Freshwater Crustaceans, Part One: Shrimps
Author: Kenneth Wingerter
The first of a two-part series, an expert on crustaceans reviews the biology and basic care requirements of freshwater shrimps, as well as some great species for a freshwater tank.
Finding Freshwater Crustaceans
As a freshwater hobbyist, you are probably aware of the long-term scarcity of invertebrates for the freshwater aquarium in contrast to the marine hobby, where a choice of various shrimps and crabs has long been available. What you may not know is that there are highly desirable and marketable species of invertebrates that live in the same habitats as popular aquarium fishes do. In fact, new species are now being introduced to hobbyists at a remarkable pace, especially decapod crustaceans. This steady influx of new (and often poorly identified) species of freshwater shrimp, crabs, and crayfish into the hobby can be as confusing as it is exciting.
What are these creatures? Shrimp, crabs, and crayfish are all arthropods or jointed-leg animals. They are further described as crustaceans or animals with a rigid exoskeleton that is soft immediately after molting. They are also decapods, or animals with 10 legs. If you’re going to do any research into these crustaceans—and you definitely should before purchasing any—a basic familiarity with their biology and the terms used to describe them will be beneficial.
The arthropods are named for their jointed legs, and they are a dominant group on this planet, with two principle subgroups—the insects and the crustaceans—filling terrestrial and aquatic habitats worldwide. The primitive arthropod body plan is characterized by a repetitive, linear succession of body sections (somites), which are often connected or fused to form specialized structures: a clearly defined cephalon (head), a thorax (midsection), and an abdomen (hindsection).
Arthropods are fitted with a variety of jointed, double-branched appendages and are encased in a nonliving exoskeleton. This protective covering is composed of a material known as chitin, and it may also incorporate some calcium carbonate. The exoskeleton is periodically shed and replaced to accommodate the growing animal.
Crustaceans are named for their hard exoskeletons, which may appear shell-like. They comprise about 39,000 species in 10 classes, are overwhelmingly aquatic, and are referred to as the insects of the seas. These animals have a nauplius larval stage, paired legs, two pairs of antennae, and three types of chewing appendages.
The first two body sections are fused to form a cephalothorax. A thickened and reinforced dorsal cuticle (carapace) covers this part of the body. Sharp spines may protrude from the anterior carapace. The heavy, multilayered, plate-like cuticles of the larger crustaceans are high in calcareous content, making them rather shell-like. To allow for flexibility of movement, the cuticle is thinner and more pliable around joints between the body sections.
A crustacean must regularly cast off and replace the rigid, nonliving, nongrowing, nonhealing exoskeleton as it grows and develops, though with a decreasing frequency following maturity. Its behavior and physiology are strongly influenced by the molting cycle; the molting cycle itself is governed by signals (via hormones) from the central nervous system in response to particular environmental stimuli.
Prior to molting, the epidermis undergoes a growth spurt and then secretes a new inner exoskeleton. Next it releases enzymes that break down the inner layers of the old exoskeleton. As the exoskeleton disintegrates, inorganic (especially calcium) salts are recovered and stored in the stomach wall. Finally the old exoskeleton splits open, and the creature slips out, backward usually. The creature then swells as it takes in great amounts of water, stretching its new, soft cuticle to the new full size. Once stretched, the exoskeleton hardens and is strengthened with recycled inorganic salts.
Crustaceans have a relatively complex and responsive nervous system. The majority develop compound eyes by adulthood. Their antennae serve as both tactile and chemoreceptive organs. Statocysts, sense organs that detect gravity, are found in the more advanced members of the group.
Crustaceans usually have separate sexes. Males possess modified appendages that are used to deliver sperm to females. Fertilized eggs, and sometimes hatchlings, are typically carried on the female’s abdomen. However great its adult size or degree of specialization, every crustacean undergoes some part of its early development as a nauplius—a simple, ancestral form that lacks all but the posterior three pairs of appendages.
At the class level of taxonomy, animals are divided into broad groups like mammals, birds, reptiles, etc. Among the crustaceans, the animals we are interested in here comprise the class called Malacostraca, whose name comes from Greek roots meaning “soft” and “shell.” The reference is to the soft nature of the exoskeleton immediately after molting, but it can be misleading.
The name, however, is nowhere near as important as the characteristics of the animals. The malacostracan body plan is characterized as a trunk composed of eight thoracic and six abdominal somites. A pair of appendages extends from each somite. This very common arrangement is generally accepted to be the primitive body plan of the group.
This class contains the largest, most diverse, and most advanced group of crustaceans. It is accordingly ordered into about 360 families and approximately 23,000 species. The most important subgroups (orders) in this class are Amphipoda (amphipods), Euphausiacea (krill), Isopoda (isopods), and Decapoda (decapods: shrimp, crayfish, lobsters, and crabs).
Important characters of the decapods include a remarkably consistent number of body segments, as well as a likeness of overall body plan. On the head, the foremost appendages are usually the two pairs of antennae.
Three pairs of appendages on the first three thoracic segments, the maxillipeds, are used for feeding. The first pair—a set of sturdy, jaw-like mandibles—have opposing surfaces that aid in grinding food. The next two pairs, the first and second maxillae, are situated just behind the mandibles and are used primarily for manipulating food and passing it to the mouth.
As the name “decapod” indicates, these animals all have five pairs of (usually seven-jointed) walking legs, each of which terminates in a hand-like structure with moveable finger-like structures. The first set of walking legs may be enlarged with chelipeds (crushing pinchers) or claws. The first abdominal segment carries appendages associated with the reproductive system, and the remaining abdominal segments are equipped with paired, paddle-like appendages called swimmerets. Finally the uropods (appendages of the last abdominal segment) and the flattened posterior end of the abdomen together form a fan-like swimming apparatus oftentimes referred to in common terms as the tail, though in culinary terms the “tail” is the entire abdomen.
The excretory organs of adult decapods are paired tubes near the esophagus called green glands (or antennal glands). The major nitrogenous waste product is ammonia, which is usually excreted by simple diffusion through thin sections in the cuticle.
Princes of Crustaceans
The decapods have been described as the princes of crustaceans, considered the largest and most highly advanced in their class. This is a highly successful group, accounting for a full third of its class with over 10,000 species and 1000 genera in 105 families. It is divided into two suborders: shrimps on the one hand, and crayfishes, lobsters, crabs, and hermit crabs on the other.
Though it is a primarily marine order of animals, it is abundantly and diversely represented in nearly every freshwater ecosystem (there are nine families of freshwater decapods in North America alone). The freshwater decapods have traditionally been divided into three basic groups: shrimps, crabs, and crayfishes (there are no true freshwater lobsters).
While a few shrimps are nektonic (free-swimming in the water column), decapods are for the most part benthic (bottom-dwelling) creatures. Representatives may occur in either lentic (lake and pond) or lotic (river and stream) habitats, exploiting submerged or emergent vegetation, rocky stream banks, or muddy flood plains, as well as many singular environments (e.g., cave pools).
Provided with a few basic resources, most of these highly adaptive animals can be enjoyed (or even studied) in the home aquarium with minimal effort and expense, notwithstanding special considerations that must be addressed to culture certain species.
Decapods in the Aquarium
There are many different reasons for maintaining these creatures in the home aquarium. The burrowing behaviors of some species contribute to the turning of the aquarium substrate and the cycling of detritus. Their herbivory can play a central role in fighting nuisance algae. They are generally fascinating and attractive animals, and so are excellent subjects to display. As they regularly assume important and unique ecological roles in their native habitats, they can be particularly interesting and beneficial additions to many biotope aquaria.
Although there are exceptions, they are generally much easier to culture than their marine counterparts, and so present a wealth of opportunities for small-scale breeders. For many, all you need to do is give a group a planted tank of their own, and there will soon be shrimp of all ages in the colony.
The “true” shrimps (or prawns) comprise the infraorder Caridea, the caridean decapods. (These animals are split off from other animals also called shrimps, such as tadpole shrimp or fairy shrimp.) The carideans include over 2,000 species in 230 genera. These are further subdivided into three major families: Crangonidae (typically called shrimp), Palaemonidae (typically called prawns), and Penaeidae (typically called tropical prawns). Note that these common names are sometimes loosely used, and that culinary traditions and market naming do not respect the biological divisions.
Carideans share a uniquely branched gill form and have a translucent body. They are elongated and somewhat laterally compressed. The contour of the carapace varies greatly between genera and even species. They may have functional chelipeds on the first three pairs of legs. They are excellent swimmers, having a rather wide fantail and a very flexible abdomen. In terms of average size, shrimps are the smallest decapods, with a range of less than an inch to about 8 inches.
Many freshwater shrimps flourish in a wide variety of captive conditions, while others simply will not tolerate conditions that differ much from their natural habitats. Any serious effort to successfully maintain (much less culture) these highly specialized creatures in a closed system should begin with thorough preliminary research and a determination to spare no effort or expense in providing their basic care.
Varieties of Caridean Decapods
Let’s look at some of the many caridean decapods that are suitable for the home aquarium and can be found in the trade.
Probably the oldest in the hobby is the ghost or glass shrimp Palaemonetes paludosus, which is native to the southern United States. It is used as both an ornamental and a feeder. Growing to 2 inches, this shrimp can be kept in fresh, brackish, or marine aquaria at 68° to 85°F and is easily cultured.
You will see many species labeled under the genus Caridina or Neocaridina, with little agreement among sources as to the correct names. Most well known is the Amano shrimp C. japonica, one of the first freshwater shrimps to enjoy widespread popularity in the hobby due to its ability to help eradicate algae in planted tanks.
This 2-inch shrimp is native to Japan and tolerates 60° to 80°F. Unlike most of the other popular shrimps, this one is very difficult to culture and will not reproduce in a regular freshwater tank. The heavy demand for this species, however, has supported commercial production, which has kept the price of this species reasonable.
Reds, Blues, Yellows, Greens, and More
Then there are a rainbow of easily cultured species from China, Malaysia, and Indonesia that are known by names such as (Neo)Caridina sp. “crystal red,” (Neo)Caridina sp. “Chinese zebra,” etc. They top out at about an inch and like water between 70° and 80°F. There is some variation in their native habitats in terms of pH and hardness, but most will adapt quite well to a range of water chemistries.
Many of the color forms are selectively bred strains, like the red and white shrimp bred from wild black and white bee shrimp. If you mix different strains in a tank, they will crossbreed, with most of the offspring winding up looking like the wild type. If they are different species (very hard to know), they are likely to hybridize. It is best to keep only one color shrimp per tank to avoid this. Many hobbyists have produced their own selectively bred strains with intensified color.
Sometimes seen is the Hawai‘ian red shrimp Halocaridina rubra that grows to ½ inch and therefore is rarely kept with any but the tiniest of fish. They have enjoyed some popularity as inhabitants of allegedly permanent closed-system biotope displays. In any case, they are reported to be extremely long lived. This is a brackish species most often found in tidal pools in lava fields, but it can be acclimated to fresh water. They feed on algae, bacteria, and detritus.
Much larger, African and Asian shrimps of the genera Atya and Atyopsis are usually 3 to 6 inches long and equipped with special pompom-tipped appendages that are used to filter feed. They are called pompom, bamboo, wood, flower, or fan shrimps. In the aquarium they usually need supplemental feedings of daphnia or baby brine shrimp to provide ample nourishment. Peaceful and not a threat to anything larger than a daphnia, they have not yet been bred, and at present all aquarium specimens are wild caught.
See the full article on TFH Digital http://www.tfhdigital.com/tfh/201104#pg87