Cholinergic mechanism in Liriope tetraphylla (Cnidaria, Hydrozoa)

Crude whole body homogenates of Liriope tetraphylla exhibit a cholinesterase particularly active on acetylthiocholine but not on butyrylthiocholine. The acetylthiocholine hydrolysis is completely blocked by neostigmine. The Michaelis-Menten constant for acetylthiocholine is 0.14 mM. The pharmacological analysis of the responses to the choline esters nicotine and atropine suggests the involvement in Liriope tetraphylla of a cholinergic mechanism in the pointing reflex. Butyrylcholine, nicotine and atropine (but not muscarinic agonists) caused the contraction of the subumbrellar radial muscles. The effects of atropine were dose-dependent and were depressed in competition with muscarinic agonists. MgCl2 interfered with the action of atropine. The results were explained by suggesting the existence, at least at the neuromuscular junction, of excitatory (nicotinic) and inhibitory (muscarinic) pre-synaptic receptors modulating the release of the (unknown) transmitter acting post-synaptically.     

Unusual trophic strategies of Hydractinia angusta (Cnidaria, Hydrozoa) from Terra Nova Bay, Antarctica

Hydractinia angusta Hartlaub, 1904 has been recorded at Terra Nova Bay (Ross Sea, Antarctica) as epizoic on shells of the Antarctic scallop Adamussium colbecki. The species can exploit different trophic resources: first, polyps are able to detach and ingest tube feet and pedicellariae from the sea urchins Sterechinus neumayeri, grazing on the scallop shell, and second, they also eat masses of benthic diatoms settled among the hydrorhiza of the colony. The particular relationship observed between the hydroid and one of the most common Antarctic sea urchins may prevent or reduce the damage to A. colbecki shells, otherwise caused by the grazing of sea urchins on the algal film of the upper valve of the scallops. H. angusta is the first known species of hydroid that exploits prey several times its own size and the second that does not ingest entire prey but portions of them. The use of benthic diatoms as a food resource has previously been documented for the sub-Antarctic marine hydroid Silicularia rosea. Accepted: 20 December 1999     

Character evolution in Hydrozoa (phylum Cnidaria)

The diversity of hydrozoan life cycles, as manifested in the wide range of polyp, colony, and medusa morphologies, has been appreciated for centuries. Unraveling the complex history of characters involved in this diversity is critical for understanding the processes driving hydrozoan evolution. In this study, we use a phylogenetic approach to investigate the evolution of morphological characters in Hydrozoa. A molecular phylogeny is reconstructed using ribosomal DNA sequence data. Several characters involving polyp, colony, and medusa morphology are coded in the terminal taxa. These characters are mapped onto the phylogeny and then the ancestral character states are reconstructed. This study confirms the complex evolutionary history of hydrozoan morphological characters. Many of the characters involving polyp, colony, and medusa morphology appear as synapomorphies for major hydrozoan clades, yet homoplasy is commonplace.     

Density regulation inSarsia tubulosa (Hydrozoa)

The majority of wild-caughtSarsia tubulosa M. Sars medusae are less dense than the surrounding water. The bell ofS. tubulosa is the buoyant structure; the tentacles and manubrium sink if cut off from the bell.S. tubulosa individuals placed in dilute seawater sink initially but recover positive or neutral buoyancy and normal activity within a couple of hours. In all cases observed animals were able to achieve positive buoyancy in seawater of 20.25 S and some individuals were able to adjust to lower salinities. In most cases where positive buoyancy was not attained within two hours the animal did not achieve positive buoyancy within twelve hours and died within that period. While the mechanism of regulation is not known, ionic pumping, possibly involving the extrusion of sulphate ion, has been suggested to be responsible for the buoyancy of mesoglea in other jellyfishes.     

Orientiertes Wachstum beiMillepora dichotoma (Hydrozoa)

The orientation of fans ofMillepora dichotoma was investigated on the west coast of the Gulf of Akabah. Three transects were sampled by measuring equal areas in three different depths. The total depth range of the transect was determined in each case by the occurrence of the milleporids. Orientation of fans was determined by compass and the deviation from north for each colony noted. In all cases the orientation was uniform in the lower two thirds of the transect. The upper third shows a different orientation. Orientation is correlated with the prevailing direction of water movement and the plane of the fans grows perpendicular to the water current. Broken-off colonies lying flat on the bottom orient their new growth again perpendicular to the prevailing current. Further evidence for perpendicular fan orientation to the prevailing current is reported. Some fans exhibit two planes of orientation thus suggesting a high sensitivity to local deflection of reef current from the main direction of water movement.     

Ontogeny of vision in the Antarctic fish Pagothenia borchgrevinki (Nototheniidae)

Summary Pagothenia borchgrevinki ranging in size from 63 to 245 mm were captured from beneath sea ice in McMurdo Sound by fishing and diver collection. Changes in ocular morphology with increasing body size were measured, and assessed in relation to ingested prey. Relative eye size was highest among smaller fish (<100mm total length), and declined with increasing fish size. This was accompanied by a decrease in cone density in the retina from a maximum of 14,200 mm^–2 in the smallest fish examined (63 mm), to 1000 mm^–2 in a 220 mm long fish. Theoretical acuity was lowest among fish at either end of the size range examined (minimum separable angle 40–50) but approximately constant over the remainder of the size range (25–40). Rod density also decreased with increasing body size but rod numbers per unit visual arc were relatively constant, except in the smallest fish, where angular rod density was low. The same prey taxa occurred in fish of all sizes; however, prey items smaller than about 1.5 mm were not taken by fish of any body size.     

Gastrulation in Halocordyle disticha (Hydrozoa,Athecata)

Summary

Previous reports of development in Halocordyle disticha have described gastrulation as occurring by gradual differentiation of the inner and outer cells of the stereoblastula. In 1984, however, Martin and Thomas described an indentation on the surface of the embryo at the time of gastrulation. They hypothesized from morphological data that the indentation represented a blastopore. Here we provide results of marking studies which demonstrate that the indentation is in fact a site of cellular ingression. This is the first example known of gastrulation that involves unipolar ingression in a form with a stereoblastula. Possible functions of gastrulation by unipolar ingression are discussed, and the possible phylogenetic significance of the occurrence of such a mode of gastrulation in H. disticha is considered.     

Role of central histaminergic mechanism in behavioural depression (swimming despair) in mice

The role of the central histaminergic system in depression was studied by using swimming despair test in mice - a behavioural model of depression. In this test, immobility of mice reflects a state of depression. Intracerebral (ic) injection of histamine (50-200 micrograms) increased significantly the immobility. The H1-receptor blocker mepyramine (2.5-20 mg/kg ip) had no effect while H2-receptor blocker cimetidine (100-200 micrograms ic) caused a significant decrease in immobility. The histamine induced facilitation was blocked completely by cimetidine and antidepressant drugs-imipramine and desipramine, but remained unaffected in mice pretreated with mepyramine or atropine. The H2 agonist impromidine (20-40 micrograms ic) also enhanced significantly, the immobility which was blocked by cimetidine and antidepressant drugs. It has been concluded that central H2-receptors facilitate depression and antidepressant drugs block central H2-receptors.     

Apoptosis--a death-inducing mechanism tightly linked with morphogenesis in Hydractina echinata (Cnidaria, Hydrozoa).

Programmed cell death is not only known as a mechanism mediating tissue destruction, but also as an organismic tool for body shaping and regulation of morphological events during development. Here we report the tight and vital link of the most prominent form of programmed cell death, apoptosis, to one of the oldest, most basic, and most radical developmental processes, the metamorphosis of the marine hydrozoon Hydractinia echinata. Apoptosis, represented by DNA fragmentation, appears very early during metamorphosis, approximately 20 minutes post induction. It is then executed in a very distinct spatial and temporal pattern, including the removal or phagocytosis of a large number of larval cells prior to the appearance of stolons and tentacles. Our data indicate a developmental program striving to reduce all body parts that are no longer necessary, before reaching a distinct turning point, when the development of adult features is initiated. During these events, morphogenesis of basal and apical structures correlates with recycling of that particular larval region, indicated by the presence of apoptosis. Based on these data, the necessity of apoptosis for normal development of adult patterns is inferred and a fundamental association of apoptosis with developmental processes can be stated.     

Planktivory in Pagothenia borchgrevinki (Pisces: Nototheniidae) in McMurdo sound,Antarctica

Summary Specimens of the Antarctic fish P. borchgrevinki were collected from just beneath the sea-ice in McMurdo Sound over the period 8 November to 6 December, 1985. The stomach contents of 200 fish were analysed and compared with the co-occurrence of the prey species in the plankton. Numerically, the dominant prey was a thecosomatid pteropod Limacina helicina (83%), followed by a hyperiid amphipod Hyperiella dilatata (5%). Other prey species in decreasing order of occurrence were Euphausia crystallorophias, Euchaeta antarctica, other copepods, a decapod crustacean larva, chaetognaths, the amphipods Orchomene plebs and Epimiriella macronyx, and unidentified juvenile fish. The order of frequency of occurrence of these prey in the guts of all the fish was H. dilatata (in 73% of the fish), L. helicina (71%), calanoid copepods (55%), chaetognaths (51%), E. crystallorophias (42%), decapod crustacean larva (32%), O. plebs (27%), juvenile fish (20%), and E. macronyx (14%). In volumetric terms, the dominant diet contributors were O. plebs (38%), L. helicina (17%) and chaetognaths (15%). All prey except O. plebs and E. macronyx were taken in concurrent plankton samples. Although planktivory of Pagothenia borchgrevinki is confirmed, it is not certain whether feeding is confined to the immediate sub-ice environment.     

Benthic hydroids (Cnidaria,Hydrozoa) from the Balleny Islands (Antarctica)

Twenty-two species of benthic hydroids, belonging to ten families and 14 genera, were found in a hydroid collection obtained in the Balleny Islands during the BioRoss expedition with the NIWA research vessel Tangaroa in 2004. Twenty of those species constitute new records for the Balleny Islands, raising the total number of known species in the area to 25. Most are members of the subclass Leptothecata, although the subclass Anthoathecata is also relatively well represented. Kirchenpaueriidae and Sertulariidae constitute families with the greatest numbers of species in the collection, with five species (20%) each. Oswaldella with five species (20%) and Staurotheca with four (16%), were the most diverse genera. Twelve species (63%) are endemic to Antarctic waters, most of them with a circum-Antarctic distribution, and 17 (89%) are restricted to Antarctic or Antarctic/sub-Antarctic waters. Although the Balleny Islands hydroid fauna seems to be a typical Antarctic assemblage, it has some striking peculiarities, namely the absence or low representation of some typical and widespread Antarctic genera (Antarctoscyphus and Schizotricha/Symplectoscyphus, respectively).     

Transmission Genetics of Allorecognition in Hydractinia Symbiolongicarpus (Cnidaria: Hydrozoa)

Allorecognition is ubiquitous, or nearly so, amongst colonial invertebrates. Despite the prominent role that such phenomena have played both in evolutionary theory and in speculations on the origin of the vertebrate immune system, unambiguous data on the transmission genetics of fusibility (i.e., the ability of two individuals to fuse upon tissue contact) is lacking for any metazoan outside of the phylum Chordata. We have developed lines of the hydroid Hydractinia symbiolongicarpus (Phylum Cnidaria) inbred for fusibility and here report results of breeding experiments establishing that fusibility segregates as expected for a single locus with codominantly expressed alleles, with one shared allele producing a fusible phenotype. Surveys of fusibility in field populations and additional breeding experiments indicate the presence of an extensive allele series.     

Allorecognition in the Colonial Marine Hydroid Hydractinia (Cnidaria/Hydrozoa)

The colonial marine hydroid Hydractinia has a sophisticatedallorecognition and effector system. Unlike many unitary organisms(i.e., vertebrates) which lack a current context for allorecognition,there is the potential for strong selection pressure for allorecognitionand response in Hydractinia. Hydractinia colonies use allorecognitionin intraspecific competition for two dimensional space; spaceis an absolute requirement for Hydractinia to successfully completeits life-cycle and thus interactions for space are of centralimportance for Hydractinia. Studies of the mechanisms, molecules,and genes involved in allorecognition in Hydractinia may contributeto our understanding of the evolution of allorecognition inthe metazoa.     

Benthic hydroids (Cnidaria, Hydrozoa) from off Bouvet Island (Antarctic Ocean)

Twenty-seven species of benthic hydroids have been found in a small collection from off Bouvet Island (Antarctic Ocean). The material was obtained during the XXI/2 German Antarctic expedition with R.V. Polarstern 2003–2004. Of the 27 species collected, 21 were unknown from those waters. Consequently, the number of known species of benthic hydroids from the area has risen to 32, representing a high diversity location for this group in the Southern Ocean considering the sampling effort made. All species, including those previously known from Bouvet waters, are considered with respect to geographical distribution.The present work is an addition to the special issue “Macrobenthic studies at Bouvet island”, presenting work carried out from on board RV Polarstern in 2003/2004 and published in Polar Biology 29 (2006)     

Vibrio harveyi Associated with Aglaophenia octodonta (Hydrozoa, Cnidaria)

A previously unknown association between a luminous bacterium, Vibrio harveyi, and a benthic hydrozoan, Aglaophenia octodonta, is described. Aglaophenia hydrocladia showed a clear fluorescence in the folds along the hydrocaulus and at the base of the hydrotheca, suggesting the presence of luminous bacteria. This hypothesis was confirmed by isolation of luminous bacteria from Aglaophenia homogenates. Phenotypic characterization of bacterial isolates was performed by several morphological, biochemical, and cultural tests, completed with 16S rDNA sequence analysis. All the isolates were referred to a single species: V. harveyi. The association between V. harveyi and A. octodonta has epidemiological as well as ecological significance. Therefore, A. octodonta may function as habitat “islands” providing a unique set of environmental conditions for luminous bacteria colonization, quite different from those already recorded from the plankton for other Vibrio species.     

Evolution and taxonomy in capitate hydroids and medusae (Cnidaria: Hydrozoa)

A cladistic analysis of Capitata groups the families in four suborders based on medusa characters (such as manubrium morphology, position of gonads, and position and number of marginal tentacles) and hydroid characters (such as presence or absence of an oral tentacle whorl, and the different development of the tentacles of the oral and aboral whorls). On the family and generic levels, the revision results in changes which unite the separate hydroid and medusa taxonomic systems, defining genera which are not based on characters solely relating to the reduction of medusae to fixed gonophores. In those families where the reduction of the medusa can be analysed, it is shown that the reduction occurred after all synapomorphies defining the genera had evolved and usually affected individual species within a genus rather than the original species from which the other species in the genus evolved. This supports the view that medusa reduction is not in itself a valid generic character. A discussion of the theories of 'inconsistent' or 'mosaic' evolution concludes that no difference in evolutionary rate or degree of specialization can be demonstrated among taxa with free medusae and taxa with gonophores.     

Prospective fate of early blastomeres in Hydractinia echinata (Cnidaria,Hydrozoa)

Summary Blastomeres of two-cell, four-cell, and eight-cell embryos of Hydractinia echinata were injected with horseradish-peroxidase (HRP) or fluorescein isothiocyanate (FITC)-dextran. The fate of the descendants of the injected blastomeres was followed until the planula larva had developed. The results obtained after HRP or FITC-dextran injection were essentially the same. Blastomeres are equivalent up to the four-cell stage, i.e. half-blastomeres produce half of the ectoderm of the planula larva and quarter-blastomeres give rise to one quarter of the larval ectoderm. During normal embryogenesis, the larval anterior-posterior axis corresponds to the animal-vegetal axis of the zygote. Thus, the labelled areas of larvae consisting of the progeny of injected half or quarter blastomeres normally stretch along the larval anterior-posterior axis. Normally, material giving rise to anterior or posterior larval parts, respectively, is separated at the third cleavage. Irrespective of the type of experiment, the progeny of injected blastomeres always contributed to endoderm formation, i.e. in larvae resulting from injected embryos the endoderm was more or less uniformly labelled. Application of vital stains locally to the exterior of zygotes and following these markers through first and second cleavage, produced evidence that in the vast majority of cases, the second cleavage is meridional.Offprint requests to: A. Schlawny