APPLICATIONS OF OSTRACODA

RosQlie F. Maddocks, editor

Proceedings of the Eighth International Symposium on Ostracoda

July 26-29, 1-982

Published 1983 by the Department of Geosciences University of Houston-University Park

Houston, Texas 77004

APPLICATIONS OF OSTRACODA (R. F. Maddocks, ed.) Univ. Houston Geosc. 1983: 649-658.

ULTRASTRUCTURE OF CARAPACE-SENSILLA IN AURILA CONVEXA (BAIRD, 1850) (OSTRACODA, CRUSTACEA)

Dietmar Keyser Zoologisches Institut und Museum, Martin-Luther-King-Platz 3,

2000 Hamburg 13, F. R. Germany

ABSTRACT--The family Hemicytheridae within the podocopid Ostracoda arouses many questions about its systematic connections. It is thought that a morphological survey of the sense organs of the shell might help to clarify part of these problems. A first study of a hemicytherid ostracod, Aurila convexa, has been undertaken. Aurila convexa is known to have three distinct types of pores and associated bristles on the carapace. A fourth type of bristle emerges from the marginal pore-canals. The inner organisation of the bristles with their neural elements is studied and described. The formation of the marginal bristles during molting is shown. Comparisons are made to the known information from other ostracods and crustaceans. The possible function of the bristles is discussed.

INTRODUCTION

Ostracods possess a calcified shell. Sensory bristles penetrate this carapace to give information to the encased animal. The morphologic structures of these bristles resemble fully the morphologic features already known of the bristles in insects. One or several sensory cells are encased by a thecogen, a trichogen, and a tormogen cell (Altner, 1977; Andersson, 1977; Keyser, 1981).

The Hemicytheridae in the podocopid ostracods are thought to be highly evolved. They have developed a typical system of sensory bristles. Four types can be distinguished, the marginal bristles, the pore cone, the ridge, and the mesh bristles (Keyser, 1980). All hemicytherids s. str. have the same set-up. The aim of this study is to characterise and describe the nervous apparatus of these bristles and note the possible differences, to get an idea of the effectiveness of such an organisation, and to judge the systematic value of these structures.

MATERIAL AND METHODS

Aurila convexa (Baird, 1850) was collected at 20 m depth in the Gulf of Naples. Hirschmannia viridis (0. F. MUller, 1785), Leptocythere lacertosa (Hirschmann, 1912), L. psammophila Guinaume, 1976, Cyprideis torosa (Jones, 1857), Cytherura striata Sars, 186"5', Semicytherura nigrescens (Baird, 1838), and Heterocythereis albomaculata (Baird, 1850) were collected· from the North and Baltic seas. This material, together with Cyprideis beaveni Tressler and Smith, 1948, from South Florida, was used for additional information.· ,specimens used for study in the scanning electron microscope (SEM) were brushed clean, air-drJec::lf glued to a stub with double-sided sticking tape, and gold-coated. The micrographs were,t~~~nwith a Cambridge S4 SEM (this SEM is a loan from the Deutsche Forschungsgemei~haft~ whose help is gratefully acknowledged).

The material for light- and transmission-electron microscopy was fix~d li~e i~ 2.596 glutardialdehyde in phosphate-buffer after Slirensen or in 3% GA and 8% tanmc aCId WIth 0.5 M. Sarensen phosphate-buffer at pH 7. After rinsing in buffer it was post fixed in 2% Os04 in phosphate-buffer. The material was then decalcified in an aqueous solution of EDTA, washed in buffer, dehydrated in graded acetone, and embedded. in .Spurr'~ medium (S~urr, 1969). Sections were cut on an OM/UII Reichert microtome. Seml-thm sectIons were stamed

650 with toluidine blue and pyronine according to the method of Holstein and Wulfhenkel (1971). The ultra-thin sections were stained with uranyl-acetate (Stemper and Ward, 1964) and lead citrate (Reynolds, 1963) and viewed in a Zeiss EM 9. .

RESULTS

Marginal pore-canals and bristles.-The whole family Hemicytheridae is characterized by a margin with many pore canals (Hartmann, 1967). The pore canals are either branched or straight. They are thought to mark the line of concrescence between the outer and inner lamella. The separation during moulting in the larval stages takes place at the boundary of the calcified and noncalcified inner lamella (Figure 6). The radial pore canals in Aurila convexa (Baird, 1850) (Figure 1) contain bristles with feather-like appearance like the ones seen in Figure 2 of Heterocythereis albomaculata (Baird, 1850). They have three dendrites. Two of them reach through the whole bristle, while the third terminates at the base of the bristle (Figure 8). No outer receptor lymph cavity is present; therefore a chemical sense is not likely (Heimann, 1979; Altner et aI, 1981). During moulting the new bristle is formed completely just beneath the old one, encased by a fold of the new cuticle (Figures 7-10). The dendrites are encased by a chitinous (?) cylindrical tube, formed either by the thecogen or the trichogen cell. They penetrate the tip of the new bristle and run free in the channel protected by the cylindric tube to the old bristle, so ensuring the function of the old bristle even during moulting (Figure 7). The feather-like branching of the bristle is achieved by the activity of the second enveloping cell (tormogen cell), which separates distally from the trichogen cell and ramifies, thus building the cuticle of the branches of the bristle (Figures 8-10). The outer dendritic segments are connected by a small rootlet to the inner dendritic sediments. The inner receptor lymph cavity is small. The axons of several bristles run together as a bundle of nerve fibres, between 12 and 16 axons per bundle at the base of the epidermal layer in the body cavity (Figure 17). An enveloping structure has not been observed.

Mesh-pores and bristles.--The mesh-pores have a diameter of 10 pm and are situated at the base of circular depressions on the valve with a depth of 3 to 4 }Am (Figure 11). They are formed as sieve-plates containing between 70 and 80 single pores with a diameter of 0.5 pm. The sensory bristle emerges from a pore of about 1 pm. The bristle itself has a stout stem of 6 pm length and a diameter of 1 pm. The stem has at the base several (4-9) slender branches with a diameter of 0.1-0.2 pm and a length of 20 pm (Figure 3).

EXPLANATION OF FIGURES 1-5

Abbreviations: b = bristle, bc = body cavity, cil = calcified inner lamella, fec = first enveloping cell (trichogen cell), iec = inner epidermal cell layer, il = noncalcified inner lamella, Ip = lateral pore canal, m = mitochondria, n = nerve, ocl = outer calcified layer, oec = outer epidermal cell layer, rp = radial pore canal, sec = second enveloping cell (tormogen cell), sp = sieve pore, tc = thecogen cell. Scale: bar = 1 pm unless otherwise indicated.

Fig. l-Aurila convexa male, right valve.

Fig. 2--Heterocythereis albomaculata, frontal marginal bristles.

Fig. 3--Aurila convexa, mesh-pore and bristle; note the pore-cone bristle at lower left side ..

Fig. 4-Aurila convexa, ridge-pore and bristle.

Fig. 5--Aurila convexa, pore-cone pore and bristle, upper left side mesh-pore with bristle.

651

663 In adult animals the shell shows about 90 to 100 of these mesh pores. They have

distinct places on the shell within this species. The internal organization of the mesh bristles resembles the one already described in Hirschmannia viridis by Keyser (1981). Four dendrites lie crosswise together, forming two pairs facing each other. These pairs have different lengths. One of them terminates at the chambering of the chitinous (?) theca (Figure 12), the other extends to the end of the bristle, although it is hard to distinguish it at the distal part. All three types of enveloping cells, the thecogen, trichogen, and tormogen cell, are well developed (Figure 12. The distal part of the tormogen cell surrounds the base of the sieve pores. These pores narrow to the base and contain an electron-dense substance (Figure 13, 14). This substance is either still formed by the animal or the ostracod uses it in a certain way, for the surrounding tormogen cell has a concentration of mitochondria at this place. This means that a high metabolic activity takes place in this part of the cell. No reservoir of any substance was found.

Ridge-pores and bristles (Figure 4).-The ridge-pores have a diameter of 6 pm, and the depression in which the sieve-plate lies is about 7 ,urn. The number of single sieve­pores could not be counted exactly, but it was estimated at about 70 single pores. The sensory bristle emerges from a pore of about 1 pm diameter. The bristle has a length of 35 ",m and a diameter at the base of 1 ,urn, thickening instantly to 3 ,urn and then gradually diminishing to the end. It does not branch at all (Figures 4, 18). The internal structure resembles that of the mesh-pores and bristles. The number of ridge bristles counted on adult specimens was between 55 and 65. They also have distinct places on the carapace.

DISCUSSION

Aurila convexa has been chosen as a typical hemicytherid to study the sensilla of the carapace. The results of this study show that the morphology of the sensilla does not exhibit a fundamental difference from the already-known facts (Gnatzy and Tautz, 1980; Guse, 1980a, 1980b), although it has to be pointed out that there are slight differences that might lead to further information about the role of sensilla in the phylogeny of Ostracoda.

One result is that a different number of dendrites seem to be present in different bristles. The marginal bristles have three, mesh and ridge bristles have four, and the pore­cone bristles have one. A t this time is cannot be said whether this is consistent through at least all hemicytherids. The inner structure of the ridge and mesh bristles was found in all cytherid species studied so far. These two crosswise-lying pairs of dendrites terminating in different places were found in Hirschmannia viridis (see Keyser, 1981, 1982), in Leptocythere

EXPLANATION OF FIGURES 6-10

All illustrations are of Aurila convexa. See Figures 1-5 for explanation of abbreviations.

Fig. 6-Section through marginal area showing an animal prior to moulting. Arrow shows the area where the inner lamella breaks and the animal with the new shell moves out.

Fig. 7--Section through marginal pore-canal. It shows at the upper left side the marginal bristle. In the pore-canal the still-folded new bristle is recognizable and also the connection of the nerve from the old bristle to the newly formed one (n).

Fig. 8--A perpendicular section through the newly formed bristle. It shows that the branching of the bristle is done by the second enveloping cell (tormogen cell), while the first enveloping cell (trichogen cell) is only surrounding the dendrites.

Fig. 9--Same as Fig. 8, section more distal.

Fig. I~-Same as Figs. 8, 9, section more distal, showing single branches of bristle. See also Fig. 2 for comparison of bristle.

654 lacertosa, ~. psammophila, Cyprideis torosa, C. beaveni, Cytherura striata, and Semi­cytherura nigrescens.

The only record of a similar feature in other Arthropoda could be found in the trichobothria of the spider Tegenaria derhami (Scopili) (Christian, 1971a) , although the resemblance is limited only to the proximal arrangement of the cilia. The attachment and other structural features are entirely different.

The explanation of the difference in the number of dendrites is not yet clear. There might be a functional aspect to it. The ones with more than one dendrite might indicate the stimulus direction, while the ones with only one dendrite are only coarse mechano­receptors and are, therefore, restricted to the nodes, which rise above the surface of the valves. A chemical sense has to be ruled out, for no outer receptor lymph cavity is present, and no pores in the bristle were found. The possibility of temperature sensing is likely but cannot be proved. Only electro-physiologic investigations can confirm these speculations.

In Aurila the sieve pores seem to function also out of the moulting time. In Hirschmannia, Cyprideis, Xestoleberis, and Leptocythere the sieve pores are merely left­overs from the moulting process (Keyser, 1982). The sieve-pore function in Aurila of secreting or excreting a substance is confirmed by two facts: first, the high number of mitochondria at the base of these sieve pores, and, second, the electron-dense substance in every pore-canal, together with the deep nesting of the base in the cell, even in the adult stage. The explanation here could possibly be a release of a hydrophobic substance, in order to keep the vicinity of the mechanically-sensitive bristles free of dirt or other creatures such as diatoms. The observations that in living Aurila the meshes with sieve pores are always clean tends to favour this opinion.

It is noteworthy that the sieve-pores in the hemicytherids are active throughout their life. How the release of the substance is furnished is another interesting question not treated herein. Besides the development and use of the sieve-pores in the moulting cycle, the patterns of moulting the sensilla seem to be the same among insects, Malacostraca, and Ostracoda. The development of bristles in a pocket of the new cuticle has been described (Gnatzy and Romer, 1980; Guse, 1980a, 1980b; Dexter, 1981).

Summarizing, it can be said that the hemicytherids have some unique features among the other ostracode families. They possess four different types of sensilla on their carapace. Besides mesh and ridge bristles, they have a different number of dendrites. The sieve pores seem to function the whole lifetime as secretion organs.

EXPLANATION OF FIGURES 11-14

All illustrations are of Aurila convexa. See Figures 1-5 for explanation of abbreviations.

Fig. ll--Section through sieve-pore opening of a ridge-pore.

Fig. 12--Section through the same pore at a more proximal region. Note the electron-dense material in the sieve-pores and the large quantity of mitochondria in the second enveloping cell. The chambering of the theca is clearly seen, and only two dendrites are surrounded by it.

Fig. 13--Section through a mesh pore, showing essentially the same structures as the preceding figure.

Fig. 14--Part of a mesh-pore, showing the filling of the sieve pores and the base in the second enveloping cell. Note the thecogen cell at the lower right still encasing four dendrites; three are visible at the edge of the photograph.

655

ACKNOWLEDGEMENTS 657

I would like to thank Prof. Dr. Hartmann, Hamburg, for his information and help. Dr. G. Bonaduce was so nice to help me with the material and with many fruitful discussions. Thanks are also due to Miss E. Gans for her steady and friendly technical help.

REFERENCES

Altner, H. 1977. Insektensensillen. Bau und Funktionsprinzipien. Verhandlungen der Deuschen Zoologischen Gesellschaft 1977: 139-153.

Altner, H., Ch. Routil, R. Loftus. 1981. The structure of bimodal chemo-, thermo-, and hygroreceptive sensilla. Cell and Tissue Research 215: 289-308.

Andersson, A. 1977. The organ of Bellonci in ostracodes: An ultrastructural study of the rodshaped or frontal organ. Acta Zoologica 58: 197-204.

Christian, U. 1971. Zur Feinstruktur der Trichobothrien der Winkelspinne Tegenaria derhami (Scopoli), (Agelenidea, Aranea). Cytobiology 4: 172-185.

Dexter, B. L. 1981. Setogenesis and molting in planktonic crustaceans. Journal of Planktonic Research 3: 1-13.

Gnatzy, W., and J. Tautz. 1980. Ultrastructure and mechanical properties of an insect mechanoreceptor: Stimulus-transmitting structures and sensory apparatus of the cercal filiform hairs of Gryllus. Cell and Tissue Research 213: 441-463.

Gnatzy, W., and F. Romer. 1980. Morphogenesis of mechanoreceptor and epidermal cells of crickets during the last instar, and its relation to molting-hormone level. Cell and Tissue Research 213: 369-391.

Guse, F. W. 1980a. Fine structure of sensilla during moulting in Neomysis integer (leach) (Crustacea, Mysidacea). Experientia 36: 1382-1384.

Guse, G. W. 1980b. Development of antennal sensilla during moulting in Neomysis integer (Leach) (Crustacea, Mysidacea). Protoplasma 105: 53-67.

EXPLANATION OF FIGURES 15-18

All illustrations are of Aurila convexa. See Figures 1-5 for explanation of abbreviations.

Fig. 15--Section through a pore-cone pore, showing second enveloping cell, first enveloping cell, the chambering thecogen cell, and in the center only one dendrite. Note the scale, which shows that this channel is only half the diameter of the preceding ones.

Fig. 16--Section near the base of the cuticular bristle, showing in the lower right an opening to the outside, in the middle the sensory bristle with its nervous elements, still connected to the shell. In the upper left part of the pore-cone the bulb is cut, showing that it is built by the second enveloping cell. Note the few sieve pores (sp).

Fig. 17--Section through the marginal area, perpendicular to the shell, showing outer and in~er epidermal cell layer connected by a small bridge at which a bundle of nerves runs. Jp th~s case there are 12 single nerve fibres, indicating that they come from four different brIstles. No enveloping structure is present.

Fig. 18--Section along a ridge bri,stle showing the narrow base of this bristle. Note the sieve pores present.

658 Hartmann, G. 1967-1975. Ostracoda. In, H. Bronn, Klassen und Ordnungen des Tierreiches, 5, 1.- Abteilung, 2. Buch, 4. Teil, 1-5 Lieferung.

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Keyser, D. 1982. Development of the sieve pores in Hirschmannia viridis (0. F. MUller, 1785). In, R. H. Bate, E. Robinson, and L. Sheppard, eds., Fossil and Recent Ostracods. British Micropaleontological Society Series, p. 51-60.

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