Population ecology, vertical migration and feeding of the Ponto‐Caspian invader Hemimysis anomala in a gravel‐pit lake connected to the River Rhine

Abstract

Summary 1. The Ponto‐Caspian invader, Hemimysis anomala, was recently found in large numbers in a gravel‐pit lake connected to the Lower Rhine. Mysids were sampled with Perspex traps between September 2002 and April 2003 to study the population dynamics, vertical migration and feeding. 2. The abundance (as catch per unit effort, CPUE) of H. anomala declined from 270 individuals (ind.) trap−1 (4 h)−1 in December to below 4 ind. trap−1 (4 h)−1 in April. Average lengths ranged from 4.9 mm in autumn to 9.9 mm in March. The length–weight relationship of virgin females changed throughout the sampling period, from a size‐corrected wet weight of 5.1 mg in September to 16.6 mg in April for a female of 7.8 mm. 3. Successive monthly samples taken over 24 h revealed that H. anomala preferred the surface at twilight and night. During dawn the mysids migrated to the middle and bottom layers and were hardly found during day. Diel vertical migration clearly depends on the proximate factor light. 4. Stomach analyses revealed that larger H. anomala preferred zooplankton, whereas small individuals fed more on phytoplankton. The seasonal comparison showed an increasing percentage of zooplankton with increasing length. The proportion of zooplankton in the stomachs of large H. anomala individuals was highest during night and lowest during day. 5. The results suggest that H. anomala may become an important link between primary/secondary production and higher trophic levels in the food web of its new environments at the Lower Rhine.

Benthic macroinvertebrates along the Czech part of the Labe and lower section of the Vltava rivers from 1996–2005, with a particular focus on rare and alien species

“…From there it invaded in-flowing rivers and spread into continental Europe. It poses a threat rather to plankton (Ketelaars et al 1999;Borcherding et al 2006) than to macrozoobenthos, as the species acts like nekton during the night. However, H. anomala so far occurs in Central Europe only in river ecosystems, and no influence has yet been demonstrated (Petrusek 2006).…”

The First Occurrence of the Ponto-Caspian Invader, Hemimysis Anomala G. O. Sars, 1907 (Mysidacea) in the U.K.

“…These findings, including initial records from the current distribution survey, were recently reported by Holdich et al (2006). Whilst invasive populations of H. anomala are widely distributed across Europe, current observations are scattered, reflecting H. anomala's concealed daytime behaviour and thus its preclusion from standard sampling methods (Ketelaars et al, various techniques involving diving (Salemaa & Hietalahti, 1993;Dumont, 2006), hand nets (Verslycke et al, 2000), vertical plankton hauls (Ketelaars et al, 1999) and trapping (Borcherding et al, 2006). However, none of these methods are appropriate for conducting a large-scale distribution study of H. anomala in shallow water habitats.…”

“…Ketelaars et al (1999), for example, compared H. anomala's positioning in the water column at 10.00 hrs and 22.00 hrs, finding it exclusively at depths >7.5 m during daylight, whilst at night some individuals dispersed into waters of <2.5 m deep. DVM can be accompanied by horizontal dispersal of swarms into open water after dark (Borcherding et al, 2006). Mysids including H. anomala are sensitive to elevated light levels (Lindstrom, 2000), indicating that diurnal migrations are a negatively phototaxic response that has evolved to reduce predation by fish (e.g., Naesjeetal., 2003).…”

“…Consideration of this regeneration period alongside the species' mean brood size, determined as 20-29 eggs or larvae per female (Borcherding et al, 2006), indicates that an introduced H. anomala population could reach high densities within 3 years. As an adaptable omnivore with a high feeding rate (Ketelaars et al, 1999), populations that attain high densities may have profound effects on the receiving ecosystem (Ketelaars et al, 1999;Borcherding et al, 2006).…”

“…H. anomala is a euryhaline species, native to both brackish and fresh waters (Mauchline, 1980), and as such the salinity of an invaded ecosystem offers no barrier to its colonization (Pienimaki & Leppakoski, 2004). H. anomala is thought to produce two or three generations annually (Borcherding et al, 2006). Consideration of this regeneration period alongside the species' mean brood size, determined as 20-29 eggs or larvae per female (Borcherding et al, 2006), indicates that an introduced H. anomala population could reach high densities within 3 years.…”