Asellus aquaticus, also known as the Water louse, Water shrimp, Water slate, or Aquatic sowbug is a species of small, freshwater crustacean in the family Asellidae. They look like something in between woodlice and cockroaches, which they are actually related to.
These small crustaceans are pretty hardy creatures. They can live in extremely polluted, acidic, or alkaline waters, water with low oxygen, and even brackish conditions.
Asellus aquaticus can be introduced into home tanks along with live food and can be found inhabiting small crevices and crawling on the substrate.
So, if you found them in your tank, or simply want to know more about Asellus aquaticus, in this article, I will provide a comprehensive overview of mosquito larvae including their habitat, lifecycle, and more.
Distribution of Asellus Aquaticus
Asellus aquaticus is one of the most common freshwater crustaceans and is widely distributed throughout Europe and North America.
This species is commonly associated with a temperate climate, it has been recorded as far south as the Mediterranean, and as far north as Scandinavia
Note: It is hypothesized that Asellus aquaticus arrived in Europe around 8 to 12 million years ago through the brackish Paratethys basin.
Habitat of Asellus Aquaticus
High densities of Asellus aquaticus can be found in stagnant or slow-flowing waters with organic pollution, especially in the vicinity of sewage outfalls. However, they can be also found in relatively clean waters in areas with high levels of naturally occurring organic matter.
Asellus aquaticus is eurybiont.
These crustaceans are highly adaptable and capable of thriving in a wide range of environmental conditions including slightly brackish surface waters, as well as in subterranean freshwater habitats.
The species is absent from marine waters and from fast-flowing streams and rivers, so seas and high mountain ranges are reported as significant barriers to its dispersal.
Description of Asellus Aquaticus
- Size: Water Louse is a small aquatic crustacean that typically ranges from 0.2 – 0.4 inches (5 – 10 mm) in length, with males being slightly larger than females.
Note: Despite some scientific sources stating that this species can grow to an impressive 0.8 inches (2 cm), this is more of an exception than a rule. The size of Asellus aquaticus is also slightly related to the environment in which it lives, as individuals from clean and cold water sites tend to be larger than those from warm and polluted sites. - Body Shape: They have flattened, segmented, and relatively elongated shapes with a series of overlapping plates.
- Color: Their body may exhibit shades of brown or gray, sometimes with mottled or speckled patterns. Their coloration can vary depending on environmental factors, for example, cave forms show a decrease in the degree of body and eye pigmentation.
Note: Water Louse has evolved cryptic pigmentation presumably as a consequence of visual predation, whereby close matching of isopod body color and microhabitat background color has been found - Segments: The body consists of 5 cephalic (head), 8 thoracic, and 7 abdominal segments. The segments are arranged in a linear fashion along the body.
- Telson: The last segment of the abdomen, often ending in a pointed or rounded tail-like structure called the telson.
- Eyes: They have two pairs of compound eyes, which are often quite small and not easily visible. The eyes of the water louse are sessile (stationary).
- Antennae: Possesses a pair of long, segmented antennae that are used for sensory perception and feeding.
- Mouth. The upper lip of the water louse is small, and the mandibles are strong, with two projections on their inner side – a basal one, with a ridged surface for chewing, and a distal one, with sharp teeth used by the louse to bite and gnaw its food.
- Appendages: Equipped with seven pairs of walking legs, each with several segments. The thoracic legs are arranged in such a way that the first two pairs are directed forward, the legs of the third pair are directed to the sides, and the three last pairs of legs are directed backward.
| Asellus aquaticus belongs to the Isopoda, which contains more than 10,000 species with extensive intraspecific morphological variation, sexual dimorphism, sequential hermaphroditism, and with a global distribution. |
Lifespan of Asellus Aquaticus
On average, their lifespan is about 1 year. However, depending on temperature and diet, it can be slightly longer (up to 1.5 years).
Typical Behavior of Asellus Aquaticus
The water louse moves slowly using its legs in a rhythmic, crawling motion along the substrate or decaying plant debris.
They are fully aquatic animals. The water louse is practically helpless in the air, as its long and slender limbs are incapable of supporting its body weight in the air.
They do not swim. However, they can use their appendages to propel themselves through the water. In reality, it looks more like a jerky and hopping motion that only looks like swimming.
This species has two defensive mechanisms, they can either survive by remaining motionless, making them difficult to notice, or use autotomy (when captured, the animal can easily shed its limbs, which subsequently regenerate).
In dry seasons, Asellus aquaticus buries itself in the mud and enters a state of torpor until the rains return.
Diet of Asellus Aquaticus
Asellus aquaticus is an efficient detritivore with a broad spectrum of diets. This species is known to feed on leaf litter, biofilm, microorganisms, fungi, algae, etc.
Although these crustaceans are excellent scavengers, they still have some preferences.
Their primary food source is decaying vegetation, which positions them as major contributors to nutrient and biomass recycling within their ecosystem. It also serves as a shelter from predators such as fish, aquatic beetles, their larvae, nymphs of dragonflies and damselflies, and amphibians like newts, axolotls, frogs, etc.
Additionally, if we compare other food sources to each other, according to the study, there is compelling evidence that this species prefers fungal food sources, and even specific species of fungi, over other parts of the biofilm, possibly because higher growth rates can be sustained when feeding on phosphorus and nitrogen-rich fungi.
In aquarium.
If you keep them in an aquarium, there will not be a problem with feeding as they will consume everything.
However, if you wish to indulge them and recreate the natural conditions they are accustomed to, simply add a few dry leaves to the aquarium periodically. You will observe that only the veins of these leaves will remain.
| Important: It is not advisable to add green leaves. They still contain lots of sugar. The decomposition process can lead to an increase in organic waste and potentially affect water quality.
Additionally, ensure that these leaves are clean and not contaminated or treated with anything. Therefore, collecting them in the city could pose a risk. |
Are Asellus Aquaticus Plants Safe?
Yes, it is safe to keep them in planted tanks. Although Asellus aquaticus is a nonselective feeder that can consume a variety of food items, they prefer and thrive on decaying vegetation.
Unfortunately, people sometimes confuse grazing on with eating the plant. Even if you see them eating a “healthy” plant, it is not as it seems. It means that the plant is already dying, although, it has not been shown yet.
Keeping Asellus Aquaticus
These crustaceans are true survivalists in freshwater tanks. Let me give you some examples.
- Asellus aquaticus is tolerant to poor water quality, and organic pollution, and is able to bioaccumulate metals.
- It has a significant salinity tolerance which may facilitate an extension of its range into brackish conditions.
- Oxygen Levels: This species is resilient to both low oxygen levels and high levels of unionized ammonia.
- pH: They are adaptable to a considerable range of pH levels. Results of the experiments showed 100% survival in 4.5–11.0 pH regimes, with an optimum pH of 6.5–9.0.
- Temperature: They can tolerate a relatively wide range of temperatures, typically between 41 – 86°F (5 – 30°C), although they are most active in the cooler months.
Note: High temperature has a negative effect on rates of growth, survival, and reproduction.
It is also worth noting that this temperature range does not imply that they can acclimate immediately.
In other words, temperature changes should be gradual. In the case of abrupt fluctuations (e.g., more than 5 degrees), this can lead to temperature shock, as demonstrated in a study. Additionally, the initial predisposition of the organisms should be taken into account. For instance, if they were collected in warmer locations, their tolerance to higher temperatures might be greater compared to organisms collected in colder areas.
| I’ll add a bit more for comparison.
If you ever hard scuds in your tank, you probably know how hardy these creatures are. Well, with Asellus aquaticus, the situation is even more impressive. Experiments showed that Asellus aquaticus was 5 times more resistant to low oxygen levels and 2 times more resistant to unionized ammonia than Gammarus pulex. |
Breeding Asellus Aquaticus
Asellus aquaticus is a sexually reproducing arthropod. This specie usually produces two complete generations per year. These give rise to spring and autumn cohorts. Breeding starts in spring when the water temperature reaches about 44 – 46°F (7–8°C).
Maturity:
Sexual maturity is reached at 1.5 to 3 months (depending on environmental conditions) and at approximately 0.12 – 0.16 inches (3–4 mm) in length. Growth is continuous after sexual maturation.
Sexual dimorphism:
- In males, the genitals consist of pairs of testes that are located in the region that binds together the pereon and the pleon. In females, the ovaries are paired and lie parallel to the hindgut with oviducal openings on the fifth thoracic segment. When a female is about to produce eggs, the ovaries enlarge and extend along the length of the thorax.
- Besides the gonads, males are larger than females.
- In males, the 1st pair of legs in males have special structures called apophyses, which are not present in females. The 4th pair of legs is smaller and curved, helping them hold and support the female while guarding her.
- Brood pouch. Females have specialized limbs called oostegites that grow larger to create a pouch on the front underside of their bodies. This pouch is used to carry and protect eggs, covering the openings for laying eggs.
Mating:
Mating is preceded by a pre-copulatory stage (called amplexus or mate guarding) during which the male guards the female by carrying her until insemination becomes possible.
Males grasp the females with specialized legs (the 4th pair of pereopods) and wait for 5-7 days until the female molts into a fertile state.
It was observed that when mating, larger males often pair up with larger females within the population.
Fertilization:
Fertilization is only possible during approximately 24 hours, while the female oviducal openings are free (right after she molts).
The male transfers sperm to the female’s genital openings using his copulatory abdominal legs. The inner branches of the second pair of legs in the male serve as a kind of syringe.
After fertilization, the male immediately releases the female.
Eggs:
Within 2-10 hours after mating, the female develops a brood pouch that appears as a greenish swelling.
In this pouch, orange eggs are deposited.
Females can produce a variable number of eggs (usually between 30 – 150). Large females tend to produce more eggs.
Embryogenesis:
These pouches serve as a developmental environment where the eggs mature and undergo several stages of growth.
Initially, eggs are round and surrounded by the chorion and the vitelline membrane. Early embryogenesis is characterized by the appearance of a dorsal curvature and the start of the incorporation of the yolk into the digestive glands.
By late embryogenesis, the yolk is fully incorporated, the appendages are well developed, and the embryo has lengthened along the ventral curvature.
In the brood pouch, the larva undergoes three molting stages.
The estimated incubation time ranges from 23 to 35 days, depending on the temperature.
Embryos:
Embryos develop into small juveniles inside the marsupium and are released from the brood pouch at approximately 1 mm in length.
They are tiny copies of adults and are completely independent.
As in other crustaceans, growth is dependent on molt (shedding old exoskeleton), which involves tissue growth and the synthesis of a new exoskeleton.
Asellus Aquaticus in Fish Tanks
These small crustaceans are sometimes used as live food for medium to large bottom-feeding fish, but breeding them as live food has not gained widespread popularity.
Their movement and presence can trigger predatory instincts, encouraging fish to hunt and feed actively. This can be especially beneficial for fish that thrive on live prey.
| Although it does offer potential benefits for certain fish and other aquatic animals, wild-caught Asellus aquaticus can act as intermediate hosts for parasitic worms (such as acanthocephalosis).
However, infection of aquarium fish by the latter is rare, so there is no cause for concern. |
As scavengers, they will be a clean-up crew and contribute to the breakdown of organic matter in the aquarium. They help clean up decaying plant material and other detritus, contributing to a healthier and more balanced ecosystem.
If you have egg scatterers in the tank, do not worry, they will not damage the eggs.
Asellus Aquaticus in Shrimp Tanks
Having Asellus aquaticus in a shrimp tank can add diversity to your setup. However, it is important to note that if your aquarium is specifically designed for breeding shrimp, keeping these two crustaceans together is not recommended.
This is not due to aggression, which is unlikely, but rather because of potential food competition.
Food Competition. Asellus aquaticus and dwarf shrimp share common food resources (detritus, algae, biofilm, etc.). Thus, having competitors, especially for shrimplets, can have an impact on the survival and growth of young shrimp.
For example, after hatching, baby shrimp are not very active and often prefer to stay for several days in one area where they can hide. They do not dare to swim into the open and can get only floating particles, algae, or biofilm nearby.
Asellus aquaticus do not care. Adult or young they will venture into the open to find anything they can eat.
Breeding rate. Additionally, Asellus aquaticus reproduces at a much faster rate than shrimp, which could the situation even worse over time.
Tolerance. Asellus aquaticus is a highly tolerant species. They can survive in environments where shrimp cannot. Basically, anything that can kill them will also definitely kill shrimp.
How To Remove Asellus Aquaticus from the Tank?
Asellus aquaticus does not really like a clean environment.
- Remove hiding places: Asellus aquaticus often hide under stones or other tank decorations. By doing so, it will be easier to locate and remove them.
- Deep cleaning: Do a nice deep clean in your tank. Clean your filter. If your tank has a substrate, vacuum the substrate.
- Increase water flow: Asellus aquaticus are not particularly adapted to fast-flowing water environments.
- Adjust feeding: They mostly feed on detritus and decaying vegetation. Adjusting the feeding regimen and reducing the amount of excess food in the tank can help discourage their presence. Control the amount of algae in your tank. Scrape it down.
- Use Traps: Place a leaf or vegetable in the tank in the evening. The Asellus aquaticus will gather on the bait, allowing you to remove them more easily. Additionally, you can also buy or make traps that are designed to capture snails. They should work as well.
In Conclusion
Water louse are very undemanding, easy to care for, and don’t require large tanks. They can also be used as live food for aquarium fish.
They are not a foe to aquarium setups. Through their daily activities, these tiny crustaceans contribute positively to the tank’s cleanliness and that’s commendable.
Related article:
References:
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- Lafuente Elvira, Lürig Moritz D., Rövekamp Moritz, Matthews Blake, Buser Claudia, Vorburger Christoph, Räsänen Katja “Building on 150 Years of Knowledge: The Freshwater Isopod Asellus aquaticus as an Integrative Eco-Evolutionary Model System” JOURNAL Frontiers in Ecology and Evolution. volume 9.2021
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