Freshwater Invertebrates

Author: Chris Lukhaup

Freshwater invertebrates are on the rise in the aquarium world, providing hobbyists with the opportunity to keep something more than the usual fish and plants—namely, snails, crayfish, crabs, and shrimps. Some of these species are quite colourful and have attractive patterns; however, the true fascination comes from their bizarre body shape and their small size, which makes them ideal inhabitants for nano tanks. Almost every week a new variant or species of freshwater invert is brought to the market. Some of them become established in the hobby, while others disappear from the wholesalers’ lists as fast as they entered them. More often than not, unknown requirements for keeping and breeding them coupled with high loss rates and a lack of offspring are responsible for this phenomenon. Moreover, some invert species turn out to be quite incompatible with other aquarium inhabitants and thus soon cease to grace dealers’ stock tanks. The basic precondition for keeping any animal in captivity is the exact knowledge of the species and its requirements. Unfortunately, careless mistakes happen, and some crustacean species are often mistaken for each other in trade, sometimes as early as at the importer’s or wholesaler’s, and then bad information gets passed down to the customer. Sometimes, shrimp are even sold as crayfish and vice versa, and the outlook is not much brighter when it comes to crabs or snails. 

Freshwater Shrimp

Since the late 1990s, ornamental freshwater shrimp have been an integral part of the hobby. Before that turning point, pet stores sold them from time to time, usually as live food. But then a veritable boom set in, and today shrimp may be the most popular inverts in our aquaria. The shrimp commonly found in the hobby belong to different genera and families, scientifically speaking, but what unites them is that they spend all of or at least most of their lives, especially as adults, in freshwater. Other species have not become entirely independent of the original habitat of their ancestors, the sea, and they need brackish or marine water for reproduction. These species are known as the primitive types, and they produce large numbers of very small eggs per batch. The larvae hatching from these eggs are released into the open water, where they form part of the plankton and go through many stages of development. Only toward the end of their time as larvae do they start a benthic life on the ground. Around this time, they also migrate back to pure freshwater. During the course of evolution, freshwater dwarf shrimp have spread from the estuaries of larger rivers to rapidly flowing creeks in the mountains, enormous lakes, the smallest trickles of water, and even to cave waters. They are found in tropical to temperate climates today, although they have not conquered the arctic regions yet.This abundance of different habitats has resulted in a great variability in shrimp species, and in stunning forms. Their sometimes truly impressive colors and patterns are the result of their adaptation to the different living conditions in their habitats. From all the different shrimp in the world, only representatives from three groups have managed to find their way into our aquaria: dwarf ornamental shrimp, fan shrimp, and long-arm shrimp. They differ in body size and form, as well as in their habits. The requirements regarding their environment do not differ much between shrimp belonging to one of these groups. Practically every shrimp available in the trade belongs in one of the three groups, under systematical aspects. Dwarf shrimp are probably the most prominent and also the most popular among the freshwater shrimps. They have found their way into the aquaria and hearts of their keepers in almost every country around the world. The shrimp offered in the trade today are rather variable in size. Dwarf shrimp with a total body length of around a half-inch (15 mm) and long-arm shrimp of the genus Macrobrachium, some of which can grow to a body size of over 20 inches (50 cm) in length, of course differ considerably when it comes to their keeping requirements. For this reason, it is practically impossible to describe a “typical” shrimp tank; you’ll have to decide which species or at least group of species you want to keep. Dwarf ornamental shrimp can be kept in tanks as small as 2½ gallons (10 liters) and up. Many shrimp species are excellent algae eaters, able to keep a tank free from that unsightly scourge, and moreover, their behavior is highly interesting to watch. However, contrary to what is often believed, they do not eat long filamentous or staghorn algae that have already become a problem in an aquarium. However, a shrimp population in a tank may prevent algae from taking over, as they eat the young growth on plants and other surfaces. Shrimp are omnivores. Besides algae and vegetable matter, their diet should contain flake food and any kind of frozen fish food like blackworms or brine shrimp. All shrimp species eat algae, zooplankton, detritus, and soaked fish food. Tablet and flake food, as well as special plankton food made by various manufacturers, are very well suited for this latter purpose. Many other products have been brought to the market as well that have been specifically tailored to the needs of ornamental shrimp. One of the most important inventions when it comes to shrimp-keeping in my view is the shrimp salts, which have been specially developed to improve the growth of bacteria in the shrimp aquarium that in turn gets eaten by the shrimp. Some species that grow to a larger size, such as Macrobrachium rosenbergii or other long-arm shrimp, have been reported to prey on smaller or bottom-dwelling fish. And some shrimp species live in brackish water in nature, so their larvae need marine water to grow up, which makes breeding these species difficult to almost impossible.
Otherwise, these robust inverts are impressive and highly enjoyable companions for an ornamental tank and will develop nicely when kept in the right conditions. Most species are quite tolerant with regard to water parameters. They endure temperatures from 45° to 82°F (7° to 28°C), and they tolerate a considerable range of pH values from 5.5 to 7.6. However, there are some species that require very specific conditions. Highly important for the entirety of shrimp species is the oxygen content of the water. Insufficient oxygen can result in diseased or even dead shrimp, which makes a well-aerated or filtered tank a must for the successful shrimp keeper. Moreover, these animals like low light and many hideaways where they can stay during the day. Like all crustaceans, shrimp have to shed the old exoskeleton they have outgrown at regular intervals and form a new one. The old exoskeleton breaks open between cephalothorax and abdomen, and the shrimp frees itself by jerky, sudden movements. This process is also called molting. After the molt, the shrimp’s body is still very soft, which makes the animal highly vulnerable. Shrimp will usually hide from possible predators after shedding their old exoskeleton to avoid being disturbed while the new one hardens.  

Shrimp Tank Setup

Most shrimp that can be kept in an aquarium are good swimmers. Unlike fish, however, they don’t need open water to swim, but rather prefer a really well-structured tank that provides various surfaces to sit on. For this purpose, you can use all kinds of aquarium decorations. Dwarf shrimp will even perch on the vertical glass panes of your aquarium if a little algae and biofilm have had the chance to form on them. Fan shrimp need a sufficient number of places to sit, depending on the size of their group, where they can make full use of the current and still find some shelter. If there are not enough of such places in the tank, they may start fighting for a good place. Long-arm shrimp need a well-structured aquarium, especially if you are planning on keeping more than one (which is not possible for all long-arm shrimp species) or breeding them. As these larger shrimp are not really great swimmers, especially when they grow bigger, it is important to provide them with three-dimensional structures in the tank where they can climb. They also like caves and hideaways under wood and rocks where they can retreat. Depending on the type of shrimp, their requirements regarding filtration differ considerably, too. Tanks dedicated to dwarf shrimp can be equipped with small internal filters, air-driven sponge filters, or undergravel filters, for example. A slight flow is preferable. Even though many dwarf shrimp species originate from small creeks, it is not necessary to provide them with a strong current, and, in fact, a strong filter brings the risk of shrimp being sucked into its intake. A strong current in a shrimp tank is only possible if the filter intake surface is sufficiently large like that of an undergravel filter or a large filter mat, for example. This kind of filtration and filter intake is especially suitable for shrimp tanks with fan shrimp. Most fan shrimp originate from rapid or even torrential creeks, and they just love sitting in a good, strong current where they can catch food from the water column with their filtering fans. The risk of them being sucked into a filter intake is practically nonexistent, but their food will be removed from the water too quickly if the filter intake current is overly strong. The keeping requirements of long-arm shrimp can be compared to those of crayfish. A relatively large, strong filtering system, be it internal or external, is the way to go. Most shrimp do not eat aquatic plants, so you can plant your tank just as you please. Even though many shrimp originate from bodies of water with little to no aquatic plant growth, they do not mind living in a densely planted tank at all. In a tank dedicated to fan shrimp, you should make sure these somewhat plumper shrimp still have room to move without hindrance, though. They clearly prefer unplanted areas with rocks or stones.  

Types of Shrimp

DWARF SHRIMPWith over 290 species, shrimp of the genus Caridina are one of the most diverse groups within the Atyidae family. They are also the most widespread shrimp in the aquarium hobby. However, recent research has found that this genus is in urgent need of a scientific review and restructuring, as there are many discrepancies. The genus Neocaridina has up until now been represented by 30 species and has also found wide distribution in the hobby.  FAN SHRIMPAtyid fan shrimp can be subdivided into five genera and around 20 species. However, only three genera have found their way into the aquarium hobby. All fan shrimp have long bristles on their first and second leg pairs that look a lot like hand-held Chinese fans with which they filter edible particles from the current in fast-flowing waters.  LONG-ARM SHRIMPLong-arm shrimp belong to the superfamily Palaemonoidea. They differ from all other shrimp species by their second pair of walking legs, which are considerably larger. Sometimes, especially in male specimens, they can be even longer than the body of the shrimp itself. They are used for catching food, and, secondarily, for defense. Long-arm shrimp are often mistaken for river crayfish, as in some species, the elongated chelipeds are thicker, too.  

Freshwater Crayfish 

There are approximately 660 crayfish species in the world, and new species are discovered every year. Crayfish inhabit creeks, streams, lakes, swamps, and wet meadows. Some species live in water bodies that go dry during a part of the year, and some are even completely independent of surface water, as they dig tunnels in the ground that are several yards (meters) deep and lead to groundwater, where the crayfish get all the water they need. On the surface, nothing would make a layman suspect that crayfish can live in such a place, but experts find their burrows because of the chimneys they build, which look a bit like molehills. Even in cave waters almost devoid of nutrients, several crayfish species are found. Some of them have been living in these lightless biotopes for so long that they have lost all color. In adapting to life in eternal darkness, some species’ eyes are almost or even entirely degenerated. Other crayfish species have conquered dry land and spend most of their lives out of the water, only returning to moisten their gills and release their offspring. If you consider the wide area of this group’s distribution, the various climate zones and altitudes they inhabit, and the very different biotopes where they can live, you soon realize that general statements about the requirements of crayfish and the conditions they should be kept in are practically of no use. If you want to create the best artificial habitat possible in your aquarium for your crayfish, it is crucial to know exactly which species you have. Only then can you be totally sure of what its individual requirements are. There are large differences in crayfish body size, too. The bodies of the smallest specimens grow to less than half an inch (1 cm) in length, and the largest crayfish, up to 24 inches (60 cm) and 11 pounds (5 kg). In the aquarium hobby, crayfish have been a late addition. The first ones to end up in an aquarium were originally imported as food for human consumption. Among them, hobbyists found very colorful specimens that kindled their interest. One of the first crayfish in a pet shop tank was the Louisiana crayfish (Procambarus clarkii) from the southern United States. Since 2000, a veritable aquarium-hobby run on crayfish has taken place in Europe, Japan, and the U.S., and the species pet shops have on offer have become more varied. All crayfish are bound to freshwater for their entire lifecycle. Their embryonic development takes place inside the egg, with the exception of the last one or two stages. In contrast to most marine invertebrates, crayfish do not go through free-floating, planktonic, or independent larval stages. The eggs are carried under the pleon of the mother crayfish and hang from a tiny filament called a telson thread. The young crayfish that have already hatched are still connected to their mother with by this thread so they do not get lost. It is fixed to the last body segment of the larvae, the telson, and goes through the egg shell to the swimmerets of the berried (egg-laden) female. No other representative of the crustaceans has developed a comparable system. There is one all-important aspect of keeping crayfish that goes far beyond the aquarium hobby and needs to be brought to the general public’s attention. As global trade with these invertebrates is booming, they have started to become an increasing danger for the environment: Should crayfish be released into nature by irresponsible aquarists, they pose a grave danger for the native crayfish population and other aquatic organisms. The entire biotope may be seriously compromised.  

Crayfish Tank Setup

When the volume is the same, an aquarium with a larger footprint is always preferable, since water depth is not as important for crayfish as the usable surface, which is what dictates how many crayfish you can keep in a tank. Depending on the size and the aggressiveness of a species, tank décor is also a very important factor. Crayfish need structure, horizontal as well as vertical, and a sufficient number of places to hide. Moreover, they are true escape artists, and they will use every tube, hose, and cable leading out of the tank to have a closer look at their surroundings—which, unfortunately, often ends in their death. Even a tad of silicone jutting out at the corner of the tank can serve as a way to escape the aquarium. Thus, a tank cover is not only absolutely necessary, but it also needs to be very tight. Make sure you close off every cable feedthrough with foam or netting. If your crayfish are of a larger species, keep in mind that they have enormous strength and can lift tank covers seemingly without effort if they are set on leaving the aquarium. The majority of crayfish need a cave for their well-being. Some species also take cover in dense vegetation and only build tunnels when their aquatic habitat threatens to become dry, so they need a moist, well-protected retreat.  As it is hardly possible to have a loamy stable ground in the aquarium for the crayfish to burrow into, we need to provide them with other materials that they can use to build their caves. A slab on a sufficiently thick layer of sand and gravel propped up by smaller rocks is the simplest option. The crays can dig out the gravel below and create a tunnel for themselves.  Driftwood, coconut shells, clay flowerpots, and clay tubes are suitable hiding places, too. There are hardly any limits for a designer’s imagination. The important thing is that each crayfish can build and live in its own cave. It is even better if the number of hiding places exceeds the number of crayfish so they can choose.  Many crayfish species are nocturnal or crepuscular, which makes lighting somewhat less important outside the aesthetic considerations. A strong filter is great in a crayfish tank. Crayfish break down their food into very small parts when eating, whether they have tablet food or a piece of fish, and small particle clouds tend to spread and spoil the water.Some crayfish species may be socialized with small fish or dwarf shrimp with identical requirements regarding the water parameters. These fellow occupants soon learn how to make use of these additional sources of food and take care of a large part of these food particles, which would otherwise go to waste. Their requirements regarding water quality differ from species to species, though crayfish generally tolerate a rather large range of water parameters. The great robustness and adaptability of some crayfish species should, however, not lead their keepers to the assumption that regular water maintenance is unnecessary. On the contrary, good filtration and regular water changes are a must in every crayfish tank. Crayfish have a very hard exoskeleton, and thus cannot grow continuously. When they shed their old skin (or molt), they experience a growth leap. A crayfish is never fully grown but will continue to shed its skin and grow immediately after every molt. Of course, they molt more frequently and thus grow faster while still young, and their relative size and weight gain per molt is larger than for older crays. Very old crayfish molt, but they do not grow significantly at all. Instead, they just renew their old skin that is sometimes overgrown with algae and other micro-organisms.  Every molt is life-threatening for a crayfish. Often, an unsuccessful molt marks the end of a crayfish’s life. If the old skin gets too tight, it is shed in order to make room for a new, larger one. For this purpose, the old exoskeleton gets softer and more elastic as hormones trigger metabolic processes that remove calcium from it. After that, the crayfish can slip out of its old skin.  As crayfish inhabit most different biotopes, it is hard to make a general statement about their diet. Usually, crayfish are omnivores, which means they eat food of vegetable and animal origin. Most species will eat anything organic, in fact. The choice of foodstuffs they eat in nature well demonstrates their variability and adaptability regarding food: rotting wood, detritus, algae, fungi, bacterial layers, aquatic plants, dead tree leaves, terrestrial plants, fruit, seeds, insects, crustaceans, worms, mollusks, fish, and other vertebrates.  

Freshwater Crabs 

Keeping freshwater crabs is by no means easy. They are intelligent invertebrates with highly developed sensory organs, and they need special environments. As crabs have quite individual temperaments, their keeper needs to observe them closely to be able to meet their needs. Crabs are highly interesting crustaceans to keep and can fill an aquaterrarium with life. They are quite susceptible to changing habitat conditions and stress, though, so they need a lot of room and good hiding places under and above the waterline. Many species require a section of dry land in the tank, as their life strategy is amphibian. Unlike other crustaceans, the size of crabs is indicated by their carapace diameter, not body length. A dedicated aquaterrarium for dwarf land crabs can be decorated with mosses, rocks, and driftwood. The air humidity should be as high as possible. Some Geosesarma species are highly endangered in their countries of origin, as their areas of distribution are limited to small forest zones that are in danger of being cut down for lumber or drained. So the population density of these crabs in their natural habitats is low, leading the males to stake out territories that they defend against intruders.  Keep this in mind when stocking your aquaterrarium: Either put in only one male and two females, or at most a small group of five to six crabs—two males and three to four females, assuming that the aquaterrarium is sufficiently large for the bioload.  Different species should not be housed together, as the more dominant species will attack and even kill the weaker crabs. In general, female dwarf land crabs carry relatively few but rather large eggs from which fully developed young crabs will hatch after up to four months. However, Geosesarma maculatum and G. perracae are known to release larvae into freshwater that develop into young crabs after a short time. Separating the offspring from their parents is highly recommended when raising them in captivity, as the adults will prey on them, and the young crabs will have difficulty avoiding them in the confinement of the aquaterrarium. Most freshwater crab species are carnivorous, hunting above and below the waterline. They can be offered live house crickets or other insects on the above-water section of the tank, and, in fact, any live food from the terrarium trade will do nicely here. Under the waterline, they have been observed to prey on small fish or shrimp and other small inverts. Even though some of these species look very much alike, there are huge differences regarding behavior, especially in reproduction and brood care. Some of them reproduce exclusively on firm ground, others need freshwater, some undergo direct development (where fully developed young crabs hatch from the eggs), and yet others belong to the unspecialized type whose larvae require water—and most frequently even marine conditions—to develop. Very often you can find fiddler crabs offered in the trade. These mangrove crabs of the genus Uca, which comprises about 100 scientifically described species, lead a semi-terrestrial life and inhabit burrows they dig in the sandy or muddy ground at the waterside of tropical or subtropical coasts. Some of these species are really beautiful in color, and children in the tropics love catching them for fun. They are frequently offered for stocking an aquarium, where they will not do very well—they really need an aquaterrarium.   

Interesting Inverts

Freshwater invertebrates add a bit of the unusual to an aquarium, and their colors and patterns make them stand out, especially against the greenery of a planted setup. Along with their generally small size, it’s no wonder they are increasingly in demand as aquarists look for something different with which to stock the nano tanks that are increasingly popular in today’s aquatics hobby.  Just be sure you know the species you are getting and research their requirements before adding them to any setup, and these colorful crustaceans will reward you amply with their beauty, unique forms, and natural charm.