The Ostracod a Neglected Little Crustacean
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Reprinted from Turtox Kews, Vol. ;k, 110. 4, VO~.34, Ilo. 5 and Vol. 34, No. 6, April, May and June, 1956 The Ostracod A Neglected Little Crustacean By Robert V. Kesling CURATOR OF MlCROPAlEONTOLOGY MUSEUM OF PALEONTOLOGY UNIVERSITY OF MICHIGAN O~racotls li1.e in Inany en\ironmen~s. ANN ARBOR, MICHIGAN Tlxy are found in open oceans, bays, swamps, lakes, streams, ponds, temporary pools, and springs. Some are pumped out of wells from underground waters. One species has even been reported in the datnp leaf mdd of a tropical fcuest. Snnx osua- Textbooks 011 invertebrate ztnlogy say cods live in the Arctic and Antarctic re- very little about ostracods, if indeed tlxy gions, whereas others abound in very warm nrentinn them at all. Most texts state that springs. Most ostracods are bentlx~nic,but the carapace is bivalved, the body lacks seg- sorne swim free nearly all of tlreir lives. mentation, and crustacean characteristics are Only a few species are cm~rnetwalon other e\.ident in the jointed appendages. With crustaceans and fish, and none is knr)wn to rare exceptions, the illustrations are of dis- be parasitic. Many fresh-water species occur nlembered parts of ostracods, and show both in Europe and in North America be- little or nothing on the organization of the cause of a peculiarity in their life cycle. animal. Several statements that have ap- Their eggs can withstand desiccation for peared in literature are erroneous, and the long periods of time; it was discovered that figure in at least two textbooks has the sonic eggs remain viable after drying for mouth above the upper lip instead of hc- many years. Eggs blown by the wind or hind it. carried in mud on the feet of birds have For one reason or another, most zoolo- only to settk in fresh water to hatch ancl gists have avoided studying ostracods. Per- start the species in a new locality. haps some have been discouraged by the Sonie species reproduce by synganiy and s~nallsize of the animals. Adults of many others by parthenogenesis; none is herma- species are less than 1 millimeter long. iZ phroditic. One species is known to he svn- more serious hinderance to study, however, pa~iiicin one environment and partheno- is the ostracod's general body plan. The genetic in another, but, insofar as indicated diagnostic structures are enclosed in a bi- I,y meager reports, all other species have valved carapace. It is absolutely necessary only one metld of reproduction. Many to remove the valvcs from around the body syngamic species show dinwrphism in the and appendages to classify an ostracod. Rut form of the carapace. One of the unusual as we shall see presently, the dissection is features of ostracods is the size of the nnt as great an obstacle as many people be- spermatozca: in males of some species, they lieve, and the difficulties are rewarded by are several times as long as the carapace. acquaintance with the biology and morplwl- These spermatozoa are absolutely, a5 well as ogy of this not-very-well-known crustacean. relatively, the largest in the animal kingdom. F'q 1. Condona suburbans Hoff. a. Female with left valve removed. b. Male with left valve removed. In each figure the maxilla is turned to show the endites, which normally lie medial to the palp. Both figures to scale at the left. The Genus Candona bottom dwellers, crawling on or l~urrowing in the detritus on the floor of ponds and Cnrzdona is ideal for laboratory study. lakes. Some are living within a few lniles Ostracods of this genus live in quiet bodies of nearly every reader of this page. Cat2doza of \\.ater in temperate latitudes. They are can be separated from nearly all other Fig. 2. Candona suburbana. a, b. lnner faces of distal podomeres of female and mole antenna. c. Outer face of male left 1st thoracic leg. d. lnner face of endopod of male right 1st thoracic leg. Fig. 3. Candona suburbana. lnner face of female right valve. fresh-water -genera bv its white or very mature in about one month. pale golden carapace, the shape of the valves (Figs 1, 3), and by its inability to Observing the Living swim. he approximately 100 species of Certain features are best observed in liv- the genus have only slight differences in de- ing specimens. A few adults can be re- tails of the appendages and in the shape moved from the aquarium to shallow dishes and size of the valves. Males are known in by means of a pipette. As can be seen with only about half of the species, and the rest a binocular microscope at about 36 diame- are assumed to be parthenogenetic. ters magnification, the two valves of each Caadoncl species are easily maintained in ostracod are hinged only along their dorsal aquaria and rnultiply rapidly. They thrive borders, and have free edges around the rest best on diatoms and algae, but most do of their periphery. The hinge is covered by well on dead leaves. The aquarium should a thin ligament, which by contraction opens be aerated: it can be stocked with water the carapace. If the animal is disturbed by plants if an air hose is not available Very gentle poking with a needle, it will close the young specinlens can be isolated in individ- valves quickly, hermetically sealing itself ual \ ials and their ontogeny followed. They inside. Although the valves are composed of seminal receptacle opening seminal vesicle ~enker'sorgan Fig. 4. Candona suburbana. Sex systems. a. Female gonad in the hypo- dermis. b. Right half of female system in the body. c. Right half of male system. a layer of calcite between layers of chitin, They are the adductor muscles, which ex- they are thin enough to reveal faintly some tend through the body from one valve to structures of the animal within. The animal the other and close the carapace by their can be held on its side by a thin cover glass. contraction (Fig. 3). I'osteroventral to the The body shows up as a pouchlike mass adductor muscles, a rapid rhythll~icmotion suspended from the dorsal region of the can be observed; this is the beating of :I carapace. In the anterodorsal region, just long exopod plate on the maxilla. Antero- below the front end of the hinge, there is a ventral to the adductors there is a very dark dark spot that now and then brilliantly re- area, which includes the teeth and setae of flects light. This is the single median eye. the oral region, stained with juices of algae Near the center of the valve, slightly ante- on which the animal feeds. rior, several spots can be discerned through If the specimen happens to be a female, the valve, arranged with one above and the several eggs can be seen in the posterior others closely spaced and forming a rosette. half of the body. If it is a male, seven sllort parallel bars silo\\. through the poster- completely anesthetized run it through alco- odorsal area of the \-alve; these are wreaths hol to xylol, being careful not to expose the of chitin spines inside the ejaculatory ap- upper valve. If any part of the specimen parati, or Zenker's organs (Figs. 16, 4c). In dries during the procedure, air bubbles will the postero\-entral region of the valve the adhere and cause difficulty in the dissection. gonads can be recognized. They lie in the When the ostracod is in xylol, slip the tip hypodernlis, a thin flap of tissue extending of a bent needle under the edge of the from the body and lining the inner sufxe upper valve and break the adductor muscles. of the \-all-e. The female has a wide band, The upper \alve can then be removed entire the ovar!., in each valve, whereas the male and placed to one side for further examinil- has four narro\\. parallel bands, the testes, tion. This kind of prepar~ttionshows many in each valve (Figs. 4a, c). The long hairs features of the body plan that "smear" on the surface of the valve are setae, which preparations do not A little practice will lead through the shell to nerves in the enal~leone to make "half shell" dissections underlying hypodermis and warn the ostra- accurately and efficiently. cod of contact with other objects. If undisturbed for a minute or two, the General Body Plan ostracod \vill open the valves and thrust out two pairs of appendages along the ventral As can now be readily determined. the border, the antennae near the front and the body of the ostracod has no distinct bound- second thoracic legs near the rear. These ary between head and thorax. It lacks an are the \valking appendages. When the ani- abdomen. The body narrows abruptly be- mal rights itself, two thin appendages are hind the head, and the thorax tapers to the swung out from the anterodorsal region, posterior end of the body. The anterodorsal equipped Ivith long delicate filan~ents,and part of the head is the forehead, the antero- used to "feel" the area in front of the ani- ventral part the upper lip, the ventral part mal. They are the antennules, which prob- the mouth, and the posteroventral the hy- ably sene as tactile and balancing organs. postome (Fig. 4c). The forehead and upper lip are joined and rigid, with a framework of chitin rods. The uppcr lip is thick and Needle Dissection serrate on the rear edge, which forms the A very instructive dissection can Oe made anterior border of the mouth.