The Ostracod a Neglected Little Crustacean

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 hypodermis. 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. The hypo- of the ostracod "on the half shell". Remove stm is a movable sternum-shaped struc- an adult by a small pipette and place it on ture behind the mouth. The long uniramous a blotter. The animal, finding itself out of antennules are attached to the forehead. water, will close the valves, and the outside The antennae are joined to the sides of the of the carapace can be thoroughly dried. head near the junction of the forehead and The ostracod can be handled easily by a uppcr lip. The mandibles are equipped small sable-hair brush moistened slightly. In with tceth that meet in the mouth and are the field of a binocular microscope, put a used in n~astication. The maxillae lie along small quantity of Duco or other quick-dry- the sides of the hypostome, which they ing, ~,ater-insolublecement on a small glass more or less hide. There are three pairs of slide and immediately place the ostracod thoracic legs, each of different shape, and a in the cement on its side. Be certain the terminal pair of furcae in the rear half of cement attaches the ostracod by only one the ostracod. Thc genitalia lie between the valve. Submerge slide and ostracod in a Zd thoracic legs and the Furcae. shallow dish containing a chloretone solu- Appendages and Furcae tion. If a few small bubbles adhere to the carapace, they can be dislodged with the The antennule (Figs. la, 6) has seven brush. After several minutes, the animal podomeres, which are interpreted as coxa, will relax its adductor n~usclesand gradual- basis, and five podomeres of the endopod. ly open the valves. When the specimen is The four distal podomeres are provided with long setae used to explore the area in front of the animal. The antenna is a walking is a clasping organ used to hold fast to the leg, equipped with strong claws at the end. edge of the female va1L.e during copulation. It has a short, strong coxa, a long basis, The flexible terillinal part is called the finger. and an endopod of three podomeres. The long seta at the end of the basis may be a The 2d thoracic leg (Figs. la, 6) is long vestigial exopod. The antennae are dimor- and uniramous. It is used in walking. The phic (Figs. 2a, 6). In the male the penulti- endopod is long, tapering, directed back- mate podomere is nearly divided in half ward, and ends in a strong, curved claw. and bears, on its inner face, two heavy club- This appendage in some species of Candona shaped setae thought to be sensory and is slightly dimorphic in the lengths of the used in copulation. On the rear edge of the podomeres and claw. first podomere of the endopod there is a The 3d thoracic leg is modified as a sense club, a special sensory sera said to be cleaning organ. The undivided protopod a chemoreceptor. hangs vertical, t&e first podomere of the en- The mandible is a powerful appendage. dopod is directed backward, and the rest of The long coxa is strongly chitinized and the leg is directed dorsally. The protopod provided with masticating teeth at its ven- is extremely flexible; it can pivot and enroll tral end. The palp, which extends forward on its axis to bring the leg to many posi- and includes the basis and the three- tions. The very small terminal podomere podomere endopod, has small terminal has three long setae, which are used to claws; it sweeps food particles backward to sweep out foreign particles from inside the the nlouth. A thin, delicate, setiferous plate carapace. Each furca has a long rodlike ralnus and on the top of the hasis is the exopod. It is two terminal claws. The width of the said to be respiratory, but this may be rainus and the size and curvature of the questioned (probably all the surface of the claws differ slightly in male and female. body takes part in respiration). The furca is used to clear foreign objects The maxilla is difficult to study unless from the posterior part of the carapace. It removed from the rest of the animal. Its does not appear to be locomotor. voluminous base, hanging down over the side of the hypostome, consists of an undi- The Valves vided protopod. The endopod is a palp of Each valve contains two calcareous parts. two podomeres, which extends forward to The distal part is the outer lamella; it is the the rear edge of the mouth. Medial to the - only part seen when the carapace is closed. palp, in a horizontal row, there are three The proximal part, a narrow flange just endites, often called the "masticatory proc- inside the free edge, is the duplicature esses," although they do not masticate. The (Fig. 3), so-called because it duplicates tile palp and endites have numerous setae at chitin-calcite-chitin structure of the outer their ends, which convey food particles to lamella. The two parts are joined together the mouth. The exopod is a long, delicate, by a thin layer of chitin, the adhesive strip. setiferous plate; it moves rapidly, several The open channel between the duplicature times a second, and creates currents along and the outer larnella is called the vestibule the side of the body that aid respiration and (Fig. 3). It extends from the line of con- carry food particles into the carapace. crescence (the inner border of the adhesi\-e The 1st thoracic leg is pivoted to a lateral process on the posterodorsal end of the strip) to the inner margin (the inner edge hypostome. The protopod is large, with the of the duplicature). The duplicature has proximal part vertical and the distal part thin chitin ridges on the medial face, which directed forward; the terminal setae convey fit into grooves of the opposite duplicature food particles forward towards the mouth. and hermetically seal the valves when The exopod is an inconspicuous little proc- drawn together. ess with a few setae. The endopod is di- The valves are attached to the adductor morphic. In the feinale (Fig. la) it is a muscles, which draw them together and to long lobe on the rear part of the leg, pro- muscles that operate several anterior ap- vided with a few terminal setae. In the pendages, lead to the endoskeleton, and ad- male (Figs. 2c, d) the left and right endo- just the body. The muscles leave permanent pods differ slightly in size and shape; each ston food hnll 1

esop unub

Fig. 5. Candona suburbana. Digestive system. Right half of the system in ihe male. The body framework of the left side is added in solid black.

circumesophogeal

cerebrum HI-ventral chain of ganglia upperlip] paragnath' peribuccal ring

Fig. 6. Candona suburbana. Nervous system. Right half of the system in the male; the body framework of the left side is added as in Figure 5. scars at their points of attachment. New valves are formed after each ecdysis. Sex Systems They are secreted by the hypodermis. Nerves In both female and male, the sex system in the hypodermis are connected to setae on is paired, and the left half is not connected the outside of the valve by pore canals. The to the right. The female system (Figs. 4a, small round pores through the outer lamel- b) includes ovaries, uteri, uterine openings, la are known as normal pore canals, and vaginae, and seminal receptacles. The ex- those through the adhesive strip as radial ternal parts are a pair of genital lobes be- pore canals. tween the 3d thoracic legs and the furcae. The male (Fig. 16) has much greater Each lobe is pointed at the rear in Candona, volume in the posterior half of its carapace and the shape of the tip is regarded as a than the female (Fig. la). The additional specific character. Each genital lobe con- space accommodates the large penes and tains two openings, a uterine opening and the Zenker's organs. a vagina. 24 thoracic leg

Fig. 7. Candona suburbans. Musculature. a. Female with muscles attached to dorsal part of right valve. b. Male with muscles attached to the endoskeleton, adductor muscles, dimorphic ventral muscle, and furcal muscles in the right half of the body.

To study details of the internal female study of oogenesis, because stages of dif- organs it is necessary to make serial sections. ferentiation are present in positional, as The specimen must be decalcified before em- well as chronological, sequence. The ovary bedding. Sections show clear differentiation starts in the posterior part of the hypo- of tissues with Ehrlich's haematoxylin and dermis, curves down, forivard, and up, and eosin. enters the body as the uterus without a The ovary (Fig. 4a) is often left in the demarcation. hypodermis when the valve is removed. If The uterus (Fig. 46) is distended with not, it can be found in a smear of the body eggs in every adult female. In zhe front contents or identified in serial sections. The section of the uterus, the shell is added to posterior end is the syncytium. Eggs are the eggs, first as snlall granules and finally formed and push forward with growth. The as a strongly chitinized coating. The eggs ostracod ovary is particularly good for are laid through the uterine openings on the inner edge of the genital lobe. tube and through the inner lobe of the The vagina is both the adit and exit for penis. spermatozoa. The vagina leads into a spiral canal, which can be seen through the body Digestive System wall, and theretiy into the seminal receptacle. Seminal receptacles in bred females The ostracod has an elaborate set of are distended with scores of spermatozoa. food-gathering devices. The upper lip is (In syngamic species, unbred adult females fixed, except for a median movable process are hard to find.) The seminal receptacles in its posterior region, but the oral edge is are not connected to the uteri. The genital serrate. The hypostome, however, is swung lobe contains numerous muscles that draw back and forth by muscles and serves to the vagina and uterine opening together, concentrate particles in the mouth (Fig. 5). and spermatozoa are thought to pass into Along its front edges the hypostome has a the uterus by this route. pair of soft setiferous lobes, the paragnaths The male penis is strongly modified in (Fig. 6), which move food forward. The the ostracod by the carapace. It must be rakes (Fig. 5), two chitinous serrate struc- long enough to reach out from the carapace tures embedded in the front part of the of the male into that of the female, but it hypostome, lift particles upward. must also be retracted when the male closes Food is swept backward to the mouth by his valves. It is very complex, and not coni- the mandibular palp and forward to the pletely understood. Each of the paired mouth by the palp and endites of the penes has lateral chitinous shields and three maxilla and by the protopod of the 1st lobes inside (Figs. IB, 4r). Only the front thoracic leg. end is joined to the body. The only masticatory organs are the man- The four testes in each half of the hypo- dibles, which have strong teeth at the ven- dermis are curved and parallel; they lie in tral ends of the coxae. The teeth meet in about the same position as the ovary in the the center of the mouth. female. They enter the body and unite tc The mouth, esophagus, pharynx, and ven- form a vas deferens. Each vas deferens has tral pharynx are covered with chitin and a devious circuit before it reaches the form the stomadaeum. A pair of glands in penis. It re-enters the hypodermis in the the upper lip, called the salivary glands, anterodorsal region, curves parallel to the empty into the mouth and are supposed to free edge, and extends to the posterodorsal secrete digestive fluid. Food is moved up- region as a blind section. This part of the ivard in the esophagus by muscular action vas deferens is sometimes confused with the and itched into the stomach by setae on testes, and some accounts list five testes in the movable pharynx and ventral pharynx. the hypodermis. Spermatozoa reaching the A liver or hepatopancreas empties into blind end reverse their direction. In the each side of the stomach. The food forms ventral region the vas deferens divides, a food ball in the stomach, surrounded by with the second branch running parallel to digestive fluid, presun~ablysupplied entirely the first and entering the body in the an- by the liiers. Later it passes through a nar- terodorsal region (Fig. 4c). Thence, the ro\\. intestine into the rear gut and becomes vas deferens extends to the posterior half a fecal pellet. Most, if not all, digestion of the body, loops back and forth a few takes place in the ston~ach. There is little times, encircles the Zenker's organ, and en- proctodaeum, and the anus is a simple larges ?o form a seminal receptacle. In unmuscled opening. smear preparations, the senlinal receptacles are usually broken open, releasing the mass Nervous System of long spermatozoa. The nervous system resembles that of In back of the seminal receptacles are the other crustaceans. The cerebrum is located Each Zenker's organ sur- Zenker's organs. in the lower part of the forehead. It can be rounds a narrow Dart of the vas deferens. subdivided only by the nerves leading into It consists of seven wreaths of chitin spines it. The circumesophageal ganglia join the and an enclosing membrane. Hundreds of cerebrum to the ventral chain of ganglia n~usclesconnect the spines in such a tiny (Fig. 6). From the front end of the ventral \ray that the organ can rhythmically con- chain there is a peribuccal ring of nerves tract to pump spermatozoa out by a narrow extending into the upper lip. The ventral chain tapers posteriorly and ends at the In addition to the central nervous system, literature there are ganglia in the basal parts of the Gunnar Alm, "Monographie der appendages. Sensory nerves lead from the Schwedischen Siisswasser-Ostrocoden appendages, furcae, digestive system, and nebst systemotischen Besprechungen hypodermis to the central systen:. der Tribus Podocopo," Zoologisko Bidrag fran Uppsala, 4 (19161, 1-248. Carl Clous, "Beitrage zur Kenntniss Musculature der Siisswosser-Ostracoden," Arbeiten aus den zoologischen lnstituten der Muscles in the ostracod are attached to Universitot Wien und der zoologischen the valves, the framework of the body, and Stotion in Triest, Port 1: 10 (18931, 147-216, 12 pls.; Part II: 11 (18951, the endoskeleton. Their arrangement can be 17-48, 5 pls. worked out from serial sections. From the C. C. Hoff, "The Ostracods of Illinois, dorsal part of the valve (Fig. 7a) muscles Their Biology and Taxonomy," Illinois Biological Monographs, 19 /1942), 1- extend to the antennules, antennae, mandi- 196, 9 pls., 1 fig. bles, hypostome, and rear part of the body. Walter Klie, "Ostracodo. Muschel- Other nluscles attached to the valve (Fig. krebse," Biologie der Tiere Deutsch- 7b) are those to the endoskeleton, the ad- lands, 16 (19261, 1-56, 43 figs. ductor muscles, and (in the male only) Dom Remacle Rome, "Herpetocypris ventral muscles. The ventral muscles are reptons (ostracode), Etude morpho- logique et histologique, I. Morpho- used in copulation; by contraction they logie externe et systeme nerveux," Lo strongly compress the body, turn the Cellule, 51 (1947), 51-152, 15 pls., 20 Zenker's organs about 90 degrees, and figs. swing the penes out of the body and for- G. 0. Sars, An Account of the Crus- ward. They also unfold the complicated tacea of Norway with Short Descrip- tions and Figures of All the Species, penes, which contain no muscles. Vol. 9, Ostrocoda (Bergen: The Bergen The endoskeleton is an elaborate chitin Museum, 1922-19281, 277 pp., 119 pls. structure behind the esophagus and below the stomach. It is embedded within the body, and suspended from the dorsal part of the carapace. It has muscles leading to the appendages, the hypostome, the rakes, and the adductor muscles. The muscles to the mandibles are exceptionally well de- veloped. The furcae are operated by muscles attached to the chitin framework on the rear part of the body (Fig. 76). Ontogeny The ostracod grows by ecdysis, like other arthropods. It meticulously sheds the chitin covering of the appendages and body as well as the valves. Candona has nine instars, of which only the last is mature. Space precludes a detailed account of the succession of appendages. In general, the animal hatches with antennules, antennae, and mandibles, and other appendages are added in later instars. Appendages first appear as anlagen and later add podomeres and setae to assume the definitive form. Sex organs appear in the eighth instar, but are neither clearly differentiated nor func- tional until the final, or adult, instar.