Consequently, the foam is solid provided that the stress exerted by the gastropod is smaller than the yield stress, which is plausible. The resolution of this specific issue warrants the need for further studies to establish whether sea foam has a higher yield stress than the applied stress of gastropods. From a purely behavioural perspective, the study sensory responses and mechanoreceptor reactions to varying levels of structure softness could offer valuable insights on the differences in behaviour between a hard wall sensu stricto and the surface of aged foam. Alternatively, the presence of more toxic repellent compounds in fresh seafoam could lead to an inhibition of soft-thigmotactic behaviour in the presence of this foam, indicating that snails are able to follow soft
discontinuities with low resistance.

## 5. Conclusion

Taken together, our results unequivocally show that the common periwinkle *Littorina littorea* responds to both direct and indirect signals from the accumulation of naturally formed seafoam in its environment. We have shown that seafoam, particularly when freshly formed, leads to changes in activity and thigmotactic behaviour. More specifically, this study shows that our understanding of the thigmotactic behaviour of gastropods, which has typically been described on the basics of contact with hard surface discontinuities in both marine (*Moisez and Seuront, 2020*; *Jethva et al., 2022*) and in freshwater environments (*Bose et al., 2022*; *Fogliano et al., 2023*) may still be incomplete. Specifically, our observations indicate that *L. littorea* behaviourally respond to surfaces as soft as seafoam, which further suggests the presence of a soft-surface thigmotactic behaviour, although further work is required under laboratory-controlled conditions using biologically neutral artificial foam to support our soft-surface thigmotaxis hypothesis. Our results also show that seafoam accumulation can have significant consequences for species that depend on aerial chemosensory and tactile cues for movement, feeding and mating in rocky intertidal ecosystems where the presence of long-term seafoam is enhanced by strong hydrodynamics and a high organic matter content. The structure and composition of the seafoam also have a significant impact on the gastropod's responses as illustrated by the different responses observed in the presence of fresh and aged seafoam, although these observations warrant the need for further work to decipher the mechanisms behind the differences in phytoplankton composition between fresh and aged seafoam. Further work is also needed to infer the potential involvement of natural and anthropogenic surfactants in the formation of persistent foam, given their known toxicity to aquatic life (*Lechuga et al., 2016*). Finally, our results emphasize the need for extensive interdisciplinary research to delve into the effects of seafoam on organisms inhabiting rocky shores and to assess the extent of its impact and the associated underlying mechanisms. In a era of global change, further work is needed to integrate the study of seafoam with other stressors (e.g. heat waves and flooding), which would yield a more complete understanding of the behavioural responses of intertidal gastropods to their shifting environment. This knowledge is crucial for the conservation and management of these coastal environments of high ecological and economic value, as well as for responding to potential pollution issues linked to seafoam accumulation.

### CRediT authorship contribution statement

**Solène Cazalis-Henry:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Nicolas Spilmont:** Writing – review & editing, Supervision, Resources, Conceptualization. **Elsa Breton:** Writing – review & editing, Data curation. **Florence Elias:** Writing – review & editing, Funding acquisition. **Laurent Seuront:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no know competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was financially supported by the Agence Nationale de la Recherche (ANR) through the project ECUME (ANR-22-06CE-0029). S. Cazalis-Henry is financially supported by a PhD scholarship from the

University of Lille, France.

### Data availability

Data will be made available on request.

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