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Ch. 3. Ecology of Fairy Shrimps Life Stage and Species (Mertens Et Al

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Ch. 3. Ecology of Fairy Shrimps Life Stage and Species (Mertens Et Al Ch. 3. Ecology of fairy shrimps life stage and species (Mertens et al. 1991). As face of the fused 12th and 13th segments of the appendages return forward from a beat, a low thorax (Fig. 1.2, p. 2). Yes, you read correctly, pressure is created between them causing water to males actually have two penes. Each lateral testis flow in (be "sucked" in) with its dead particles, empties through only the penis on its side of the and living organisms too if they are unable to body. swim against the flow. When the phyllopodia In females, oviducts extend from the ovaries beat once more, whether in graceful swimming or and enlarge at their distal ends into what are bottom scraping, increased pressure forces water called lateral pouches. These join one even through the setal meshwork and particles too large larger structure called an ovisac which also re- to pass through its pores are sieved out, whether ceives ducts from several shell glands. The living or dead, plant, animal, or mineral, nutri- ovisac expands within the brood pouch which is a tious or not. These particles are moved to a food centrally placed and posteriorly directed protru- groove between the bases of the phyllopodia, then sion from the ventral body wall of the fused geni- passed along to the jaws and mouth by specialized tal segments (Fig. 1.3, p. 3). The opening of the spiny lobes at the base of each appendage. For an ovisac to the outside is the female's genital pore, excellent discussion of anostracan feeding mech- the site that male sexual behavior is aimed at en- anisms, see Fryer (1983). tering for insemination. Rather detailed illustra- For those of you who enjoy history, it may be tions of this system can be seen in a paper by of interest to know that until just recently a fairy Brendonck (1991). shrimp's two fused genital segments were The head, the most anterior portion of the thought to be part of its abdomen because, like body, is situated immediately in front of the tho- that portion of the body, they lacked phyllopodia. rax, and bears the mandibles (jaws) and other However, certain perceptive and dogged scientists tools. Rotating action of the mandibles helps (e.g., Walossek 1993), noting that sex ducts of crush and push food into the mouth (Fryer 1966). related fossil and living branchiopods all open on The grub is moved to the teeth-like mandibles phyllopodia, that the embryology of the genital from the thoracic food groove by two pairs of segments is more similar to thoracic than ab- small head appendages located immediately be- dominal development, and that the penes of the hind the mandibles, the maxillae (Manton 1977). male are actually derived through evolutionary Unlike the teeth of humans and your pet goldfish, modification of a pair of phyllopodia, have finally but similar to a grasshopper or lobster, a fairy persuaded skeptics that the fused genital region shrimp's mandibles are not within its mouth cav- truly belongs to the thorax! That issue settled, we ity, rather they flank and ultimately cover the can now rest in peace with the notion that the mouth. Immediately anterior to the mouth, and posterior portion of the thorax is the focal point overhanging the mandibles, is a lobe or mound for the reproductive organs and their accessory called the labrum, or "upper lip", which effec- internal and external structures. tively prevents leakage from between the mandi- Internally, ovaries and testes are paired, blind bles and serves as a backstop for food that might tubules that lie alongside the gut, mainly in the otherwise be pushed further forward. This device abdomen, but in some species an anterior branch thus assures that mandibular action gets all of the extends further into the thorax as well. The food into the mouth. No messy face here! male's two penes protrude from the ventral sur- More forward yet are one pair of large, 12 Ch. 3. Ecology of fairy shrimps stalked, compound eyes and two pairs of anten- there are no arteries, capillaries, or veins, and the nae, referred to as antennules and antennae (Fig. heart is not the only propulsive force (Greene 1.3, p. 3). In males, the antennae are large, and 1924). Blood thus surrounds and bathes all body possess a variety of processes and appendages, as organs, flows out into all appendages where the well as ridges and mounds which may be orna- majority of oxygen uptake from the environment mented with spines or denticles (check out the is thought to occur, then generally moves posteri- many examples on pages 143-165). Because an- orly where it re-enters the heart via valved open- tennal shape and ornamentation apparently aid a ings called ostia. female in choosing a mature mate, such features If you want to see a thing of beauty and per- differ from species to species (Belk 1991a). As petual motion, isolate a fairy shrimp in a small might be imagined, the crafty fairy shrimp tax- volume of water (a clear-plastic box 2-4 cm across onomists have learned to use the male antennae, works fine) and observe it carefully with a magni- with their bumps, processes, and acne-like orna- fying glass or dissecting microscope. If you can mentations, as the main distinguishing character- take your eyes off the continuous motion of ap- istics of species. pendages and look inside through the thin, semi- The abdomen is continuous with the posterior transparent exoskeleton, you will not see defined end of the thorax and bears at its terminus a pair body spaces filled with blood. What you will ob- of setae-fringed, fluke-like cercopods (Fig. 7.1, p. serve above the dark, food-filled gut is the dorsal, 140). These cercopods are used in concert with elongate heart pumping rhythmically and con- abdominal musculature in turns as well as in rapid tinuously to the beat of its unseen metronome - escape locomotion much as beavers, whales, or the heart pacemaker. lobsters would employ their tails and bodies. The blood, called hemolymph, is usually col- Should you touch a cercopod you may stimulate a orless, and it contains cells, the movement of flurry of such behavior, so it seems these struc- which will allow you to trace the general direction tures serve a rear-guard sensory function as well. of blood flow (Greene 1924). At times the hemolymph may contain dissolved, not cell- Circulatory system bound, hemoglobin, which of course is a red, iron-based respiratory pigment. Because the exo- So far our discussion has dealt with fairy skeleton is so thin, this internal color makes the shrimp parts that "meet the eye". But how about animal appear reddish. Under conditions of its "innards"? We begin with the circulatory sys- plentiful oxygen hemoglobin is absent (why make tem, a fascinating component of internal structure. it if you don't have to?), but in those species hav- Unlike your closed system of blood vessels, the ing the genetic ability, hemoglobin appears to be fairy shrimp has an open circulatory system to synthesized in response to low environmental transport oxygen, digested food materials, hor- oxygen levels, allowing the animals to increase mones, and wastes through its body. The system the supply of oxygen carried to their tissues. is said to be "open" because the long, dorsal, tubu- lar heart pumps blood anteriorly into the head Oxygen: its procurement and regulation region, whence it flows posteriorly through body spaces largely under the force of muscles contract- Oxygen (02) is required for metabolic proc- ing for other body functions. In other words, esses from which energy is released to accomplish 13 Ch. 3. Ecology of fairy shrimps bodily work like locomotion, growth, egg and which are the concentration of its dissolved salts sperm production, and all that sexual activity we and, therefore, the further reduction of its 02 will speak about with biological frankness later in content - strikingly so in briny waters. The big the chapter. Aquatic creatures must not only take problem is that in the face of these reductions the in sufficient 02, but most must also maintain the consumption of oxygen by pool creatures increases rate of its intake, though some species may possi- dramatically as their metabolic rate is elevated by bly just "tough it out" in the face of a pool's de- higher temperatures. To make matters worse, creasing supply; and a reality of pool life is that atmospheric oxygen is not redistributed to the the quantity of 02 does rise and fall, sometimes pool's volume at times of atmospheric calm. drastically, even within a few hours. Oxygen, Should the habitat contain phytoplankton or sub- when in its greatest concentration in water, is pre- merged aquatic vegetation, 02 is added to the wa- sent in minuscule amounts compared to air. ter by photosynthesis during daylight hours; but at While the atmosphere contains approximately night all of the pool organisms, plants included, 200,000 parts of 02 for every million parts of air continue to consume the absolutely necessary but (ppm), water, at its maximum, is capable of dis- limited supply of dissolved oxygen. As a conse- solving only slightly less than 15 ppm, and the quence, 02 may decrease dramatically until the amount actually available dwindles from there! time "ol' Sol" begins again to brighten the morn- Scary, eh? ing sky. There is importance as well as fascination in Having said all of that, we must now advise understanding the physical and biological rela- that not much is known about how fairy shrimps tionship between the amount of 02 in an aquatic gain their oxygen and cope with drastically environment, and the latter's temperature, salin- changing concentrations.
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