Limnomysis benedeni is found primarily in shallow (0.5–5 m), near-shore locations, though it has been reported in waters as deep as 33 m (Steinmann 2009). Kelleher et al. (1999) note high densities at depths of 0–0.5 m and a preference for still water. Wittmann (1995) reports a maximum current velocity tolerance of 0.5 m/s, though this species has occasionally been found at higher velocities (1.5 m/s); L. benedeni may tolerate such conditions by maintaining physical contact with the substrate (Wittmann 2007). Under bright conditions, this species prefers habitats with stands or spots of dense submerged vegetation, including macrophytes (especially stonewort), submerged tree roots, and flooded terrestrial weeds (Weish and Türkay 1975; Wittmann 1995; Wittmann et al. 1999; Gergs et al. 2008). Limnomysis benedeni shows great habitat plasticity and will select from a variety of other structured habitats—such as spaces between stones or boulders, stones overgrown by mussels, empty shells, branches of submerged trees, and coarse debris—when sufficient plant cover is unavailable. High densities of mysids have been observed in and on coats of filamentous algae covering concrete walls, and occasionally, individuals are even observed on the bare surface of soft sediment or concrete walls (Wittmann 2007). In shallow habitats, L. benedeni is usually solitary or found in loosely aggregated groups, tending to stay a few centimeters above the substrate or rest directly on it. In the turbid waters of coastal lakes or in deeper waters of clear continental lakes, this species may form dense aggregations of hundreds or thousands of individuals (Wittmann 2009). At night, its distribution becomes more scattered. Part of the population is found in surface waters while others remain at greater depths (i.e., it undergoes vertical diel migration) (Wittmann et al. 1999). Salinity is a primary limiting factor to the distribution of L. benedeni. Most populations occur in freshwater; however, mass occurrences have been observed in coastal and continental lakes with salinities of 0.5–5 PSU (Wittmann 1995, 2007), and a few populations have been reported in habitats with 6–14 PSU (Bacescu 1954; Komarova 1991; Ovcarenko et al. 2006). Wittmann (2009) reports that it prefers a salinity range of 0.1–3 ppt while it is able to tolerate 0–14 ppt. In a laboratory study by Bacescu (1940), a low tolerance was exhibited for salinities above 10 PSU. In contrast, Ovcarenko et al. (2006) report that L. benedeni is largely unaffected by salinity increases up to 19 PSU and shows only about 25% mortality when exposed to 23 PSU for 24 hours. It is not until 24 hours of exposure to 34 PSU that a 100% mortality rate is observed.
This species favors slightly alkaline waters, as mass occurrences are found only in areas with pH >7.7 (Wittmann 2007). Preference for alkaline waters is also suggested by reduced juvenile oxygen consumption at pH 8.4 compared to pH 5.4 (Szalontai et al. 2003). Overall, it is able to tolerate a pH range of 5.5–9.6, with values of 7.3–8.6 comprising its optimal range (Wittmann 2009). The lower oxygen limit tolerated by L. benedeni in freshwater is 3.75 mg/L, which is relatively high among freshwater invertebrates though lower than lethal concentrations demonstrated for other freshwater mysids (Bacescu 1940; Wittmann 2007). Dissolved oxygen concentrations > 5.9 mg/L is reported as its optimal range by Wittmann (2009). Oxygen consumption rates for this species are 0.53 and 3.20 µg O2/mg dw/h at 0°C and 13°C, respectively (Szalontai et al. 2003). Limnomysis benedeni tolerates a wide range of temperature (0–31°C), while ideally inhabiting waters of 10–25°C (Wittmann 2009).
Limnomysis benedeni is primarily microphagous, with a diet consisting of phytoplankton, epilithon, detritus, and biofilms on macrophytes, while animal prey (chironomids, etc.) play a minor role (Dediu 1966; Wittmann and Ariani 2000; Gergs et al. 2008). Like other freshwater mysid species, L. benedeni plays an important role in the diets of fish (Bacescu 1940, 1954; Zhadin and Gerd 1961; Mordukhai-Boltovskoi 1979; Hanselmann et al. 2011; Specziár and Eros 2014).
As in all mysid species, L. benedeni shows strictly sexual reproduction. Eggs are fertilized upon or shortly after deposition in the brood pouch. A single female with fertilized eggs may be sufficient for establishing an entirely new population (Wittmann 2009). Breeding females are found from March/April to October/November (Bacescu 1954), with an overwintering generation reproducing in spring/summer, followed by one or two summer generations reproducing in summer to autumn (Wittmann 1984, 2009) for a maximum of 5 generations per year. The shortest generation time recorded was 6 weeks yielding 5 generations in a year in the Middle Danube River (Borza et al. 2014). Newly hatched individuals undergo two larval stages while within the brood pouch and molt to the fully mobile juvenile stage upon release. The number of eggs or larvae carried in the brood pouch increases with parent body size, with females typically carrying 12–40 eggs on average (range 2–46) (Wittmann and Ariani 2000; Gergs et al. 2008). Additionally, egg numbers and parent body size vary by season, with the spring generation carrying nearly three times the amount of eggs as the summer (Dediu 1965; Gergs et al. 2008). A single female is capable of producing several subsequent egg clutches (Wittmann 2009).