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Sperm bundles of the ephemeropteran Cloeon dipterum (L.) were found in the seminal vesicle. They are apically protected by cyst cells and sperm anterior regions are embedded in a secretion produced by these cells. Cyst cells further degenerate making sperm cells free. The sperm bundles are able to perform a forward progression only when sperm are stick together with their tail posterior tips. On the contrary, when sperm have their tail ends free, they beat but are unable to progress. The functional significance of the sperm bundles is discussed., (Copyright © 2011. Published by Elsevier Ltd.)

Fertile male polyps of three colonies of the black coral Rhipidipathes reticulata (Aphanipathidae) from Togian Islands (Indonesia) have been the source of the sperm investigated at ultrastructural level, in order to compare their organization with that of other members belonging to the family Antipathidae and Myriopathidae. The extension of the studies to a representative of the family Aphanipathidae stresses once more the structural similarity of the male gametes in antipatharians. A sketch of the sperm model reports the similarity and differences in the examined taxa. Among the micro-characters, the cup-like body linked to the pericentriolar apparatus, is exclusive of the antipatharians. Other inclusions concern the more common pro-acrosomal vesicles or the acrosomelike structure observed only in Antipathella subpinnata and Myriopathes japonica. Lipid vesicles are occasionally present. A typical inclusion, the electron-dense content of which has a C-shaped configuration, is restricted to Rhipidipathes reticulata and is associated to the cup-like body or to the mitochondrion., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Libellula depressa (Odonata, Libellulidae) is an exophytic dragonfly ovidepositing eggs in clutches on the surface of floating plants and algae. The present work investigates, at ultrastructural level, the gradual differentiation of the egg envelopes and the chorionic changes after egg deposition in water. The ovary of the mature female of L. depressa is composed of numerous strings of panoistic ovarioles, where the eggshell formation takes place gradually throughout the activity of the follicle cells. The present data show that the egg envelopes are constituted of a very thick electrondense vitelline envelope, a thin endochorion and an extremely thick exochorion composed of a fibrillar matrix resting on a thin electrondense layer. After deposition in water, L. depressa eggs, initially white and almost transparent, gradually become brown spots in a semitransparent jelly coat, rich of incorporated debris. The jelly coat enveloping the eggs of L. depressa derives exclusively from the exochorion, constituted of a fibrillar matrix, which swell at contact with water. The jelly-like coat performs an adhesive function and presumably a protective role during egg segmentation and ensuing larval hatching.

This study investigated the development of the larvae of Spongia officinalis in experimental conditions, after settlement on plastic substrates, using electron and light microscopy. The released larvae show a dark pigmented ring distinguishes the posterior larval pole. The youngest larvae, covered with a flagellate epithelium, move onwards by rotating on their longitudinal axis. Over time a creeping-like motion prevails, probably linked to the need for settlement. After a free-swimming period of 24-48 h, larvae settle on the artificial substrate by the anterior pole. At settlement, the flagellate epithelium is substituted by flattened cells, which delimit the outermost surface. Post-larvae were reared to about three months. The early phase of post-larval differentiation shows a solid interior mainly consisting of granular cells varying in shape and size. They are included in a dense collagen matrix that contains a conspicuous amount of bacteria. Lacunae are already evident in the initial phase of metamorphosis. In several of them, cell debris and nucleate cells are visible. This feature is consistent with a progressive reduction of the cell mass (autolysis). Neither choanocyte chambers nor canals differentiate. The morphogenetic process leads to a metamorph only consisting of vacuolated cells and collagen fibrils included in a thin fibrous coat.

The association between the aquatic phases of the caddisfly Ceraclea fulva (Trichoptera, Leptoceridae) and the freshwater sponge Ephydatia fluviatilis (Porifera, Spongillidae) has been investigated by means of scanning and transmission electron microscopy (SEM, TEM). Ceraclea fulva habitually feeds on sponges and builds its case by using the siliceous spicules of the sponge, which are arranged side by side, inter-crossed, cemented with silk, and organised in layers. In the newly hatched larva, the case is strengthened exclusively by cemented siliceous spicules, while during growth, the insect adds sponge fragments to it. The fine organisation of the sponge tissues growing on the case proves that the sponge is functional. Inter-spaced, small protrusions, derived from the outermost compact silk layer, form a series of "bridges" enhancing case/sponge adhesion. Tube-case shape varies according to the aquatic developmental phase of the insect: in the mature larva and pupa, this shelter carries larger sponge fragments dorsally. The caddisfly acts as carrier of the sponge, thus facilitating its dispersal and the colonisation of new habitats. This justifies regarding this association as a successful mutualistic relationship, and not as a unilateral parasitic behaviour on the part of the insect., (Copyright 2003 Elsevier Science Ltd.)

The caddisfly Ceraclea fulva feeds exclusively on the freshwater sponge Ephydatia fluviatilis. Sponge spicules are accumulated in the insect midgut and arranged perpendicularly to the longitudinal axis of its gut. The peritrophic membrane of the midgut is so thick that it prevents spicules from damaging the epithelium during their transit. The digestion process of the endocellular zoochlorellae, which are vehiculated by the sponge cells, was examined by transmission electron microscopy (TEM). Zoochlorellae were seen in the midgut lumen, close to the peritrophic membrane and in the underlying space. Discrete algal cells became evident in tight apposition to the brush border of the midgut cells and were enveloped by the microvilli. Digestion progressed to the final transformation of the organism into membrane-delimited vacuoles.

To study interactions between sponge cells and components of the extracellular matrix (ECM), cells of the calcareous sponge Clathrina cerebrum were investigated in vitro by scanning electron microscopy. Cells were settled on glass coverslips, used as controls, and on coverslips coated with various ECM components (laminin, collagens and fibronectin), and with an adhesive substance (polylysine). Cells tended to conserve a rounded shape, producing thin, stiff processes (scleropodia) and lamellipodia, whose shape and extension varied according to the substrata. Spreading was observed only on polylysine, inducing cells to assume a fibroblast-like aspect. On laminin, cell adhesion was assured only by scleropodia. On fibronectin, scleropodia and lamellipodia were present, but reduced in size and length. On collagens, laminar processes occurred among prevailing scleropodia. Measurements of cell area and perimeter allowed statistical comparison of substrata, on the basis of their induction of cell flattening and protuberance formation. In summary, sponge cells were found to modulate their morphology in response to the external environment, expressing features for dynamic activities most fully in the presence of substances close to their natural ECM constituents. These results are discussed in the context of tissue rearrangement as a basic adaptation occurring throughout the life span of these organisms.