Postembryonic eye growth in the seashore isopod Ligia exotica (Crustacea,Isopoda)

The eye of Ligia exotica is of the apposition type and has open rhabdoms. The facets are hexagonal, and the dioptric apparatus consists of a flat cornea and a spherical crystalline cone placed in the center of two large cone cells. Each ommatidium has seven regular retinula cells and one eccentric cell; a basement membrane forms the proximal boundary of the retina. With increases in body size from 0.6 to almost 4.0 cm, facet numbers and ommatidial diameters increased from 800 to 1500 and 35 microm to 100 microm, respectively; eye length and width grew from 1.2 to 3.2 and 0.9 to 2.5 mm, respectively; and length of dioptric apparatus and width of retinal layer changed from 70 microm to 180 microm and about 70 microm to 120 microm. Visual angles and interommatidial angles of centrally located ommatidia remained constant at about 30 and 6.9 degrees, respectively. An almost perfect linear relationship was found when eye length was plotted against the product between the square root of the total number of ommatidia and the ommatidial diameter. No difference between males and females was observed in any of the relationships, but the results suggest that, compared with smaller specimens, larger ones possess increased absolute sensitivity in single ommatidia, increased sensitivity to point sources, and overall larger angular visual fields for the eye in its totality. This means that larger individuals of L. exotica (which are also faster) have an advantage over smaller individuals at night, but that smaller individuals may cope better with bright lights. Vision in L. exotica seems useful not only in detecting potential danger, but also in locating and approaching cliffs from a distance of 2-4 m when swimming in seawater.     

Histological and histochemical studies of the male reproductive system of Ligia exotica Roux (Crustacea: Isopoda)

The male reproductive system of Ligia exotica consists of a pair of testes, a pair of vasa deferentia and a pair of genital pores. The testes are tube-like, unpigmented and translucent and each is composed of three elongate, fusiform follicles. The follicular lumen of the mature testis contains spermatogonia, spermatocytes, spermatids and spermatozoa. The histochemical reactions of the testis and the vas deferens show the presence of acidic sulphated mucopolysaccharides and neutral mucopolysaccharides. In addition, they contain basic proteins, tyrosine, disulphide groups, SH-groups, SH-groups, lipids, phospholipids, RNA and DNA.     

Water uptake via two pairs of specialized legs in Ligia exotica (Crustacea, Isopoda)

When individuals of Ligia exotica were exposed to dry environments, their weight decreased within 3 h to 90% of the initial weight. When the animals were subsequently presented with wet paper, pereiopods VI and VII were firmly apposed and moved around until their tips were stationary for some seconds. Subsequently the pleopods became soaked in water and the body weight recovered. Morphological observations revealed hollow structures on the surface of the dactylus and propodus of pereiopods VI and VII, and a series of thin cuticular protrusions, oriented in several parallel lines, from the propodite to the ischiopodite of pereiopod VI and on the basipodite of pereiopod VII. The width between the parallel lines varied little, but the total width of the regular lines increased linearly with increasing body size. When isolated caudal pereiopods were immersed, beginning at the distal end, in water, water flowed along pereiopod VI as far as the proximal end of the ischiopodite, but water flow along pereiopod VII occurred only in conjunction with the basipodite. This means that water uptake with the legs requires both pereiopods VI and VII and can be achieved only when these legs are closely apposed.     

The hemocytes of an lsopod Ligia exotica Roux

The morphological features of the hemocytes of the crustacean Ligia exotica are similar to hemocytes of insects and millipedes. Jones system of hemocyte classification is extended to crustacean hemocytes. As in insects, seven classes of hemocytes, identified as prohemocytes, plasmatocytes, granular hemocytes, cystocytes, oenocytoids, spherule cells and adipohemocytes, occur. The prohemocytes can be subdivided into five categories that probably represent the precursor of major cell types. The structural and chemical features of other major cell classes are distinct and support the concept of Jones ('62) that these types might have different lineages and might not be capable of transforming into one another. Some of the prohemocytes, plasmatocytes and granular hemocytes are amoeboid. Cystocytes do not bring about any visible plasma coagulation similar to their counterpart in millipedes. Oneocytoids and adipohemocytes are rare. Plasmatocytes, cystocytes and oenocytoids occur in conglomerates, the significance of which is discussed. The cell types are compared with those of the hemocytes of other crustaceans. It is suggested that the nomenclature based on morphological characters is more suited for crustacean hemocytes than a nomenclature based on behavioural and physiological characters.     

Photosensitive neurogenic heart of the isopod crustacean Ligia exotica

The heart of animals is regulated through the central nervous system in response to external sensory stimuli. We found, however, that the adult neurogenic heart of the isopod crustacean Ligia exotica has photosensitivity. The beat frequency of the isolated heart decreased in response to a light stimulus. Magnitude of the response was stimulus intensity dependent and the heartbeat frequency decreased to less than 80% of the dark value during illumination of the white light with an intensity of 6.0 mW cm-2. The spectral sensitivity curve of the heart photoresponse peaked at a wavelength around 520 nm. In response to 530 nm monochromatic light, the relationship between light intensity and response magnitude was linear and the threshold intensity was 7.26 x 1012 quanta cm-2 s-1. Bursting activity of the cardiac ganglion, which is located in the heart and acts as the cardiac pacemaker deceased in frequency in response to illumination by white light. This fact suggests that the heart photoresponse of L. exotica results from the photosensitivity of the cardiac ganglion neurons. The photoresponse of the heart therefore contributes to regulation of cardiac output in addition to other regulatory systems.     

Desiccation resistance of two semiterrestrial isopods, Ligia exotica and Ligia taiwanensis (Crustacea) in Taiwan

The ability to resist desiccation stress was examined in two semiterrestrial Ligia species, Ligia exotica Roux and L. taiwanensis Lee, in Taiwan, under a certain desiccation condition. L. exotica exhibited the longer survival time, lower weight-specific rates of water loss, and the slightly higher ability of tolerance to water loss, compared to L. taiwanensis. In each species, the animal size displays a positive correlation to the survival time and total ability to resist desiccation, yet this size effects on the weight-specific water loss rate is negative. Neither water content nor maximum tolerance to water loss shows the association with the animal size in both species. The path ways and magnitudes of the interactions between these traits of desiccation resistance are analyzed and diagrammed using a stepwise regression model. In this model, the body sizes of animal can explain the most part of the variations in the survival time. The body size has a direct effect and an indirect effect, through the effect on water loss rate, on the time that the experimental animals can survival under this desiccated condition. These results suggest that L. exotica attains larger size than does L. taiwanensis, a lower transpiration rate and, consequently, a greater ability in desiccation resistance. The performances of these interactions in the desiccated resistance are more advantageous for L. exotica to migrate and colonize in variable land habitats within a certain limit, and as a result that L. exotica shows a broader distribution pattern than did L. taiwanensis in Taiwan.     

Developmental changes in heart photosensitivity of the isopod crustacean Ligia exotica

During juvenile development, the cardiac pacemaker of the isopod crustacean Ligia exotica is transferred from the myocardium to the cardiac ganglion of the neurogenic heart. In adult, light stimulus decreases the beat frequency of the heart. To elucidate developmental changes in the photosensitivity of the juvenile Ligia heart, we examined the effect of a light stimulus on the semi-isolated heart of juveniles at various developmental stages by the recording membrane potential of the myocardium. We also examined the effect of hyperpolarizing current injection into the myocardium, because this causes different effects on the beat frequency between myogenic and neurogenic hearts. In newly hatched juveniles, beat frequency decreased upon current injection but exhibited no response to white light. In contrast, 10 days after hatching, beat frequency did not change upon current injection, but decreased in response to white light. The heart photoresponse of juveniles was reversibly eliminated by application of tetrodotoxin, which changes the heartbeat from neurogenic to myogenic by suppressing cardiac ganglion activity. The proportion of juveniles exhibiting a heart photoresponse increased gradually up to 100% during the period between 3 and 10 days after hatching. The results suggest that the heart photoresponse of L. exotica appears in association with transfer of the cardiac pacemaker from the myocardium to the cardiac ganglion during juvenile development.     

Dual effects of dopamine on the adult heart of the isopod crustacean Ligia exotica

In the adult heart of the isopod crustacean Ligia exotica, the cardiac ganglion acts as the primary pacemaker with the myocardium having a latent pacemaker property. We show several lines of evidence that dopamine modulates the heartbeat of adult L. exotica affecting both pacemaker sites in the heart. Dopamine caused positive chronotropic (frequency increase) and inotropic (amplitude increase) effects on the heartbeat in a concentration dependent manner. The time courses of these effects were considerably different and the inotropic effect appeared later and lasted longer than the chronotropic effect. Dopamine rapidly increased the frequency of the bursting activity in the cardiac ganglion neurons and each impulse burst of the cardiac ganglion was always followed by a heartbeat. Moreover, dopamine slowly increased the amplitude and duration of the action potential plateau (plateau potential) of the myocardium. When the myocardial pacemaker activity was induced by application of tetrodotoxin, which suppresses cardiac ganglion activity, dopamine slowly increased the amplitude and duration of the myocardial plateau potential while decreasing its frequency. These results suggest that dopamine modulates the heartbeat in adult L. exotica producing a dual effect on the two pacemaker sites in the heart, the cardiac ganglion and myocardium.     

Morphologie und Funktion des Wasserleitungssystems der terrestrischen Isopoden (Crustacea,Isopoda, Oniscoidea)

Zusammenfassung 1. Das Wasserleitungssystem der Oniscoidea wird einer Strukturanalyse unterzogen. Die Teilstrukturen des Systems werden dargestellt. Es wird gezeigt, daß das System seinen Anfang an der Harnöffnung des Cephalothorax nimmt, wo es den aus den Maxillarnephridien stammenden Harn aufnimmt und durch ventrale und dorsale Leitungsstrukturen zum Pleoventralraum leitet. Aus diesem wird der Harn durch den Anus wieder aufgenommen und vom Rectum resorbiert.2. In der Strukturanalyse wird gezeigt, daß innerhalb der Oniscoidea zwei Typen des Wasserleitungssystems unterschieden werden können: der Porcellio-Typ und der Ligia-Typ. Bei grundsätzlicher Gleichheit beider Typen bestehen folgende Unterschiede: der Ligia-Typ hat an den 6. und 7. Pereiopoden Leitungsstrukturen, mit denen Wasser aufgesaugt wird. Im Wasserleitungssystem des Ligia-Typs fließt Harn und Wasser, in dem des Porcellio-Typs nur Harn.3. Da die entwicklungsgeschichtlichen Extreme Ligia und Hemilepistus ein Wasserleitungssystem haben, bei 45 weiteren Arten verschiedener Familien keine Assel ohne ein solches System gefunden werden konnte, muß geschlossen werden, daß dieses System konstitutiv für alle Landasseln ist. Alle untersuchten Arten lassen sich einem der beiden System-Typen zuordnen.4. Die vorgefundenen Strukturen werden auf ihre Funktion hin interpretiert. Es wird die Hypothese vertreten, daß das Wasserleitungssystem ein geschlossenes (Porcellio-Typ), multifunktionales System, vor allem aber integraler Bestandteil des Exkretionssystems ist. Das bei der Ammoniotelie auch terrestrischer Isopoden anfallende NH₃ wird im Harn aus dem Körper entfernt. Aus dem Harn entweicht das NH₃ wegen der enormen Oberflächenvergrößerung der Harnflüssigkeit beim Durchfluß durch die Leitungsstrukturen des Wasserleitungssystems. Der Harn selber wird nach Durchfließen des Pleoventralraumes durch die Endopodite und durch den Anus in den Körper wieder aufgenommen.Der Nachteil der Ammoniotelie, der hohe Wasserbedarf, wird so aufgehoben und der Vorteil, die Energieersparnis, gewahrt. Darüber hinaus gehört das System zu den Funktionskreisen Respiration und Thermoregulation und hat außerdem Funktionen bei der Wasserleitung sowie bei der Feuchthaltung und Reinigung des Integuments.5. Das Wasserleitungssystem wird als im Tierreich einzigartiger Weg erwiesen, die physiologischen Probleme des Landganges zu bewältigen.

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Morphology and function of the water conducting system in terrestrial isopods (Crustacea, Isopoda, Oniscoidea)

Summary In terrestrial isopods there are two types of a water conducting system. The porcellio-type is a recycling system. Out of the maxillar-nephridium urine is secreted, which spreads through the ventral and dorsal structures of the water conducting system. It is then directed along the pleopods and is reabsorbed in the gut. Within this system only urine runs. The ligiatype is an open system. Here urine runs as well as water, which is absorbed by the sixth and seventh pereiopods.In all the species investigated incl. Ligia and Hemilepistus a water conducting system has been found. Apparently this system is constitutive for all terrestrial isopods. Concerning function (hypothesis): the water conducting system is multifunctional, but above all serving excretion. Isopods, also terrestrial isopods, are ammoniotelic. While the urine is spreading through the conducting structures ammonia is vapourizing off the increased surface. A special permeability of the integument needs not be assumed. After the urine having passed the anus, it is reabsorbed in the gut. The disadvantage of the ammoniotelism, the great demand of water, is avoided, the advantage, the economizing of energy, is preserved.Additionally the water conducting system allows an active thermoregulation to a certain extent. Furthermore it has functions in respiration, conducting of water and keeping the integument humid and clean.

     

Electrical studies on the compound eye of Ligia occidentalis Dana (Crustacea: Isopoda)

The ERG of the compound eye in freshly collected Ligia occidentalis, in response to high intensity light flashes of ⅛ second or longer duration, begins with a negative on-effect quickly followed by an early positive deflection, rapidly returns to the baseline during illumination, and ends with a positive off-effect. As the stimulus intensity is decreased the early positivity progressively decreases and the rapid return to the baseline is replaced by a slowing decline of the negative on-effect. Responses were recorded with one active electrode subcorneally situated in the illuminated eye, the reference electrode in the dark eye. The dark-adapted eye shows a facilitation of the amplitude and rates of rise and fall of the on-effect to a brief, high intensity light stimulus. This facilitation may persist for more than 2 minutes. Following light adaptation under conditions in which the human eye loses sensitivity by a factor of almost 40,000 the Ligia eye loses sensitivity by a factor of only 3. The flicker fusion frequency of the ERG may be as high as 120/second with a corneal illumination of 15,000 foot-candles. Bleeding an otherwise intact animal very rapidly results in a decline of amplitude, change of wave form, and loss of facilitation in the ERG. When the eye is deganglionated without bleeding the animal the isolated retina responds in the same manner as the intact eye. Histological examination of the Ligia receptor layer showed that each ommatidium contains three different retinula cell types, each of which may be responsible for a different aspect of the ERG.     

Morphologie und Funktion der tergalen Längsrippen bei Landisopoden (Oniscoidea,Isopoda, Crustacea)

Zusammenfassung Tergale Längsrippen auf dem Peräon finden sich in folgenden Isopoden-Familien: Trichoniscidae, Buddelundiellidae, Stenoniscidae, Tendosphaeridae, Oniscidae, Platyarthridae, Pentheidae (= Armadillidae). Es wird angenommen, daß sich diese Rippen durch ähnliche ökologische Bedingungen konvergent in den genannten Familien entwickelt haben.Alle behandelten Gruppen zeigen ein ähnliches Rippenmuster. Pro Tergit können 6–12 Rippen vorkommen.Drei Rippentypen können unterschieden werden: 1. derCretoniscellus aegaeus-Typ mit abgerundeten Längserhebungen ohne zusätzliche Strukturen; 2. derHaplo-phthalmus-montivagus-Typ mit einer Reihe von spitzen Höckern entlang dem Rippengrat; 3. derPlatyarthrus-schoebli-Typ mit einer Reihe von der Rippe aufgesetzten großen Schuppenborsten.Ontogenetische Beobachtungen anHaplophthalmus zeigen, daß frischgeschlüpfte Jungtiere die gleiche absolute Rippenhöhe und dieselbe Anzahl der Rippen pro Tergit besitzen wie adulte Tiere.Es wird angenommen, daß die Funktion der Rippen darin besteht, bei Kontakt der tergalen Überfläche mit feuchtem Substrat die dabei auftretenden Adhäsionskräfte zu verringern. Die Rippen müssen demnach (im Zusammenhang mit anderen strukturellen Differenzierungen) als eine wichtige morphologische Anpassung an ein bestimmtes Habitat betrachtet werden. Dieses Habitat ist in der Regel der untere Teil der Humusschicht, der durch enge Zwischenräume und durch permanent gesättigte Luftfeuchtigkeit ausgezeichnet ist.In einem Anhang werden die tergalen Strukturen innerhalb der GattungPlatyarthrus und ihre ökologischen Zusammenhänge analysiert.

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Morphology and function of the tergal ridges in terrestrial Isopods (Oniscoidea, Isopoda, Crustacea)

Summary Longitudinal tergal peraeon-ridges are found in the following isopod families: Trichoniscidae, Buddelundiellidae, Stenoniscidae, Tendosphaeridae, Oniscidae, Platyarthridae, Pentheidae (= Armadillidae). It is presumed that these ridges evolved convergently in the families cited above under similar ecological conditions.The ridge-pattern is similar in all treated groups. There are 6–12 ridges on each peraeon-tergite.Three types of ridges can be differenciated: 1) theCretoniscellus-aegaeus-type showing rounded ridges without accessory structures; 2) theHaplophthalmusmontivagus- type with a row of pointed spikes on top of the ridge; 3) thePlatyarthrus-schoebli- type with a row of big scale-spines situated on the ridge.Ontogenetic observations inHaplophthalmus show that the absolute hight of the ridge and the number of ridges per tergite are the same in newly hatched juveniles as in adults.The function of the ridges is supposed to be a means to reduce the adhesive forces in case the tergal surface comes into contact with wet substrate. The ridges are to be considered an important adaptation (seen in connection with other structural differenciations) for a special habitat which is the under-storey of the litter stratum. This habitat is characterized by very small interstices, and by a permanently saturated moisture.An appendix is dedicated to a tentative analysis of the tergal structures in the genusPlatyarthrus seen in an ecological context.

     

Characterization of polymorphic microsatellite loci in the marine isopod Sphaeroma terebrans (Crustacea, Isopoda)

Nine polymorphic microsatellites were characterized in the marine isopod Sphaeroma terebrans (Isopoda, Sphaeromatidae) for phylogeographic and parentage analyses. The number of alleles ranged from 3 to 10. Observed heterozygosity ranged from 0.428 to 0.950, while expected heterozygosity ranged from 0.532 to 0.889. Heterozygote deficiency was detected for one locus, possibly the result of null alleles.     

Male dimorphism in the harem-forming gnathiid isopod Elaphognathia discolor (Crustacea: Isopoda)

Previously unreported males of a gnathiid isopod were found in reproductive aggregations of the harem-forming gnathiid Elaphognathia discolor. Although the male gnathiids were small in size and morphologically different from E. discolor males, the male sexual organ, appendix masculina, was similar to that of E. discolor males, and possible conspecific larvae and females of the small male gnathiid were never found. In the laboratory, the small male gnathiids as well as male E. discolor successfully copulated with female E. discolor, and the development of embryos in female brood pouches was observed. Offspring of small male gnathiids develop to adults of E. discolor after molting three times, or small male gnathiids after molting two times. Thus, the small male gnathiid was concluded to be an alternative male form compared to the regular large male form of E. discolor. This male polymorphism was thought to have a genetic basis, since no small male specimens appeared in offspring of regular E. discolor males. Field sampling showed that a regular large male formed a harem composed of one large male and several females and never coexisted with other large males as previously reported. However, small males were often found together with large males. Therefore, small males are thought to be sneakers intruding into harems dominated by large males.     

Identification of two cardioacceleratory neurons in the isopod crustacean, Ligia exotica and their effects on cardiac ganglion cells

We identified two pairs of cardioacceleratory (CA1, CA2) neurons in the central nervous system of the isopod Ligiaexotica and examined their effects on the cardiac ganglion (CG). CA1 neurons had cell bodies in the 2nd thoracic ganglion and had arborizations in the subesophageal ganglion and the 1st and 2nd thoracic ganglia. CA2 neurons had cell bodies in the 3rd thoracic ganglion and had arborizations in the 2nd, 3rd and 4th thoracic ganglia. They sent axons to the heart through the ipsilateral 3rd roots of the ganglia where their cell bodies were located. Repetitive stimulation of the CA1 axon rapidly increased the burst frequency of the CG, and that of CA2 rather slowly. The increased burst rate caused by the CA1 stimulation was significantly higher than that caused by CA2. Overall depolarization of a quiescent CG cell produced by the CA1 stimulation was significantly larger in amplitude than that produced by CA2. Facilitation was obviously seen in the excitatory post-synaptic potentials evoked by the CA1 stimulation. These results show that the synaptic properties of CA1 and CA2 neurons are different, suggesting that they have different functional roles in heart regulation. Accepted: 19 July 1997