Because fairy shrimp are small (mostly less than 2.5 cm, or 1″, long) and constantly in motion, their anatomy is not obvious to a casual observer and the names of the various body parts may be unfamiliar to one accustomed only to mammals and birds. Good diagrams of fairy shrimp are given by Linder (1941, p. 107), Moore (1969, p. R177), Kaestner (1970, p. 86), and Lynch (1958, 1960, and 1964) but generally lack labels. For this web site, I will rely on my photographs rather than try to obtain permission to produce others’ figures.
This section applies only to adult fairy shrimp. Pre-adult (naupliar) stages are morphologically very different. They are also mostly too small to see.
The fairy shrimp body has a head, a thorax, and an abdomen. Because fairy shrimp swim head first, there is rarely any doubt as to where the head is. The head is the short front end of the body that lacks legs. Its most prominent visual feature is a pair of compound eyes (black in most of the species I have seen) on short stalks that stick out laterally from the head. On pale fairy shrimp, you may be able to see a tiny black spot on the front of the head midway between the eyes. This is the ocellus (Dexter, 1959, p. 559) or naupliar (pre-adult) eye (Belk, 1982). It is sensitive to light. A third eye indicative of wisdom figures prominently in Hindu symbolism. Fairy shrimp actually have a third eye but I’m not so sure about the wisdom. The head also has a maxillary gland, mandibles, maxillae, labrum, and a mouth, which all figure in eating (Kaestner, 1970, p. 86). You would probably have to dissect a fairy shrimp to see them.
A male fairy shrimp (Branchinecta coloradensis) with body parts annotated.
The head has 2 pairs of antennae, here referred to here as antennae I and antennae II. Antennae I are smaller and more toward the front of the head than antennae II. They have only one segment, or joint. They are straight and extend forward so they look like stereotypical antennae. Antennae II are different in males and females. In females, they are small and inconspicuous although fatter than antennae I. In males, antennae II are quite large and have developed as a means to grasp females before and during copulation. There are such great interspecies variations in the male antennae II that they are often the most important feature used for species identification (Belk, 1975). They have 2 segments (except Polyartemiella – Belk, 1975). The basal, or proximal, segment is fatter and mostly longer than the distal, or apical, segment and is generally more or less cylindrical. It may have a mound, pad, pulvillus, ridge, or bulge with or without a spiny roughness, or a peg-like or finger-like process or outgrowth, or some combination of these (Linder, 1941, p. 127-144; Belk, 1975). A microscope is required to see such features. The distal segment may be cylindrical, flattened, bladed, or broadened and may be curved, kinked, or twisted. It may taper to a point, terminate bluntly, or end with a feature that is shaped like a human foot, or something else. In normal swimming, antennae II project from the head back over the legs (fairy shrimp swim on their backs). In some species, the pair of antennae II may cross above the legs 1/2-3/4 of the way down the thorax.
But that’s not all, males of some species have antennal appendages. These grow from the basal segments of the male antennae II. They may be longer or shorter than antennae II. Thicknesses and cross-sections vary between species and the appendages have spines, outgrowths, fingers, branches, processes, or projections which defy a short description (Linder, 1941, p. 127-144; Belk, 1975; Pennak, 1978, p. 342-343). Belk (1975) distinguished horn-like, lamelliform, and triangular appendages. Some species have a single frontal appendage, which is interpreted as the fusion during early development of the antennal appendages of each antenna II (Belk, 1982). It emerges from the front of the male’s head. These species resemble unicorns but it is unlikely that the original conception of unicorns came from fairy shrimp. A frontal appendage can be as complex as the antennal appendages but I haven’t found any good descriptions or diagrams.
The thorax begins with the first segment behind the one with the second maxillae (Linder, 1941, p. 112). Visibly, it begins with the first pair of legs. In all but 2 genera, the thorax consists of 11 segments, each with a pair of legs. The Polyartemiella thorax has 17 segments and the Polyartemia thorax has 19 (Linder, 1941, p. 107). The legs are always in motion so the thorax generally looks fuzzy or blurred. When photographing fairy shrimp, a shutter speed of 1/125 second will just about freeze the leg motion. For the microscopist, thoracic segments may have spines, warts, or ridges, depending on species (Linder, 1941, p. 114). The thoracic segments and legs are generally all alike but Linder (1941, p. 112) found that Parartemia females lack legs on the 11th segment and Pennak (1978, p. 328) stated that the rearmost pairs of legs are smaller.
The legs are complex structures with multiple lobes, flaps, and irregularities along their margins. In the words of Johansen (1921, p. 21), the legs are “divided up into many hairy leaves or flagella”. He later referred to “ambulatory trunk-limbs, the foliaceous, hairy legs”. Clearly, they are not like the legs of familiar crustaceans, such as crabs and lobsters. The parts of the legs have since acquired names such as endites, endopodites, epipodites, exopodites, exites, and pre-epipodites (Dexter, 1959, p. 550; Kaestner, 1970, p. 86-87). Parts of the legs are fringed with arrays of bristles that are important for feeding. Gas exchange occurs principally through the legs so legs serve the same purpose as gills in other aquatic animals. The legs are stiffened by internal fluid pressure rather than by a stiff integument (Kaestner, 1970, p. 85), or outer covering. Fairy shrimp legs lack the chitin or calcification found in other arthropods.
Male fairy shrimp on its side in clayey water that makes details of legs visible. They are not stiff or jointed like crab legs.
The legs have a simple motion, beating back and forth at 140-400 beats/minute (Kaestner, 1970, p. 91). They move “as grain stalks before the wind” (Johansen, 1921, p. 22). Most of the motion is for collecting food particles and pushing them into the ventral food groove and up toward the mandibles (Kaestner, 1970, p. 90-92). Forward body motion, in contrast, is produced primarily by propeller-like motions of the terminal lobes of the legs (exopodites), which can be inclined at various angles (Pennak, 1978, p. 329). Moore (1969, p. 178) referred to experiments that showed that anostracans can regenerate their legs, but with a very low success rate.
The relatively slow, continuous swimming motion of fairy shrimp is helpful for identification. I haven’t seen anything else swim like fairy shrimp do. Clam shrimp and tadpole shrimp swim at about the same speed but they stop from time to time (within a minute or 2?). Fairy shrimp never stop unless they are scraping the bottom of the pond with their legs. Insects have much shorter swimming spurts, probably less than 20 seconds or so. Insects can be relatively slow, like dytiscid larvae (order Coleoptera, family Dytiscidae), but most are fast, like water boatmen (sub-order Heteroptera, family Corixidae). Amphipods are also fast swimmers and the ones I have seen don’t swim for more than about 30 seconds without stopping, or disappearing from my view. Ostracods are continuous swimmers (at least for as long as I can track them, which may be only 30 seconds or so) but they are faster and look like tiny clams. A nearly spherical animal with a continuous swimming pattern that is a little faster than fairy shrimp is not likely to be confused with them because they are less than 3 mm in diameter. I think such spherical animals are water mites (Arthropoda, subphylum Chelicerata, class Arachnida, subclass Acari (mites), superorder Acariformes, unranked Hydrachnida or Hydrachnidia – Thorp and Covich, 2001, p. 551).
The abdomen is long and thin and has no legs. To Johansen (1921, p. 21, 22) and me it looks like a tail. It consists of 9 segments (8 for Polyartemiella and Polyartemia). There are 2 fused segments at the front and the last segment is called a telson, or furca, and has the anus (Linder, 1941, p. 107; Kaestner, 1970, p. 85). The 2 fused segments comprise the genital region. In males, they have 2 penes (left and right) and in females, they have 2 oviducts which lead to a single ovisac (Belk, 1982). The telson has 2 short cercopods, which are also referred to as uropods, caudal appendages, or rami (Belk, 1975; Belk, 1982). They are stick out from the end of the abdomen to give it the appearance of a forked tail in some species. The cercopods may be rod-like and pointy or flattened and flap-like (Belk, 1975). The pointy ones may be straight or curved. They are generally fringed with bristles which are usually too small to see.
The abdomen may be longer or shorter than the thorax (Linder, 1941, p. 116) but the lengths do not differ by much. The intestine extends the full length of the abdomen as a thin tube. Because fairy shrimp bodies are usually translucent, the food matter in the intestine can make the fairy shrimp more visible if it is darker than pale water or paler than dark water. The ovisac of females attaches to the forward end of the abdomen and floats over the abdomen during swimming (fairy shrimp usually swim on their backs). Like everything else, the ovisac is translucent and hard to see unless it contains eggs. Eggs of a different color than the water help make fairy shrimp more visible. In some species, the ovisac is a long pouch which extends for more than half the length of the abdomen. In others, it is a short, globular pouch at the upper end of the abdomen. Although not commonly used for species identification, different types of ovisacs can alert you to the presence of different species.
A female fairy shrimp (Streptocephalus seali) with body parts annotated and abundant eggs.
A female fairy shrimp (genus Artemia) with a bulbous or globular ovisac and indistinct eggs.
Quiz: Identify which of these 2 fairy shrimp is female and which is male. Can you see the ocellus on each?
Fairy shrimp have a heart! But it’s a tube (Pennak, 1978, p. 331), like most crustaceans I guess. It lies between the intestine and the back and extends through most or all of the body segments (Pennak, 1978, p. 331). Pennak (1978, p. 331) wrote that the heart is much shorter in Notostraca and Conchostraca so it may be safe to say that fairy shrimp are the most big-hearted of branchiopods. Fairy shrimp are pretty small so they don’t need blood vessels. The blood circulated by the heart, then, isn’t really blood, it’s just the intercellular fluid that fills the spaces between the cells of the body. It is also called hemolymph or extracellular fluid.
One of the functions of the intercellular fluid is to distribute oxygen to the cells, like a vertebrate circulatory system. Respiration occurs through the branchia, also known as exites, of the legs, which are small finger-like lobes (Fig. 4-2 in Kaestner, 1970, p. 87). Pennak (1978, p. 332) added that gas exchange “probably takes place through all exposed surfaces of the body”. Oxygen diffuses from the water through the branchia and into the intercellular fluid. A figure in Kaestner (1970, p. 86) shows “blood” flowing within the legs, from the forward legs toward the rearward legs, and from each segment into the centrally located heart. There are openings in the septa between the body segments for the heart. Pennak (1978, p. 332) also mentioned “slitlike ostia”, which are the openings within the segments into the heart. Peristaltic contractions push the intercellular fluid forward through the heart into the head (Pennak, 1978, p. 332), where it is just kind of dumped. Neither Kaestner (1970) nor Pennak (1978) mentioned heart rate.
Fairy shrimp (and other branchiopods) are not exactly red-blooded but they do have hemoglobin. They certainly do not have blue intercellular fluid like crabs and many other crustaceans. The blue bloods have hemocyanin rather than hemoglobin for carrying oxygen. In high-TDS water or water with a low oxygen concentration, fairy shrimp produce more hemoglobin and become pink or red (Kaestner, 1970, p. 93). Most of the studies have been done on fairy shrimp of the genus Artemia, which live in high-TDS water and are commonly a shade of red. I have seen both red and nearly colorless Artemia. Sanchez and others (2016) found red and colorless Artemia in the same high-TDS waters (140,000-200,000 mg/L) and determined that the red ones were infected by cestode parasites but the colorless ones were not. The parasites stimulated the production of both hemoglobin and orange carotenoids in the Artemia. Rarely, other genera of fairy shrimp I have seen in low-TDS waters are a shade of orange or pink, such as those in Dead Ant Rock Pool and those in Small Rare Plant Habitat Pond.
Red fairy shrimp of the genus Artemia with pale food in their intestines. That the water has high TDS can be inferred from the presence of white mineral crusts along the shore.
Essentially colorless fairy shrimp (genus Artemia) in high-TDS water. Dark bulbous ovisacs or pale food in the intestines make some fairy shrimp easier to see. In others, the intestine along the back is dark. A female at upper left has a white ovisac. Note the bright reflections from recently precipitated mineral crystals floating on the surface of the water. TDS must be very high for minerals to precipitate.
Although most Artemia may be a shade of red, populations of other genera are commonly translucent and colorless to pale gray. This may be partly for camouflage as the waters of fairy shrimp ponds are commonly colorless and the bottoms are commonly pale (can you see these 2?). Especially in opaque, pale-brown, clay-rich water, the lack of color may help fairy shrimp avoid being eaten by wading birds which feed in such waters. In clear, shallow, alpine and arctic ponds however, pigmentation offers protection against ultra-violet radiation, which increases with elevation and is greater in the Arctic because of ozone depletion. For instance, Artemiopsis stefanssoni in arctic ponds is bright pink due to carotenoids and Branchinecta paludosa is greenish-brown from carotenoids and melanin (Rautio and others, 2009), Maybe that is why the fairy shrimp in 3,334 m (10,940′) Bivouac Lake (Wind River Mountains) are a distinctly greenish color.
Fairy shrimp have a simple nervous system. They have 2 parallel nerve cords that go all the way from the eyes to the end of the abdomen. The diagram in Pennak (1978, p. 333) shows a pair of fat nerves connecting the left and right nerve cords within each thoracic segment, pairs of small nerves extending laterally from the nerve cords outward into each thoracic segment, occasional wispy nerves extending from the nerve cords into the abdominal segments, and a thick mass of nerves connecting the eyes and ocellus to the nerve cords. Johansen (1921, p. 22) wrote that “the brain is large and well defined” but didn’t describe or locate it. Pennak (1978, p. 333) also mentioned a modified patch of “cephalic epithelium” with a “probably sensory” function known as the “dorsal organ”. Maybe fairy shrimp do have something of a brain.
The senses of fairy shrimp have not been well described. The eyes and ocellus both sense light and are used to orient the body with the back side down. Fairy shrimp will orient back side up if light comes from below. In darkness, fairy shrimp orient back side down (Pennak, 1978, p. 332). The sense of taste is localized near the mouth parts and the sense of touch is distributed throughout the body (Pennak, 1978, p. 332). What they taste and what they feel remains speculative. They have sensory bristles (“setae”) and “hairs” for touch, or sensing water motion, and chemical sensing (Kaestner, 1970, p. 87). No word on what chemicals are sensed.
I have not read articles concerning the role of vision in fairy shrimp living in water that is opaque due to suspended clay. Is their vision more sensitive because it is harder to see or have the vision neurons been co-opted by another sense that is more useful?
When the water is this opaque, what can fairy shrimp see? Are they using other senses to navigate, to keep from swimming into each other, and to find mates? Look for the dark abdomens in this photograph.
Fairy shrimp can see you. Rogers and Fugate (2001) reported that Branchinecta hiberna in one pond avoided humans so strongly that they had to be “stalked” in order to be captured. In my experience, they may or may not move away when you approach and the water is clear enough for you to see them. If you stay still and watch, they will carry on as if you are not there in most cases. Does my ability to take photographs within 0.5 m (20″) of fairy shrimp indicate that they have poor vision or that they do not see me as a threat? In some populations, such as the one in Bivouac Lake (Wind River Mountains), the fairy shrimp swim away from an approaching finger. In opaque water, you might be able to touch them but you won’t know that because they offer too little resistance to feel.