Electronic supplementary material 1 – Mysid Shrimp Behaviour Experiments
Introduction
The behavioural response of the prey, mysid shrimp (Tenagomysis spp.) used in the common bully experiments to light and dark and perch odour could influence the feeding success and behaviour of the common bullies. While no obvious differences in behaviour were noted, research on mysid shrimp species does suggests the possibility of an effect of predator cues. Lindén 2007 documented anti-predator behaviour of another species of mysid shrimp (Neomysis integer) in response to the presence of European perch. Two other species of pelagic mysids (Mysis mixta and Mysis relicta) have been shown to decrease feeding rates and alter prey selection in response to a chemical cue of herring (Clupea harengus membras) (Lehtiniemi and Lindén 2006). Observations of the mysid shrimp species in our research have also shown it to be a predominantly nocturnal species (Sutherland and Closs 2001; Larkin 2005; Lill 2005) suggesting a possible difference between activity levels in light and dark experimental periods. To confirm the results of the common bully feeding experiment, a study of the location of mysid shrimp in the water column in response to perch odour, as well as light and dark conditions was completed.
Methods
The same experimental aquaria used with the common bully experiments detailed above with the same aeration setups were used. A black line was drawn onto the sides of the aquaria prior to experimental trials 10 cm above the bottom of the tank. After a minimum of one week in a holding tank with 13ppt saltwater and ‘Speight’s’ water solution 10 mysid shrimp were placed into each of six aquaria filled to a depth of 20cm with the 13ppt saltwater and ‘Speight’s’ water solution. The odour treatment consisted of the addition of 100mL of perch odour water to each aquarium and the control treatment consisted of the addition of 100mL of ‘Speight’s’ spring water to each aquarium immediately following the addition of the shrimp. Two experimental periods were run for two hours in light and then an additional two hours of dark. Two more experimental periods were run with the opposite light/dark cycle - starting in dark and then ending in light. Spot observations of the presence of mysid shrimp above or below the 10cm line were made at the start of the experimental period, 5 min, 10 min and every 20 min thereafter for the 2 h period of the light period. This was repeated for the following alternate light/dark condition. Two desk lamps with 60-watt red bulbs, placed approximately 2 m from the aquaria were used to make observations in the dark. They were turned on only long enough to make spot observations. For statistical analysis of the results we used generalized linear mixed model (GLMMs), using the glmmPQL function in the R package MASS (Venable and Ripley 2002). Our GLMM (logit-link with quasi-binomial family) had the shrimp positions (above or below) as the response, the treatments (control or perch odour), the diel effect (light or dark) as fixed terms and shrimp groups a random factor (this is to account for repeated observations of the same group of shrimp).
Results
No statistically significant effects of light or dark, or perch odour on the location of mysid shrimp within the water column were observed (Table AI).
Table AI Results of GLMM analysis on the location of mysid shrimp in the water column of the aquaria (above or below 10cm) when exposed to perch odour or distilled water (treatment) during light and dark (time) replicates.
Response / Factor / Value / SE / d.f. / t / PLocation in water column / Intercept / -1.3795 / 0.1003 / 358 / -13.7522 / <0.0001
Treatment (odour) / 0.2813 / 0.1374 / 22 / 2.0470 / 0.0528
Diel Period (dark) / -0.0796 / 0.1137 / 358 / -0.7006 / 0.4840
Treatment x Diel Period / -0.1190 / 0.1535 / 358 / -0.7756 / 0.4385
The treatment effect here is odour and the diel period effect is dark, giving the effect of odour on location in the water column, and the difference in the location in the water column between light and dark periods.
A slightly insignificant effect of perch odour on the location of the shrimp in the water column was observed (GLMM analysis; d.f. = 22, P = 0.0528). However, this is shown to be biologically inconsequential by an average total difference of less than 1 shrimp observed above or below the 10cm line between light and dark, and treatment replicates (Fig.AI)
Fig.AI Average ± S.E. number of shrimp observed in the bottom of the aquaria (below 10cm) and in the top of the aquaria (above 10cm). (n = 96 for each replicate type – control, light; control, dark; perch odour, light; and perch odour, dark)
Discussion
The results show no significant effect of the treatments (perch odour and control) or light and dark on the location of mysid shrimp within the water column (above or below 10cm) in the aquaria. This, therefore, does not suggest any influence of mysid shrimp behaviour on common bully feeding behaviour or the use of cover in our experiments in the presence or absence of perch odour, or light and dark conditions. This finding is supported by Sutherland and Closs (2001) which found no significant difference in diel drift of this same genus of mysid shrimp. While other research has shown changes in behaviour of various mysid shrimp species in response to predatory cues, it is possible that mysid shrimp, or at least Tenagomysis spp. do not change their position in the water column as a response. Other observations of behaviours were not made as location in the water column was the key consideration for an effect on bullybehaviour. It may also be that they require a visual cue to trigger any significant behavioural response to a predator. Lindén et al. (2003) documented a response of two littoral mysid species (Neomysis integer and Praunus flexuosus) to European perch only when both chemical and visual signals of perch were present. This study not only helps to confirm the findings of the common bully feeding experiments, but also gives a better understanding of the behaviour of Tenagomysis spp.
References
Larkin GJA (2005) Hypoxia in the Kaikorai Estuary: dynamics, causes and biological impacts: Unpublished MSc thesis, University of Otago, Dunedin, New Zealand
Lehtiniemi M, Lindén E(2006) Cercopagis pengoi and Mysis spp. alter their feeding rate and prey selection under predation risk of herring (Clupea harengus membras). Mar Biol 149: 845-854
Lill AWT (2005) The distribution, life history and production of mysid shrimp (Mysidacea mysidae) in East Otago estuarine systems, in particular focusing on temporal open estuaries. Unpublished MSc thesis, University of Otago, Dunedin, New Zealand
Lindén E (2007) The more the merrier: Swarming as and antipredator strategy in the Mysid Neomysis Integer. Aqua Ecol 41, 2
Lindén E, Lehtiniemi M, Viitasalo M (2003) Predator avoidance behaviour of Baltic littoral mysids Neomysis integer and Praunus flexuosus. Mar Biol 143, 5
Sutherland DL, Closs GP (2001) Diel patterns of mysid drift (Crustacea mysidacea) in the Taieri River estuary, New Zealand. NZ J Mar Freshw Res 35: 197-200
Venable WN, Ripley BD (2002)Statistics and Computing, Modern Applied Statistics with S. Fourth Edition. Springer – Verlag, New York