I.A. Kosevich

Associations between the White Sea Invertebrates and Oxygen-Evolving Phototrophic Microorganisms

Eleven species of White Sea invertebrates (sponges, actinians, hydroids, polychaetes, and nudibranch mollusks) were tested for the presence of associated oxygen-evolving phototrophic microorganisms (OPM) (microalgae and cyanobacteria). Two main approaches were applied: (a) light and electron microscopy of intact samples and fixed preparations of invertebrates, and (b) isolation of microorganisms from samples of invertebrates after mild surface sterilization. The obtained results lead to conclusions on the formation of multicomponent associations by White Sea invertebrates and OPM based on the following data: (1) isolation of 27 cultures of OPM from eight species of invertebrates (sponges, hydroids, polychaete trochophore larva), (2) specificity of association between epibiontic communities of microorganisms and macroorganisms within the same biotope, and (3) spatial integration of micro- and macropartners resulting in the formation of morphological structures within the interorganismic contact zones.

Mechanics of Growth Pulsations as the Basis of Growth and Morphogenesis in Colonial Hydroids

Growth and shaping in colonial hydroids (Hydrozoa, Cnidaria) are realized due to the functioning of special colony elements, growing tips located at the terminuses of branched colony body. Unlike in plants, the growing tips of colonial hydroids are sites of active cell movements related to morphogenesis and lacking proliferation. The activity of hydroid growing tips is expressed as growth pulsations: cyclic repetitions of their apex extensions and retractions. The parameters of growth pulsations are species specific and related to the shape of a forming element. Here, the succession of cell movements and changes in mutual arrangement within the growing tip are described in detail at all pulsation phases. The role of the inner cell layer in the tip activity was demonstrated for the first time. Relationships between the growing tip parameters, length and diameter, and pulsations are discussed. A scheme is proposed for cyclic processes in both epithelial layers. An explanation is provided for the two-step mode of growth pulsations with relative independence of the main phases. It was proposed that successive activities of the tip ecto-and endoderm serve as driving forces provided there is a hard outer skeleton. This scheme makes it possible to explain some patterns of growth and morphogenesis in colonial hydroids, such as gradually increasing growth rate of a new tip and its maximum growth rate, differences in the parameters of growth pulsations between shoot and stolon tips, shoot base inclination towards the stolon tip, etc., and provides a basis for further improvement of the model of morphogenesis in hydroids.

Morphogenetic evolution of hydroid colony pattern

A scheme of evolution of hydrozoan colony pattern is proposed based upon the consideration of macromorphogenesis. Four main processes play decisive roles: (1) hard skeleton formation by soft tissues, (2) changes in duration of the growth phase relative to the transition to differentiation in interdependent zones of growth, (3) ratio in growth rates between adjacent zones of growth within the rudiment, the shoot, or the whole colony, and (4) spatial relationships among growth zones. The main tendency in morphological evolution of the hydroids is an increasing integration of the colony as revealed by increasing complexity of its structure. That is from a temporary colony towards the permanent one with highly organised shoots, as hydranths and branches are localised in a strictly arranged manner. An analysis of diverse data allows one to state that the main morphogenetic mechanism of increasing complexity in the hydroid colony is convergence, then fusion, of adjacent growth zones, a variant of heterochrony.

The nervous system of the lophophore in the ctenostome Amathia gracilis provides insight into the morphology of ancestral ectoprocts and the monophyly of the lophophorates.

BackgroundThe Bryozoa (=Ectoprocta) is a large group of bilaterians that exhibit great variability in the innervation of tentacles and in the organization of the cerebral ganglion. Investigations of bryozoans from different groups may contribute to the reconstruction of the bryozoan nervous system bauplan. A detailed investigation of the polypide nervous system of the ctenostome bryozoan Amathia gracilis is reported here.ResultsThe cerebral ganglion displays prominent zonality and has at least three zones: proximal, central, and distal. The proximal zone is the most developed and contains two large perikarya giving rise to the tentacle sheath nerves. The neuroepithelial organization of the cerebral ganglion is revealed. The tiny lumen of the cerebral ganglion is represented by narrow spaces between the apical projections of the perikarya of the central zone. The cerebral ganglion gives rise to five groups of main neurite bundles of the lophophore and the tentacle sheath: the circum-oral nerve ring, the lophophoral dorso-lateral nerves, the pharyngeal and visceral neurite bundles, the outer nerve ring, and the tentacle sheath nerves. Serotonin-like immunoreactive nerve system of polypide includes eight large perikarya located between tentacles bases. There are two analmost and six oralmost perikarya with prominent serotonergic “gap” between them. Based on the characteristics of their innervations, the tentacles can be subdivided into two groups: four that are near the anus and six that are near the mouth. Two longitudinal neurite bundles - medio-frontal and abfrontal - extend along each tentacle.ConclusionThe zonality of the cerebral ganglion, the presence of three commissures, and location of the main nerves emanating from each zone might have caused by directive innervation of the various parts of the body: the tentacles sheath, the lophohpore, and the digestive tract. Two alternative scenarios of bryozoan lophophore evolution are discussed. The arrangement of large serotonin-like immunoreactive perikarya differs from the pattern previously described in ctenostome bryozoans. In accordance with its position relative to the same organs (tentacles, anus, and mouth), the lophophore outer nerve ring corresponds to the brachiopod lower brachial nerve and to the phoronid tentacular nerve ring. The presence of the outer nerve ring makes the lophophore innervation within the group (clade) of lophophorates similar and provides additional morphological evidence of the lophophore homology and monophyly of the lophophorates.

Sponge Cell Reaggregation: Mechanisms and Dynamics of the Process

Sponges (Porifera) are lower metazoans whose organization is characterized by a high plasticity of anatomical and cellular structures. One of the manifestations of this plasticity is the ability of sponge cells to reaggregate after dissociation of tissues. This review brings together the available data on the reaggregation of sponge cells that have been obtained to date since the beginning of the 20th century. It considers the behavior of dissociated cells in suspension, the mechanisms and factors involved in reaggregation, and the rate and stages of this process in different representatives of this phylum. In addition, this review provides information about the histological structure of multicellular aggregates formed during reaggregation of cells and the regenerative morphogenetic processes leading to the formation of normal sponges from these multicellular aggregates.

On some features of embryonic development and metamorphosis of Aurelia aurita (Cnidaria, Scyphozoa)

Aurelia aurita is a cosmopolite species of scyphomedusae. Its anatomy and life cycle are well investigated. This work provides a detailed study on development and structure of A. aurita planula before and during its metamorphosis. Intravital observations and histology study during the settlement and metamorphosis of the planulae demonstrated that the inner manubrium lining of primary polyp (gastroderm) develops from the ectoderm of the planula posterior end. The spatial and temporal dynamics of serotonergic cells from the early embryonic stages until the formation of the primary polyp were studied for the first time. In addition, the distribution of tyrosinated tubulin and neuropeptide RF-amide at different stages of A. aurita development was traced.

Morphogenetic Foundations for Increased Evolutionary Complexity in the Organization of Thecate Hydroids Shoots (Cnidaria, Hydroidomedusa, Leptomedusae)

The morphogenetic approach is applied to analyze the diversity of spatial organization of shoots in thecate hydroids (Cnidaria, Hydroidomedusa, Leptomedusae). The main tendencies and constraints of increased evolutionary complexity in thecate hydroids colonies are uncovered.

Branching in Colonial Hydroids

Cnidarians are primitive multi-cellular animals whose body is constructed of two epithelial layers and whose gastric cavity has only one opening. Most cnidarians are colonial. Colonial hydroids with their branched body can be regarded as a model for the whole phylum and are the most-studied cnidarian group with respect to developmental biology. Their colonies are constructed by repetition of limited number of developmental modules. The new modules are formed in the course of activity of terminal elements—growing tips of stolons and shoots. The growing tips of cnidarians, in contrast to those of plants, lack cell proliferation and drive morphogenesis instead by laying down and shaping the outer skeleton and formation of new colony elements. Cell multiplication takes place proximally to the growing tips. Branching in colonial hydroids happens due to the emergence of the new growing tip within the existing structures or by subdivision of the growing tip into several rudiments. Marcomorphogenetic events associated with different variants of branching are described, and the problems of pattern control are discussed in brief. Less is known about genetic basis of branching control.

Organizer regions in marine colonial hydrozoans

Organizers are specific tissue regions of developing organisms that provide accuracy and robustness to the body plan formation. Hydrozoan cnidarians (both solitary and colonial) require organizer regions for maintaining the regular body patterning during continuous tissue dynamics during asexual reproduction and growth. While the hypostomal organizer of the solitary Hydra has been studied relatively well, our knowledge of organizers in colonial hydrozoans remains fragmentary and incomplete. As colonial hydrozoans demonstrate an amazing diversity of morphological and life history traits, it is of special interest to investigate the organizers specific for particular ontogenetic stages and particular types of colonies. In the present study we aimed to assess the inductive capacities of several candidate organizer regions in hydroids with different colony organization. We carried out grafting experiments on colonial hydrozoans belonging to Leptothecata and Anthoathecata. We confirmed that the hypostome tip is an organizer in the colonial Anthoathecata as it is in the solitary polyp Hydra. We also found that the posterior tip of the larva is an organizer in hydroids regardless of the peculiarities of their metamorphosis mode and colony structure. We show for the first time that the shoot growing tip, which can be considered a key evolutionary novelty of Leptothecata, is an organizer region. Taken together, our data demonstrate that organizers function throughout the larval and polypoid stages in colonial hydroids.

FMRF-amide immunoreactivity pattern in the planula and colony of the hydroid Gonothyraea loveni

Gonothyraea loveni (Allman, 1859) is a colonial thecate hydrozoan with a life cycle that lacks a free-swimming medusa stage. The development from zygote to planula occurs within meconidia attached to the female colony. The planula metamorphosis results in the formation of a primary hydranth. The colony then grows by development of new colony elements. In the present work, we studied the temporal pattern of the formation of FMRF-amide-positive cells during embryogenesis, in larvae and during early colony ontogeny. FMRF-amide-positive cells appear in the planula only after its maturation. However, they disappear after planula settlement. For the first time, we show that neural cells are present in the coenosarc of the hydroid colony. We also trace the process of neural net formation during the development of a new shoot internode of the G. loveni colony.