Taxonomic revision of the Mediterranean Plakina Schulze (Porifera, Demospongiae, Homoscleromorpha)

Only three species of the sponge genus Plakina Schulze have been described from the Mediterranean since 1880, in spite of a large amount of allegedly intraspecific variability in morphological characters. However, recent genetic studies based on electrophoretic techniques have revealed extensive cryptic speciation in north-western Mediterranean Plakina , demonstrating that most of this variation was interspecific rather than intraspecific. We describe in detail the morphology and anatomy of four new species of Plakina from the Mediterranean– P. crypta, P. weinbergi, P. endoumensis and P. jani –of which the latter two were discovered through allozyme electrophoresis. Plakina monolopha Schulze and P. trilopha Schulze are redescribed, and their morphological and geographical limits are discussed along with those of P. dilopha Schulze. Accurate analysis of the internal anatomy and of the shape and ramification pattern of lophose spicules in scanning electron microscopy provides new, powerful morphological criteria for species discrimination in Plakina. More widespread use of such new taxonomic characters should provide evidence against the alleged cosmopolitanism of some Plakina species, thus generating an increase in estimates of the biodiversity of plakinids.     

Morphological and genetic differences in ecologically distinct populations of Petrosia (Porifera, Demospongiae)

The taxonomic status of two sponge tnorphotypes living sympatrically in Mediterranean caves and usually ascribed to Petrosia ficiformis was elucidated on the basis of morphological, morphometric and genetic features. The two morphotypes, spherical and cylindrical, showed differences in the shape and size of spicules and in the morphology of the aquiferous system. Electrophoretic analyses demonstrated that the two morphotypes are reproductively independent and so they should be considered as distinct biological species. The spicular features allow attribution of the cylindrical morphotype to P.ficiformis and the spherical one to P. clavata. For these two species parasympatric speciation is proposed.     

Embryo development of Corticium candelabrum (Demospongiae: Homosclerophorida)

Abstract. Corticium candelabrum is a homosclerophorid sponge widespread along the rocky Mediterranean sublittoral. Scanning and transmission electron microscopy were used to describe the gametes and larval development. The species is hermaphroditic. Oocytes and spermatocytes are clearly differentiated in April. Embryos develop from June to July when the larvae are released spontaneously. Spermatic cysts originate from choanocyte chambers and spermatogonia from choanocytes by choanocyte mitosis. Oocytes have a nucleolate nucleus and a cytoplasm filled with yolk granules and some lipids. Embryos are surrounded by firmly interlaced follicular cells from the parental tissue. A thin collagen layer lies below the follicular cells. The blastocoel is formed by migration of blastomeres to the morula periphery. Collagen is spread through the whole blastocoel in the embryo, but is organized in a dense layer (basal lamina) separating cells from the blastocoel in the larva. The larva is a typical cinctoblastula. The pseudostratified larval epithelium is formed by ciliated cells. The basal zone of the ciliated cells contains lipid inclusions and some yolk granules; the intermediate zone is occupied by the nucleus; and the apical zone contains abundant electron-lucent vesicles and gives rise to cilia with a single cross-striated rootlet. Numerous paracrystalline structures are contained in vacuoles within both apical and basal zones of the ciliated cells. Several slightly differentiated cell types are present in different parts of the larva. Most cells are ciliated, and show ultrastructural particularities depending on their location in the larvae (antero-lateral, intermediate, and posterior regions). A few smaller cells are non-ciliated. Several features of the C. candelabrum larva seem to support the previously proposed paraphyletic position of homoscleromorphs with respect to the other demosponges.     

Association of the sponge Tethya orphei (Porifera, Demospongiae) with filamentous cyanobacteria

Abstract. Specimens of the sponge Tethya orphei , collected in February 2005 on the underside of coral stones on Arì Athol (Maldives), have been processed for histological and ultrastructural investigations. The cortical layer of the sponge was found to be permeated by filamentous cyanobacteria, the trichomes of which measured 45–63 μm on average and were composed of 10–14 cells. The fine organization of the filaments was consistent with their taxonomic identification as Oscillatoria spongeliae. These filaments filled the cortical region of the sponge and penetrated inward into the upper choanosomal region, where they sometimes overlapped the siliceous spicule bundles. A budding specimen of T. orphei showed that the filaments were also present in the single bud protruding from the sponge surface, demonstrating that asexual reproduction can vertically transmit these symbionts from sponge to sponge. The occurrence of filaments in all the specimens studied is consistent with the assumption that filamentous cyanobacteria are not mere intruders but mutualistic symbionts with members of T. orphei.     

Epibiontic and endobiontic polychaetes of Geodia cydonium (Porifera,Demospongiae) from the Mediterranean Sea

Polychaete assemblages associated to the sponge Geodia cydonium were investigated at two sampling sites in the Mediterranean Sea: Porto Cesareo Basin (Apulia) and Marsala Lagoon (Sicily), both characterized by sheltered hydrodynamic conditions. Samples were seasonally performed during 1997, in order to compare the assemblages coming from the two localities studied, considering separately the internal and external tissues of the sponge, and with the aim of evaluating the influence of sponge size on polychaete colonization. The examined sponge is characterized by a peculiar stratification of its tissues: an external thick and hard layer, the cortex, and an internal softer one, the choanosome. Statistical analysis showed that this was the main factor controlling polychaete assemblage, with the internal tissue, less rich and diversified, appearing impoverished with respect to the external layer. A similarity in species composition was observed between sites, even though some differences were evidenced in the abundance of some species, mainly reflecting differences in local environmental conditions. Species richness and density increased with the increasing sponge size. Such a situation is particularly evident at Porto Cesareo, where sponges are covered by an algal layer which is particularly rich on the largest specimens, thus suggesting that most of the species of polychaetes were linked more to the neighbouring environment than to the sponge itself.     

An unexpectedly sophisticated, V-shaped spermatozoon in Demospongiae (Porifera): reproductive and evolutionary implications

The demosponge Crambe crambe shows a peculiar spermatogenesis, hard to be reconciled with the basal position of sponges in the animal phylogeny. Early spermatogenesis stages showed most of the simple features expected in sponges. However, spermiogenesis departed from the anticipated process. Spermatids lengthened remarkably, forming a deep cytoplasmic pit around the cilium insertion, with the proximal axoneme bending to produce a V-shaped spermatozoon surprisingly similar to that known in the phylum Phoronida. The cytology was unexpectedly complex, with a needle-like nucleus of helically condensed chromatin, a conical acrosome with a subacrosomal rod, and a mitochondrion connected to the basal body by striated rootlets. These findings establish that the spermatozoon of broad-casting demosponges occurs in two structural categories ('primitive' and 'modified' type). This dualistic condition must necessarily have pre-dated the evolutionary apparition of higher metazoans, if we are to keep regarding sponges as the most primitive animals. We hypothesize that internal fertilization in C. crambe – and incidentally other demosponges – may depart from the general model assumed for spermcasting sponges. The V-shape of this spermatozoon suggests a design to favour autonomous penetration through the dense mesohyl to reach the oocytes, rather than engulfment and transportation by carrier cells towards the oocyte.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 413–426.     

Ultrastructural study of spermatogenesis in Oscarella lobularis (Porifera,Demospongiae)

Summary

The various phases of spermatogenesis in the demosponge Oscarella lobularis were studied by electron microscopy. Spermatogenesis occurs within spermatic cysts, which are presumed to derive from choanocyte chambers by transformation of choanocytes into spermatogonia. Germ cells develop asynchronously within spermatocysts, and cytoplasmic bridges, indicating incomplete cells division, connect several germ cells. Attached spermatogonia suggest gonial generations. Spermatocytes I typically show the presence of synaptonemal complexes indicating meiotic divisions. Spermatocytes II have a small size probably because of the meiotic divisions of spermatocytes I. Spermatids are characterized by an acrosome, a big mitochondrion and a peripheral sheath of condensed chromatin surrounding a clearer central area in the nucleus. The mature spermatozoon shows a lateral flagellum and a flattened acrosome capping the nucleus. The phylogenetic implications of some features of the spermatozoon are suggested.     

Choanocyte ultrastructure in Halisarca dujardini (Demospongiae,Halisarcida)

Understanding poriferan choanocyte ultrastructure is crucial if we are to unravel the steps of a putative evolutionary transition between choanoflagellate protists and early metazoans. Surprisingly, some aspects of choanocyte cytology still remain little investigated. This study of choanocyte ultrastructure in the halisarcid demosponge Halisarca dujardini revealed a combination of minor and major distinctive traits, some of them unknown in Porifera so far. Most significant features were 1) an asymmetrical periflagellar sleeve, 2) a battery of specialized intercellular junctions at the lateral cell surface complemented with an array of lateral interdigitations between adjacent choanocytes that provides a particular sealing system of the choanoderm, and 3) a unique, unexpectedly complex, basal apparatus. The basal apparatus consists of a basal body provided with a small basal foot and an intricate transverse skeleton of microtubules. An accessory centriole, which is not perpendicular to the basal body, is about 45°. In addition, a system of short striated rootlets (periodicity = 50–60 nm) arises from the proximal edge of the basal body and runs longitudinally to contact the nuclear apex. This is the first flagellar rootlet system ever found in a choanocyte. The accessory centriole, the rootlet system, and the nuclear apex are all encircled by a large Golgi apparatus, adding another distinctive feature to the choanocyte cytology. The set of distinct features discovered in the choanocyte of H. dujardini indicates that the ultrastructure of the poriferan choanocyte may vary substantially between sponge groups. It is necessary to improve understanding of such variation, as the cytological features of choanocytes are often coded as characters both for formulation of hypotheses on the origin of animals and inference of phylogenetic relationships at the base of the metazoan tree. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Morphological and ecological differences in two electrophoretically detected species of Cliona (Porifera, Demospongiae)

The taxonomie status of different morphotypes generally ascribed to the species of boring sponge Cliona viridis is examined. Morphometric, ecological and genetic differences between two morphotypes living sympatrically on rocky shores of the Ligurian Sea are considered. Morphotype 1 is characterized by small, well separated papillae (α stage), exclusively boring into the crust of coralline algae on the exposed rocky bottoms of the upper sublittoral zone. Morphotype 2 shows bigger papillae, often connected by portions of encrusting tissue ( β stage), and is predominantly found boring into organogenic concretions in slightly lighted habitats. Measurements of tylostyles indicate that morphotype 1 has spicules significantly smaller than morphotype 2. Electrophoretic analysis shows that the two morphotypes are fixed for different alleles at all four loci scored, indicating absence of gene flow between the two populations, i.e. they appear to be distinct biological species.     

Metamorphosis of cinctoblastula larvae (Homoscleromorpha, porifera)

The metamorphosis of the cinctoblastula of Homoscleromorpha is studied in five species belonging to three genera. The different steps of metamorphosis are similar in all species. The metamorphosis occurs by the invagination and involution of either the anterior epithelium or the posterior epithelium of the larva. During metamorphosis, morphogenetic polymorphism was observed, which has an individual character and does not depend on either external or species specific factors. In the rhagon, the development of the aquiferous system occurs only by epithelial morphogenesis and subsequent differentiation of cells. Mesohylar cells derive from flagellated cells after ingression. The formation of pinacoderm and choanoderm occurs by the differentiation of the larval flagellated epithelium. This is possibly due to the conservation of cell junctions in the external surface of the larval flagellated cells and of the basement membrane in their internal surface. The main difference in homoscleromorph metamorphosis compared with Demospongiae is the persistence of the flagellated epithelium throughout this process and even in the adult since exo- and endopinacoderm remain flagellated. The antero-posterior axis of the larva corresponds to the baso-apical axis of the adult in Homoscleromorpha.     

Zoobenthos-inhabiting Sarcotragus muscarum (Porifera: Demospongiae) from the Aegean Sea

The cavities and outer surfaces of 20 individuals of the sponge Sarcotragus muscarum collected from the Turkish Aegean coast during July–September 1993 contained a total of 148 zoobenthic species with 5299 individuals belonging to seven taxonomic groups. Among these, Polychaeta accounted for 60% of the total number of species, while Crustacea comprised 71% of the total number of individuals and 40% of the wet weight of biomass. The amphipod and decapod crustaceans Tritaeta gibbosa and Synalpheus gambarelloides, and the bivalve Hiatella arctica, were the most abundant species on the sponge samples both in terms of number of individuals and biomass values. The community structure inside the sponges was greatly influenced by the nature of the substratum around the sponge, and by the overwhelming population of the Lessepsian echinoderm Ophiactis savignyi. Number of species, biomass and diversity index values were found to be positively related to the sponge volume. Mean density and biomass of the associated taxa are 0.22 ind cm^–3 and 11.63 mg cm^–3, respectively.     

Fauna of Sponges (Porifera) of Peter the Great Bay,Sea of Japan

A complete list of sponges of Peter the Great Bay (northwestern Sea of Japan) is presented comprising 40 species belonging to 27 genera, 19 families, 9 orders, and 2 classes. Information is presented about the biogeographical composition of sponge fauna of the bay, the depths of their habitats, and their substrates.Original Russian Text Copyright ¢ 2005 by Biologiya Morya, Khodakovskaya.     

Investigation of the budding process in Tethya citrina and Tethya aurantium (Porifera, Demospongiae)

The budding process has been studied in two congeneric Mediterranean species belonging to Tethya from different sampling sites: Marsala and Venice Lagoons (Tethya citrina); Marsala Lagoon and Porto Cesareo Basin (Tethya aurantium). Buds, connected to the adult by a spiculated stalk, differ between the two species in morphology and size, since those of T. citrina are small with elongated bodies, showing only a few spicules protruding from the apical region, whereas those of T. aurantium are round, larger, and show spicules radiating from the peripheral border. In T. citrina, cells with inclusions, varying in electron density and size, represent the main cell types of the buds. In T. aurantium, the cell component shows a major diversification, resulting from spherulous cells, grey cells, vacuolar cells and peculiar micro-vesicle cells. Neither canals nor choanocyte chambers were observed in the buds of the two species. In T. citrina, bud production is similar in both sampling sites. In T. aurantium, budding occurs more rarely in Porto Cesareo Basin, probably in relation with environmental factors, such as the covering of the cortex by sediment and micro-algae. Finally, in the buds of both species, the spicule size does not differ from that of the cortex of the adult sponges, further supporting the main involvement of the cortex in organizing the skeletal architecture of the buds.     

Morphology and ultrastructure of the swimming larvae of Crambe crambe (Demospongiae, Poecilosclerida)

Abstract. We describe the morphology and ultrastructure of the free-swimming larvae of the sponge Crambe crambe , one of the most abundant encrusting sponges on shallow rocky bottoms of the western Mediterranean. Larvae of C. crambe are released in July and August. The larva is uniformly flagellated except at the posterior zone. Flagellated cells are extraordinarily slender, elongate, and sinuous and form a pseudo-stratified layer. Their distal zone contains abundant mitochondria, some small vesicles, a Golgi complex, and the basal apparatus of the flagellum. Abundant lipid droplets are present throughout the cell. The nucleus is most often in a basal position. The flagellum projects from the bottom of an asymmetrical socket formed by cytoplasmic expansions. The basal body extends in a conical tuft and a laminar rootlet in close association with the Golgi system. The cells at the posterior pole are flat and polygonal on the surface, with long overlapping pseudopodia in the typical shape of a pinacoderm. Sparse collagen is present throughout the whole larva including the flagellated layer. Archeocytes and sclerocytes are abundant in the posterior region. Typical collencytes and spherulous cells seem to be absent. Intracellular and extracellular rod-like bacteria with conspicuous fimbria occur exclusively in the posterior region of the larva. The asymmetrical cytoplasmic prolongations, which surround the flagellum, and the basal apparatus of the flagellum are suggested as the sites of stimulus reception and triggering of locomotor responses, respectively. This ultrastructural study of the larva of C. crambe has shown features directly linked to its behavior and ecology.     

The spermatogenesis ofHalichondria panicea (Porifera,Demospongiae)

Summary Spermatogenesis of the marine spongeHalichondria panicea begins with the break up of choanocyte chambers, choanocytes constituting the origin of spermatogonia. The transition from choanocytes to spermatogonia is direct, without cell division. Already the spermatogonia are flagellated. The ensuing large aggregates of spermatogonia are enclosed by spermatocyst-building cells. Further development takes place within the spermatocysts, mostly arranged in fields which, however, lack any developmental gradient. Within a single spermatocyst development is mostly synchronous. Spermatogonia transform into first order spermatocytes directly. The transition from spermatid to spermatozoon is characterized by an unusual prolongation of the chromatin, often resulting in a helical form of the chromosome material and a strong enlargement of the mitochondria which align with the nucleus, leading to an irregular shape of the spermatozoon. Another exceptional feature is the virtual absence of a Golgi apparatus during all stages of spermatogenesis. TheH. panicea investigated here contained only male reproductive elements, thus appear to be gonochorists. Some features of the spermatogenesis ofH. panicea, such as dissolving choanocyte chambers, the enclosure of spermatogonia by spermatocyst-building cells and the formation of a synaptonemal complex in first order spermatocytes occur in other sponge species as well; however, the early presence of flagella in spermatogonia, the absence of the Golgi apparatus and the later irregular development of nuclei, mitochondria and the spermatozoa themselves represent features hitherto not observed in sponges.     

Symbiosis of Mycale (Mycale) vansoesti sp. nov. (Porifera, Demospongiae) with a coralline alga from North Sulawesi (Indonesia)

Abstract. The symbiotic association between the new sponge species Mycale vansoesti and the coralline alga Amphiroa sp. from the Bunaken Marine Park (North Sulawesi, Indonesia) is described. The alga completely pervades the sponge. The color of the sponge ectosome is white, both on the external surface and on the atrial wall, but where the alga is present the sponge takes on the light pink color of the alga. The sponge spicular complement is characterized by mycalostyles, anisochelae of two types, sigmas (often "C" shaped), and extremely abundant toxas organized in bundles forming toxadragma. In the association, the sponge shows very low silicate value, and consequently the alga represents the main skeleton of the sponge. On the other hand, the sponge affects the morphology of the alga, leading to a cylindrical shape, with thalli running parallel to the sponge surface. This association seems to be obligate for the sponge, as we found no sponges of this species living in isolation.     

Morphogenesis accompanying larval metamorphosis in Plakina trilopha (Porifera, Homoscleromorpha)

Morphological investigations of morphogenesis accompanying the metamorphosis of the cinctoblastula larva of poriferan Plakina trilopha (Homoscleromorpha) have been made. The larva possesses a distinct columnar epithelium which subdivides into three cellular areas: antero-lateral, postero-lateral, and posterior one. Characteristic morphological features of the cells in each area can be used as natural markers when tracing the fate of larval cells during metamorphosis. The ciliated epithelium of the larva is transformed directly into choanoderm and pinacoderm, without losing its organization. This transformation is a peculiar feature of the metamorphosis in Homoscleromorpha. Metamorphosis in P. trilopha is based on epithelial morphogenesis and includes the mechanisms of flattening of the exopinacoderm, evagination and invagination of larval epithelium in the course of the development of the rhagon aquiferous system. The flattening of larval cells during exopinacoderm formation in metamorphosing P. trilopha is a common change of cell shape during epithelial morphogenesis of this species. The separation of proximal fragments of cells has been observed here. This phenomenon, that we have called “cytoplasmic shedding”, appears to play an important role in the change of epithelial cell shape in P. trilopha. Mechanism of epithelial–mesenchymal transition, i.e., ingression of epithelial ciliated cells into the cavity of the metamorphosing larva of P. trilopha participates in mesohylar cell origin.