Thermocyclops hyalinus, Mesocyclops hyalinus Rehberg 1880

Similar to the native cyclopoid copepod Mesocyclops edax, T. crassus is slightly smaller and can be distinguished by the lack of hairs on the medial surfaces of the caudal rami. Thermocyclops crassus’ longest caudal setae (medial median terminal caudal seta) are usually recurved ventrally at the tips in adult individuals, whereas M. edax has relatively straight setae. Thermocyclops crassus has a very narrow and smoothly edged hyaline membrane on antennule segment 17, while in M. edax this membrane is wider and coarsely serrate. The coupler of swimming leg 4 in T. crassus has  two large protrusions ornamented with 4-6 spines while in M. edax the fourth leg coupler margin has two small triangular, unornamented protrusions. In Thermocyclops crassus, leg 4 endopodite article 3 with its lateral terminal spine is less than half the length of the medial terminal spine. In contrast the fourth leg, third endopodite segment of M. edax is characterized by a terminal spine similar in length to its lateral spine. Leg five (P5) of T. crassus has spines that are approximately equal in length and extend beyond the midpoint of the genital double somite. The inner spine of T. crassus is inserted distally, which differs from M. edax whose inner spine is inserted toward the middle of the distal segment (Connolly et al. 2017).

Thermocyclops crassus is generally regarded to be omnivorous with a preference for herbaceous prey, although evidence suggests that diets may vary among separate populations in different environments. T. crassus has a fine-mesh food collection grid formed by setae and setules that allow this species to feed effectively enough on nanoplankton to subsist on a herbivorous diet (Hopp et al. 1997; Hopp and Maier 2005). Moriarty et al. (1973) described a population in Lake George, Uganda that is primarily herbivorous throughout its life and other studies have observed T. crassus to feed mainly on diatoms, cryptomonads, and cyanophyceans (Duchovnay et al. 1992). This species typically thrives in mesotrophic and eutrophic waters (Duchovnay et al. 1992) and is capable of feeding on cyanobacteria—particularly Microcystis, which is a substantial food source for T. crassus in Lake George (Moriarty et al. 1973; Haney 1987). The flexibility in this species feeding behavior might help to explain its broad distribution in temperate and tropical regions throughout the world.

This species’ life cycle appears to be dictated predominantly by water temperature. Thermocyclops crassus bury themselves in the mud and enter diapause during or before winter when water temperature is 15-17 °C and day length is around 14-15 hours (Maier 1989a, 1990).  Emergence typically occurs in April when water temperature is 9.5-14 °C and day length is 11-13 hours (Maier 1989a). Egg production occurs when water temperature is >10 °C, which typically is in April or May and lasts until October in temperate regions (Maier 1988, 1989b; Kobari and Ban 1998). In Turkey, egg production reached a maximum of 26 eggs in June (Bozkurt and Can 2014), but Maier (1989a) reported that females in the Gronne, a shallow eutrophic lake in Southern Germany, carried 18 to 32 eggs on average and produced 3 generations per year between April and October. At tropical temperatures, maturation time can be as short as a few weeks with multiple generations per year, although in temperate waters, T. crassus may have as few as two generations per year (Duchovnay et al. 1992). At 12 °C development time between copepodite stages ranges from 4-8 days. At temperatures greater than 12 °C and up to 18 °C, development times ranges from 3-6 days (Maier 1989a).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental

Potential:

In Lake George, Uganda this species owes its dominance in the zooplankton community to its ability to raptorially-feed on Microcystis (Moriarty et al. 1973). Microcystis blooms in the Great Lakes may give this species an advantage over native species that are incapable of feeding on Microcystis. However, in Lake Erie Thermocyclops crassus is much less prevalent than the most similar copepod species, Mesocyclops edax (EPA 2016; Connolly et al. 2017), suggesting that Microcystis has not yet facilitated dominance by T. crassus.

The diets and habitats of M. edax and T. crassus likely overlap with each other and both species have similar seasonal life cycles. However, in Germany T. crassus is known to coexist with other cyclopoids such as Mesocyclops leuckarti, which is closely related to M. edax. In warm years, T. crassus was more abundant than M. leuckarti in the Gronne and in some eutrophic environments this species has outcompeted and replaced other Thermocyclops spp. (Dumont 1965; Maier 1989a). It is not certain if T. crassus could outcompete M. edax and other zooplankton in the Great Lakes but the evidence suggests that this species is more competitive in warm, eutrophic waters. Therefore, we conclude T. crassus likely will not displace M. edax or other zooplankton in the Great Lakes, but rising temperatures associated with climate change may benefit this species and confer it a competitive advantage over native copepods in nearshore, productive embayments.

In the Gronne, egg production and instar duration times of T. crassus did not give this species a competitive advantage over Cyclops vicinus and M. leuckarti. However, T. crassus was found to have a competitive advantage over C. vicinus and M. leuckarti in situations with high fish predation (Maier 1989a).

Reid and Pinto-Coelho (1994) outlined various intercontinental copepod introductions and concluded that the ecological impacts of these introductions are often difficult to determine. While in some rare cases introduced exotic copepod species appeared to displace native copepod species. In most documented exotic copepod introductions to the western hemisphere no impacts on native copepod species could be directly attributed to the introduced species.

There is little or no evidence to support that Thermocyclops crassus has significant socio-economic impacts in the Great Lakes.

Potential:

Zooplankton grazing on Microcystis can recycle nutrients that help sustain the biomass of a Microcystis bloom (Paerl and Otten 2013). Thermocyclops crassus is known to graze on Microcystis (Moriarty et al. 1973) suggesting that this species could help sustain HABs. However, there is no indication in the literature that this species significantly impacts the sustainability of a bloom.

There is little or no evidence to support that Thermocyclops crassus has significant beneficial effects in the Great Lakes.

Potential:

Cyclopoid copepods have been found to be effective mosquito control agents in several cases (Marten et al. 1994; Nam et al. 1998). Several different species of Mesocyclops and Thermocyclops crassus were used to control the mosquito Aedes aegypti—the principal vector in the transmission of dengue fever—in a Vietnamese village. Within the first 12 months the copepod-treated village had 30–97% less mosquito larvae than the control village. The researchers employed a community-based approach that had community members recycle to eliminate unused and discarded containers that collected rainwater and provided breeding habitat for mosquitoes that were not treated with Mesocyclops or Thermocyclops. The use of cyclopoid copepods in combination with community recycling completely eradicated the mosquito from the village within 18 months (Nam et al. 1998).

Use of T. crassus for mosquito control would be unwarranted in the Great Lakes region as native cyclopoids (e.g. M. edax) would be the more appropriate species to use for biocontrol.

Copepods are ideal and adequate food for fish larvae in aquaculture facilities. However, another species of copepod M. aspericornis was found to be more nutritional than T. hyalinus (synonym of T. crassus) (Vidhya et al. 2014).

There are no known regulations for this species.

Note: Check federal, state, and local regulations for the most up-to-date information.

Control

Biological

There is a variety of pelagic planktivorous fish in the Great Lakes that likely feed on T. crassus including Alewife, Emerald Shiner, and Rainbow Smelt. Additionally, predacious zooplankton such as Diacyclops thomasi, Limnocalanus macrurus, and Bythotrephes longimanus will likely prey on T. crassus. Information specific to the biological control of T. crassus in the Great Lakes is not currently present in the literature, but recent trends in the zooplankton community of the Great Lakes may reveal potential factors that might limit the proliferation of T. crassus. The increase in the biomass of the predaceous Spiny waterflea (B. longimanus) is partly responsible for the crash of Great Lake cyclopoids. The increase in water clarity due to dreissenid mussels has improved the hunting efficiency of B. longimanus, a visual predator. Additionally, the phenology of T. crassus likely increases their vulnerability to B. longimanus. Thermocyclops crassus breeds during the summer to autumn months (Maier 1989a), which directly overlaps with peak abundances of B. longimanus (Vanderploeg et al. 2012). Furthermore, smaller copepods, like T. crassus, are particularly vulnerable to predation since escape speed is a function of prey size (Link 1996). However, Maier (1989a) found that T. crassus will seek refuge in the presence of predators, which may mitigate predation impacts. Overall, these trends suggest that the Great Lakes may not be a favorable environment for introduced cyclopoids based on recent trends of native cyclopoid populations.

As of August 2016 in Lake Erie’s western basin, Thermocyclops crassus is found at a rate of 23.7 individuals/m3 of water (Connolly et al. 2017), whereas M. edax is found at  ~1000 individuals/m3 (EPA 2016). At its current population densities, the probability of male-female encounters may be too low to support a sustainable population. Kramer et al. (2008) determined mate limitation was the likely mechanism that prevented a sexually reproducing calanoid diaptomid copepod from successfully recolonizing lakes when stocked at densities less than 2.6 individuals/m3. There is currently not enough information in the literature regarding a critical population density required for sustainable T. crassus populations, although it is possible that the low densities of T. crassus in the Great Lakes may limit the number of mate encounters and thus limit its proliferation. However, recent evidence of basin-wide increases in T. crassus density and range expansion in Lake Erie suggests that neither allee effects nor predation are limiting population growth (Connolly et al. 2017).


Physical

Thermocyclops crassus develops slower in colder temperatures (15 °C or less) (Maier 1989a). The thermal regime of the Great Lakes may confine this species’ range to warmer, nearshore waters.

Chemical

The Great Lakes and Mississippi River Interbasin Study (GLMRIS 2012) suggests that alteration of water quality using carbon dioxide, ozone, nitrogen, and/or sodium thiosulfate could be effective in preventing upstream and downstream movement of copepods. It should be noted that the effectiveness of these methods is likely significantly diminished against copepod ephippia.


Other

From 1994 to 2008, the percent composition of cyclopoids decreased from 11.6 to 0.1% in Lake Michigan. The synergistic effect of resource limitation, predation, and indirect effects of mussels mediated through light likely explains this crash (Vanderploeg et al. 2012). These factors may also limit population densities of species who prefer meso- and eutrophic conditions, such as T. crassus.



Note: Check state and local regulations for the most up-to-date information regarding permits for pesticide/herbicide/piscicide/insecticide use.

Thermocyclops crassus was initially found in the western basin of Lake Erie in August 2014 during the U.S. EPA Great Lakes National Program Office (GLNPO)’s Great Lakes long-term biological monitoring program. From 2014 to 2016, basin-wide densities of T. crassus increased in the western basin from 0.6 individuals/m3  to 23.7 individuals/m3. The highest density was observed in the southeastern portion of the western basin in 2016 where T. crassus were detected at 70.8 individuals/m3. While the western basin population of T. crassus is growing, this species exists in low densities relative to the more common copepods M. edax and Leptodiaptomus sicilioides, which averaged 2000 individuals/m3. However, increasing densities in the eastern part of the western basin suggests that this species’ range is expanding and approaching the currently uncolonized central basin. Additionally, females with egg sacs or spermatophores were found in 2014 and 2016 indicating that this is an established breeding population (Connolly et al. 2017).