Gregarious behaviour in crustacean micronekton (Euphausiacea, Mysidacea)
Creators: name: O'Brien, DP
Publisher: name: University Of Tasmania
Year: 2023

description: Certain species of pelagic euphausiids and hyperbenthic mysids (Crustacea) show highly integrated social behaviour comparable in many aspects to schooling in higher marine organisms such as fish, squid and mammals. The ethology, form, maintenance, occurrence and population structure of aggregations of three species of euphausiid and six species of mysid are described. An attempt is made to define the adaptive advantages and selective pressures determining the characteristics of social behaviour of marine crustaceans as a whole, between orders, and between species. The general internal structure and dynamics of aggregations were determined using a three-dimensional stereophotographic technique capable of estimating distances between individuals as small as 5 mm in length. Results show that euphausiid and mysid aggregations are similar in overall structure. Within species, schools and swarms showed similar characteristics apart from their degree of polarization. Physical parameters, e.g., current and light, had little effect on internal structure; variation between species could generally be related to differences in habitat selection, i.e., pelagic or hyperbenthic, substrate or non-substrate associated. The degree of behavioural integration was demonstrated in the array of escape responses exhibited by these crustaceans. A scheme is proposed in which the responses are divided into three categories (primary, secondary and tertiary). Manifestation of a particular level of response was dependant upon the level of integration between individuals, the size of the aggregation, and predator size and distance. With increased threat the group response (predominantly primary and secondary forms) was progressively substituted by an individual response (tertiary form). In mysids substrate attraction also influenced the form of response. External morphology of the aggregations varied according to the behavioural state, movement and/or relative substrate attraction. The direction and polarization of light can influence individual alignment and the overall direction of travel in euphausiids. Aggregations of all species were generally derived from a single size/age class, and had a relatively constant sex ratio of 1 : 1. Mysids aggregated throughout the year, with variation in the gregarious habit within a species being determined by such external influences as antipredator responses and/or strong currents, and variation between species determined in the main by habitat selection. A new scheme describing mysid aggregations based on mechanisms influencing their formation and maintenance is proposed. Evidence for the persistence of social behaviour throughout the year in euphausiids is also presented, with variation within species being related to behavioural influences such as food availability, reproduction, maintenance of contact with conspecifics and possibly maturation in sub-adults. Stranding behaviour in the local species of euphausiid, Nyctiphanes australls, is shown to occur on a regular seasonal basis, and I provide evidence for the hypothesis that the swarm migrations associated with stranding events are probably related to reproductive facilitation. A new form of aggregative behaviour in euphausiids, termed matting, is described which occurs synchronously with stranding behaviour. The high level of integration and absence of strong passive mechanisms in the formation of aggregations, together with the relatively small influence of physical parameters on internal structure suggest that micronektonic crustacean aggregations are formed in response to internal biological mechanisms. Crustaceans demonstrate the ability to adapt their social behaviour in response to variation in both intrinsic and extrinsic factors. The selective pressures determining the form of aggregative behaviour in crustaceans are generally similar for the species studied.