Hemimysis anomala
Recommended Sampling Protocols
The Ponto-Caspian mysid shrimp, Hemimysis anomala is a small crustacean (maximum size 11 mm) that has been spreading through Europe over the past several decades and has now been reported in the Great Lakes (Pothoven et al. in press). Breeding populations were discovered in Lake Ontario in May 2006, and in Lake Michigan in November 2006. Given patterns of expansion through the canals, reservoirs, inland lakes, and river systems in Europe, a similar expansion is expected in North America. This species has the potential to cause serious ecological changes (Ketelaars et al. 1999), and therefore a systematic sampling program is needed to define current distributions, track the rate of dispersal, identify vectors of spread, and to document preferred habitats. A fundamental aspect of an effective sampling program for Hemimysis is the employment of techniques that provide the best chance for collection. Such a sampling program emphasizes that, at this early stage in the invasion process, results that record its absence are as important as results that record its presence.
Hemimysis has life habits that make it a difficult organism to sample. It migrates both vertically and horizontally, occurring near the bottom in the day and dispersing through the water column at night. Being a mostly nearshore species that prefers bottom structure (rocks, boulders, shells), it lies hidden in close proximity to these structures during the day. An oddity is that, during the day, Hemimysis has also been observed in near-surface swarms in the shadows of breakwalls, jetties, and piers. The discovery of the Lake Michigan population occurred when a swarm was observed near a channel breakwall. However, it cannot be assumed that the species is absent from a particular locality if a swarm is not observed.
Despite these sampling challenges, several methods/techniques can be deployed to successfully collect Hemimysis. All methods have inherent advantages and disadvantages depending on habitat sampled and sampling objective. Vertically-hauled plankton nets have been successfully used to collect Hemimysis, but nets should have an opening diameter of at least 0.5 m and minimum mesh size of 0.5 mm (Ketelaars et al. 1999, Ketelaars personal communication). Sampling should occur at night, or at twilight/dusk when daylight is at a minimum (Borcherding et al. 2006). The net should be allowed to rest on the bottom for several minutes before it is retrieved, preferably at a rapid rate. Hemimysis is a strong swimmer and can avoid nets that are too small in diameter, or are retrieved too slowly. Abrupt disturbance, such as that caused by the preceding pressure wave of a plankton net, can induce rapid escape behavior. This net-sampling method has been used most frequently in open waters, but it can also be used in shallow water near shoreline structures. By determining the vertical distance of the haul (depth of water column), abundance per unit volume can be determined. Night-time vertical hauls with large diameter nets is the standard method used in the Great Lakes to collect the indigenous mysid, Mysis relicta (referred to as Mysis diluviana in some recent literature).
The population in Lake Ontario was incidentally collected in a trawl routinely used to sample larval fish. The trawl had a mouth opening of 1m2 and a net mesh size of 0.5 mm. The trawl was being towed near the surface, at night, and in water 3-7 m deep over a rocky bottom. Such towed devices have a distinct advantage over vertical net hauls because a large volume of water can be sampled in a short period of time, thereby diminishing the consequences of patchy distributions, and increasing spatial coverage. The disadvantage is that they cannot be effectively used in shallow waters near shoreline structures, and that they require a suitably large sampling platform.
Traps have also been successfully used to collect Hemimysis (Borcherding et al. 2006, Odenwald et al. 2005, Whitmann unpublished manuscript) and; in fact, traps are more effective at collecting Hemimysis as compared to other mysid species (Whitmann unpublished manuscript). Traps are placed overnight on the bottom, and thus this method takes advantage of the strong, nocturnal swimming behavior of this species. Traps are designed such that they are easy for individuals to enter, but difficult to escape. Some include modifications to steer swimming individuals to trap entrances, or include a bait (common fish food) to attract individuals to the trap. The advantage is that such traps can be deployed with minimum effort and resources, but a disadvantage is that traps cannot be easily deployed in open waters. Higher trapping efficiencies are reported for transparent traps, which may indicate that Hemimysis is capable of avoiding dark colored traps. A simple trap can be made using two clear 2L pop/soda bottles. We make ours by cutting the top end of one bottle off just below the point where the curved neck flattens out into the main body of the bottle (piece 1). The bottom of the second bottle is cut off just above where the main body curves into the bottom (piece 2). Piece 1, with the cap removed, is glued into the bottom end of piece 2 with silicone household glue or aquarium cement (Pictures 1 and 2). Plastic cable ties are placed around the trap at two positions and cemented in place with the silicone cement, and small-diameter mooring line is attached through the cable ties (Picture 3). The mooring harness is weighted and adjusted to suspend the trap in a horizontal position about 0.75-1 m off the bottom (Picture 4). One or two small fish food pellets are added as bait and the trap is deployed at least overnight.
Since Hemimysis is common along shoreline structures such as breakwalls, rock jetties, and piers, a long-handled dip-net can be used for collection (Whitmann unpublished manuscript). The net is rapidly moved along the bottom around and within the crevices of these structural features. Likely the best time for collection using this technique is during the day when individuals are aggregated on the bottom and in close proximity to these features. Another Ponto-Caspian invader with an affinity for hard substrates, Echinogammarus ischnus, was successfully collected using this method (Nalepa et al. 2000). However, while the method is simple, it can only be deployed in limited types of habitat.
Overall, probably the most versatile method for collecting Hemimysis is by vertical net haul. Samples can be collected in both nearshore and more open waters, the method is simple and direct, it can provide both qualitative (presence/absence) and quantitative data, and can be accomplished with a minimum amount of resources. At this early stage in the invasion process when emphasis in on presence/absence, the trap and dip-net methods can be useful as supplemental techniques in certain habitats or when convenience warrants.
References
Borcherding, J., Murawski, S., and Arndt, H. 2006. Population ecology, vertical migration and feeding of the Ponto-Caspian invader Hemimysis anomala in a gravel pit lake connected to the River Rhine. Freshwater Biology 51: 2376-2387.
Ketelaars, H. A. M., Lambregts-van de Clundert, F. E., Carpentier, C. J., Wagenvoort, A. J., and Hoogenboezem, W. 1999. Ecological effects of the mass occurrence of the Ponto-Caspian invader, Hemimysis anomala G. O. Sars, 1907 (Crustacea: Mysidacea), in a freshwater storage reservoir in the Netherlands, with notes on its autecology and new records. Hydrobiologia 394:233-248.
Odenwald, C., Krug, K., Grabow, K., Martens, A. 2005. Eine reusenfalle zum nachweis von Hemimysis anomala (Crustacea: Mysidacea) [A trap for recording Hemimysis anomala (Crustacea: Mysidacea)]. Lauterbornia 55: 97-105.
Nalepa, T. F., Schloesser, D. W., Pothoven, S. A., Hondorp, S. A., Fanslow, D. L., Tuchman, M. L., and Fleischer, G. W. 2001. First finding of the amphipod Echinogammarus ischnus and the mussel Dreissena bugensis in Lake Michigan. J. Great Lakes Res. 384-391.
Pothoven, S. A., Grigorvich, I. A., Fahnenstiel, G. L., and Balcer, M. D. 2007. Introduction of the Ponto-Caspian bloody-red mysid Hemimysis anomala into the Lake Michigan basin. Journal of Great Lakes Research. 33: 285-292.
Whittman, K. J. Continued massive invasion of Mysidae in the Rhine and Danube River systems, with first records of the order Mysidaea (Crustacea: Malacostraca:Perarida) for Switzerland. (unpublished manuscript).