The nudibranch mollusc Onchidoris bilamellata (L., 1767) is distributed around the British Isles, the Atlantic coast of France, Iceland, Greenland, North America and various locations in the North Pacific (Sea Slug Forum, 2007). It has a primarily intertidal distribution matching that of its major prey species, the barnacle Semibalanus balanoides (L., 1767) (Todd, Reference Todd1979). Spawning occurs between December and April with planktonic settlement, which is induced by S. balanoides presence (Todd, Reference Todd1979), starting in June (Todd, Reference Todd1981). An additional spawning period is sometimes observed mid-summer which has been attributed to rapidly growing June settlers which underwent precocious maturation, spawning and death within around three months of settling (Todd, Reference Todd1981). After spawning, individuals die; this species having a sub-annual life history (Todd, Reference Todd1979).

Onchidoris bilamellata and O. muricata (Müller, 1776) are known to aggregate with the onset of reproduction (Pelseneer, Reference Pelseneer1922). Other evidence of aggregation is anecdotal including several occurrences of O. bilamellata in Scotland, Ireland and California and Stylocheilus striatus (Quoy & Gaimard, 1832) in the Caribbean (Sea Slug Forum, 2007). Disappearance of nudibranch aggregations after spawning has been attributed to post-spawning death rather than offshore migrations (Costello, Reference Costello1938; Miller, Reference Miller1961; Nybakken, Reference Nybakken1978; Sea Slug Forum, 2007).

There are suggestions in the literature that nudibranchs undertake migrations. Nearly all reports address intertidal populations where migrations have been inferred from the sudden appearance and disappearance of littoral populations. These have been attributed to spawning migrations and subsequent adult death (Pelseneer, Reference Pelseneer1922). However, sudden appearances have also been attributed to colonization by new settlers (Miller, Reference Miller1961), tidal/current/wave aggregation (Costello, Reference Costello1938), the emergence of individuals from under rocks onto their upper surfaces, possibly to breed (Nybakken, Reference Nybakken1978) and shoreward movements in response to climate factors including light (Crozier, Reference Crozier1917). Reviews such as that by Nybakken (Reference Nybakken1978) have considered the possibility of migrations but conclude no such migrations occur. A review by Todd (Reference Todd1981), however, does not exclude the possibility of reproductive migration.

Observations on subtidal migrations of nudibranchs are limited. Crozier (Reference Crozier1917) attributed shoreward movements in response to physical conditions, e.g. light. More recently, anecdotal evidence of mass movements has emerged for O. bilamellata both in Ireland and California (Sea Slug Forum, 2007).

We observed subtidal aggregations and mass movements of the nudibranch O. bilamellata on a boulder substratum near Millport, Scotland (055°44.79′N 004°55.49′W) using SCUBA divers during July 2006 (–14 m Chart Datum, temperature 12°C). Initially (5 July 2006), we observed O. bilamellata aggregated on rocks and boulders. Aggregations were associated with small numbers of egg masses (Figure 1). In addition, large numbers of O. bilamellata were observed moving between and within boulders in a uni-directional belt, i.e. all moving in the same direction in belts ~50–100 individuals wide. Also, a small number of individuals were observed moving in single to triple-file lines (similar to Figure 3). Two days later these aggregations had dispersed with some individuals remaining associated with very dense egg masses (Figure 2). The belt movements had also dispersed but individuals were still observed moving in single to triple-file lines (Figure 3) which were heading in a westerly direction towards shallower water. Ten days after the initial observation few individuals were observed. Some were associated with the egg masses while others moved individually on rocky substrata but no mass movements or aggregations were observed. No dead O. bilamellata were observed on any of these occasions. No S. balanoides were observed on any rocks at these (subtidal) depths.

Fig. 1. Aggregated Onchidoris bilamellata associated with a few egg masses (indicated by arrows).

Fig. 2. Dense egg masses of Onchidoris bilamellata associated with a few spawning individuals (indicated by arrows).

Fig. 3. (A) Mass movements undertaken by Onchidoris bilamellata are indicated by the dashed lines within which individuals are visible. The direction of movement is indicated by the arrow; and (B) close-up of black box in (A) indicating O. bilamellata moving in single to triple-file.

Our observations provide photographic evidence of mass movements of O. bilamellata associated with spawning aggregations in a subtidal population. The timing of these aggregations suggest that they are composed of precocious juveniles which settled during June of the same year (see Todd, Reference Todd1981).

We suggest that these mass movements are spawning migrations as: (1) O. bilamellata is a specialist feeder on S. balanoides (Todd, Reference Todd1979) which we did not observe to occur subtidally at this locale, so feeding O. bilamellata populations are likely to be restricted to intertidal areas (Todd, Reference Todd1979); (2) aggregations of nudibranchs have often been accounted for by settlement from the plankton directly onto their food supply followed by rapid growth and appearance as aggregations rather than migrating to a site specifically to spawn (Sea Slug Forum, 2007). However, the absence of S. balanoides in this locale excludes that settlement theory; and (3) the absence of any dead O. bilamellata suggests that these individuals may have migrated back into shallower water following spawning as suggested by the post-spawning westward mass movements. Behaviourally, these mass movements may be explained by mucus ‘trail-following’ (Todd, Reference Todd1979) and/or possible responses to magnetic fields as suggested for Tritonia diomedea (Bergh, 1894) (Lohmann et al., Reference Lohmann, Willows and Pinter1991). However, we acknowledge the possibility that subtidal populations of O. bilamellata may occur and they may be dependent on a food source other than S. balanoides, thus these movements can be attributable to mass movements within a spawning area.

This photographic evidence reveals large spawning aggregations and possible associated spawning migrations of O. bilamellata described by Pelseneer (Reference Pelseneer1922). Pelseneer's (Reference Pelseneer1922) observations were strongly refuted by Thompson (Reference Thompson1984) after a visit to the site 61 years later. However, our observations corroborate the spawning migrations described by Pelseneer (Reference Pelseneer1922) for O. bilamellata, which clearly occupy a very brief period and may not be as pronounced every year.