Body structure of marine sponges

Summary Each choanocyte chamber of Petrosia ficiformis is formed by a slightly outpocked choanocyte epithelium and by a ring of three or four uniflagellated cone cells surrounding the apopyle. The apopyle opens into a small aphodus, which leads the water flow to larger excurrent canals. Pinacocytes of the incurrent canal system cover the basal surface of the choanocytes and separate them from the incurrent canals and the mesenchyme. The water flows into the chambers by pores in the pinacocyte cover and then through gaps between adjacent choanocytes. To our knowledge this is the first report of a leuconoid canal system in which choanocyte chambers are covered by a pinacocyte epithelium of the incurrent canal system that isolates the chambers from the mesenchyme. A future comprehensive revision of the types of canal systems in sponges seems to be necessary. Permanent affiliation: Department of Biology and Health Sciences, University of Hartford, West Hartford, CT 06117, USA     

Die Aufnahme partikulärer Nahrung beiReniera sp. (Porifera)

The choanocyte chambers of the marine spongeReniera sp. protrude with their curved outer surface free into the incurrent canals. The water is sucked into the chambers by cavities between the choanocytes. Particles up to 1 µm in diameter may enter the chambers with the water current. These particles are trapped on the outer surface of the choanocyte collars and are ingested by the choanocytes and processes of the pinacocyte epithelium of the incurrent canal system, which project into the chambers. Bigger particles are retained in the incurrent canals mainly on the outer surface of the choanocyte chambers. They are ingested by pinacocytes of the canal wall and transported to cells of the mesenchyme. The present investigation shows the great importance of the pinacocyte epithelium of the incurrent canal system for suspension feeding inReniera sp.     

Ultrastructure and embryonic development of a syconoid calcareous sponge

Abstract. Recent molecular data suggest that the Porifera is paraphyletic (Calcarea+Silicea) and that the Calcarea is more closely related to the Metazoa than to other sponge groups, thereby implying that a sponge‐like animal gave rise to other metazoans. One ramification of these data is that calcareous sponges could provide clues as to what features are shared among this ancestral metazoan and higher animals. Recent studies describing detailed morphology in the Calcarea are lacking. We have used a combination of microscopy techniques to study the fine structure of Syconcoactum Urban 1905, a cosmopolitan calcareous sponge. The sponge has a distinct polarity, consisting of a single tube with an apically opening osculum. Finger‐like chambers, several hundred micrometers in length, form the sides of the tube. The inner and outer layers of the chamber wall are formed by epithelia characterized by apical–basal polarity and occluding junctions between cells. The outer layer—the pinacoderm—and atrial cavity are lined by plate‐like cells (pinacocytes), and the inner choanoderm is lined by a continuous sheet of choanocytes. Incurrent openings of the sponge are formed by porocytes, tubular cells that join the pinacoderm to the choanoderm. Between these two layers lies a collagenous mesohyl that houses sclerocytes, spicules, amoeboid cells, and a progression of embryonic stages. The morphology of choanocytes and porocytes is plastic. Ostia were closed in sponges that were vigorously shaken and in sponges left in still water for over 30 min. Choanocytes, and in particular collar microvilli, varied in size and shape, depending on their location in the choanocyte chamber. Although some of the odd shapes of choanocytes and their collars can be explained by the development of large embryos first beneath and later on top of the choanocytes, the presence of many fused collar microvilli on choanocytes may reflect peculiarities of the hydrodynamics in large syconoid choanocyte chambers. The unusual formation of a hollow blastula larva and its inversion through the choanocyte epithelium are suggestive of epithelial rather than mesenchymal cell movements. These details illustrate that calcareous sponges have characteristics that allow comparison with other metazoans—one of the reasons they have long been the focus of studies of evolution and development.     

Choanoflagellates, choanocytes, and animal multicellularity

Abstract. It is widely accepted that multicellular animals (metazoans) constitute a monophyletic unit, deriving from ancestral choanoflagellate‐like protists that gave rise to simple choanocyte‐bearing metazoans. However, a re‐assessment of molecular and histological evidence on choanoflagellates, sponge choanocytes, and other metazoan cells reveals that the status of choanocytes as a fundamental cell type in metazoan evolution is unrealistic. Rather, choanocytes are specialized cells that develop from non‐collared ciliated cells during sponge embryogenesis. Although choanocytes of adult sponges have no obvious homologue among metazoans, larval cells transdifferentiating into choanocytes at metamorphosis do have such homologues. The evidence reviewed here also indicates that sponge larvae are architecturally closer than adult sponges to the remaining metazoans. This may mean that the basic multicellular organismal architecture from which diploblasts evolved, that is, the putative planktonic archimetazoan, was more similar to a modern poriferan larva lacking choanocytes than to an adult sponge. Alternatively, it may mean that other metazoans evolved from a neotenous larva of ancient sponges. Indeed, the Porifera possess some features of intriguing evolutionary significance: (1) widespread occurrence of internal fertilization and a notable diversity of gastrulation modes, (2) dispersal through architecturally complex lecithotrophic larvae, in which an ephemeral archenteron (in dispherula larvae) and multiciliated and syncytial cells (in trichimella larvae) occur, (3) acquisition of direct development by some groups, and (4) replacement of choanocyte‐based filter‐feeding by carnivory in some sponges. Together, these features strongly suggest that the Porifera may have a longer and more complicated evolutionary history than traditionally assumed, and also that the simple anatomy of modern adult sponges may have resulted from a secondary simplification. This makes the idea of a neotenous evolution less likely than that of a larva‐like choanocyte‐lacking archimetazoan. From this perspective, the view that choanoflagellates may be simplified sponge‐derived metazoans, rather than protists, emerges as a viable alternative hypothesis. This idea neither conflicts with the available evidence nor can be disproved by it, and must be specifically re‐examined by further approaches combining morphological and molecular information. Interestingly, several microbial lin°Cages lacking choanocyte‐like morphology, such as Corallochytrea, Cristidiscoidea, Ministeriida, and Mesomycetozoea, have recently been placed at the boundary between fungi and animals, becoming a promising source of information in addition to the choanoflagellates in the search for the unicellular origin of animal multicellularity.     

Feeding in a calcareous sponge: particle uptake by pseudopodia

Sponges are considered to be filter feeders like their nearest protistan relatives, the choanoflagellates. Specialized "sieve" cells (choanocytes) have an apical collar of tightly spaced, rodlike microvilli that surround a long flagellum. The beat of the flagellum is believed to draw water through this collar, but how particles caught on the collar are brought to the cell surface is unknown. We have studied the interactions that occur between choanocytes and introduced particles in the large feeding chambers of a syconoid calcareous sponge. Of all particles, only 0.1-microm latex microspheres adhered to the collar microvilli in large numbers, but these were even more numerous on the choanocyte surface. Few large particles (0.5- and 1.0-microm beads and bacteria) contacted the collar microvilli; most were phagocytosed by lamellipodia at the lateral or apical cell surface, and clumps of particles were engulfed by pseudopodial extensions several micrometers from the cell surface. Although extensions of the choanocyte apical surface up to 16 microm long were found, most were 4 microm long, twice the height of the collar microvilli. These observations offer a different view of particle uptake in sponges, and suggest that, at least in syconoid sponges, uptake of particles is less dependent on the strictly sieving function of the collar microvilli.     

Canal systems and choanocyte chambers in freshwater sponges (Porifera,Spongillidae)

Summary The spongillid species Spongilla lacustris and Ephydatia fluviatilis possess choanocyte chambers of the classical eurypylous type. They are surrounded by the mesenchymal tissue and connected to the incurrent canal system by prosopyles and to the excurrent canal system by wide apopyles. Each apopyle is sealed against spaces between the basal choanocyte collar parts by a ring of uniflagellated cone cells. The functional aspects of the choanocyte chamber and canal structure are discussed.Dedicated to Prof. Dr. K.E. Wohlfarth-Bottermann, Bonn, in honor of his 65th birthday     

Formation and construction of asexual buds of the freshwater spongeRadiospongilla cerebellata (Porifera,Spongillidae)

Summary The buds ofRadiospongilla cerebellata are formed asexually. Budding can be induced experimentally by injuring the sponge. The first sign of budding is a slight elevation of some surface areas, which proceed to rise rapidly so that they soon protrude conspicuously from the surface of the sponge. As a bud develops, the broad base joining it to the mother sponge narrows to a stalk, which finally breaks. The free buds drift in the water for 15–20 min and then settle, forming new sessile sponges. The buds, 1.5–2.5 mm in diameter, have an internal organization identical with that of the mother sponge. They are enclosed in a layer of pinacoderm perforated by dermal pores. Under the pinacorderm there is a shallow subdermal space, which is in communication with the incurrent canals leading to the choanocyte chambers. The water sucked into these chambers proceeds into the excurrent canal system and emerges from the sponge through the oscular tube. Spicules projecting radially from the bud bear apical tufts of microscleres. The skeletal spicules of the buds, like their choanocyte chambers, are smaller than those in the mother sponge. The chambers expand to their mature size by choanocyte mitosis. Buds and sponges are colored green by intracellular symbiotic algae of the genusChlorella.     

Comparative study of the choanosome of Porifera: 1. The Homoscleromorpha

The choanoderm and pinacoderm of representatives of the two families of Homoscleromorpha sponges, the Oscarellidae and Plakinidae, have been examined by transmission and scanning electron microscopy. Different fixative procedures have shown the dramatic influence of fixation conditions on the morphology of choanocytes. These two families of sponges have the following morphological features in common: flagellated endopinacocytes with short apical microvilli and basal pseudopods; the presence of a very thin and dense sheet of matrix material which limits the mesohyl. There are, however, only minor differences in the flagellar morphology, granule content, and anchoring system of their choanocytes. Two findings are of particular interest: (1) the presence of glycocalyx bridges between the microvilli of the choanocyte collar; and (2) the discovery of a new cell type, the apopylar cell, which has a morphology intermediate between that of pinacocytes and choanocytes. The apopylar cells limit the apopylar opening of the choanocyte chamber and indicate the transition between choanoderm and pinacoderm.     

Continuous cell movements rearrange anatomical structures in intact sponges.

Time-lapse cinemicrography was used to record the active movements of cells in living intact sponges. Each of the three main cell types (pinacocytes, mesohyl cells, and choanocytes) continuously moved and rearranged themselves so that the internal anatomy of the sponge was continuously remodeled. The shape and appearance of the sponges anatomical structures often changed substantially within a few hours. The most motile were the mesohyl cells, with many moving as fast as one cell-length per minute (15 microns/min). Mesohyl cell locomotion was often accompanied by displacements of spicules, canals, and choanocyte chambers; the patterns of these displacements suggested that the mesohyl cells were providing the motive forces for these rearrangements. The locomotion of the pinacocytes varied according to position: those along the outer sponge margins were most active, whereas those in other parts of the surface moved relatively little. Choanocytes were never observed to undergo independent locomotion but were always found grouped together in choanocyte chambers. These choanocyte chambers interacted with pinacocytes and mesohyl cells to form excurrent canals, which continuously moved, fused with, and branched from one another. These observations suggest that the experimental phenomenon of sponge cell-reaggregation and reconstitution, discovered by H. V. Wilson, represents an extreme version of morphogenetic processes that normally go on continuously within intact sponges. The results from the present study also suggest that these cellular rearrangements are controlled by active cell movements and behavioral responses that include but are not limited to selective cell adhesion.     

Body structure of marine sponges

Summary Specimens of Haliclona elegans (Bowerbank, 1866) are covered by a thin, double layered dermal membrane extending over large subdermal spaces. The pores in the dermal membrane are formed by single porocytes with one or sometimes several pores in the center of the cell. The subjacent tissue shows a faintly developed mesenchyme and numerous big choanocyte chambers projecting into lacunar spaces of the incurrent canal system. The outer surface of the chambers is directly covered by the pinacocyte epithelium of the incurrent canal wall, which also separates them completely from the mesenchyme. Water influx into the chambers is guaranteed by prosopylar openings in the pinacocyte cover at the outer chamber surface. The chambers are connected to the excurrent canal system in the eurypylous way by wide apopyles, each of which is surrounded by a small ring of flagellated cone cells. About 15% of the choanocyte chambers in H. elegans contain central cells, which are thought to derive from migrating pinacocytes of the canal systems.     

The architecture of the canal systems of Petrosia ficiformis and Chondrosia reniformis studied by corrosion casts (Porifera,Demospongiae)

Summary The three-dimensional organization of the canal system in two sponge species, Petrosia ficiformis and Chondrosia reniformis, was studied using corrosion casts. Casts were made of live animals, in situ, and canal replicas were analzyed by scanning electron microscopy (SEM). In P. ficiformis the incurrent system consists of a superficial canal network giving rise to large radial canals, which ramify and anastomosize forming an internal web. Excurrent canals are arranged into modular ramified systems radiating from atrial cavities opening to the exterior. Main excurrent canals run at various depths within the sponge, even through the superficial incurrent network. Both incurrent and excurrent canal replicas show smooth, blind-ending capillaries. Some large incurrent canals merge with excurrent ones, thus bypassing choanocyte chambers. In C. reniformis there is a cortical collagen layer crossed by three-like incurrent canals, the twigs of which communicate with groups of inhalant pores. The stems of tree-like canals penetrate into the sponge medulla where they ramify and anastomosize to form a web. Main excurrent canals arise from large cloacal ducts leading to the oscular openings. They give rise to a sequence of branches intersecting the incurrent web. Both incurrent and excurrent canals have sharp, blind-ending capillaries. Morphometric data functions show that diameter scaling in canal branches is exponential in Petrosia and linear in Chondrosia. Structural differences and homologies between the two species are discussed.     

STRUCTURE AND FORMATION OF THE COLLAR IN CHOANOCYTES OF TETILLA SERICA (LEBWOHL), DEMOSPONGE

There are two types of collar in the choanocytes of adult Tetilla serica : one type is a continuous cytoplasmic tube and the other consists of discontinuous microvilli. The former is found in the small flagellated chamber and is considered to belong to a young choanocyte in the process of differertiation. To confirm this idea, very young choanocytes which are about to differentiate the collar were examined during embryogenesis.
The youngest choanocytes are noticed forming aggregations of small cells in 3-day larvae. Around the flagellum in each choanocyte, there is a depression which will become wider. At first, the collar is observed as a ring of cytoplasm; next this extends outward and becomes thinner, and finally it divides into microvilli. The microvillous collar is formed by the opening of vesicles and fusion of their membranes. These vesicles are considered to be derived from the Golgi complex. The process of collar formation through fusion of vesicles is discussed.     

The filtration apparatus for food collection in freshwater sponges (Porifera,Spongillidae)

Summary The freshwater sponges (Spongillidae) feed by filtering out small particles from the water passing through them by means of strainer devices in the flagellated chambers. These are filamentous, fine-meshed structures at the distal ends of the choanocyte collars formed of a mucous material similar to that in the glycocalyx. Each strainer separates its flagellated chamber into an outer and an inner zone. The strainers are an extremely efficient filtering mechanism.     

Flagellar apparatus structure of choanocyte in Sycon sp. and its significance for phylogeny of Porifera

The problem of the origin of Metazoa has been one of the most discussed for nearly the last one and a half centuries. Some 20 years ago, morphological approaches were replaced with molecules, but the problem then became more complex. At the same time, morphological data were incomplete, and therefore, this approach can still help in comparison with choanoflagellates and sponges—two sister groups having uniflagellated cells with a collar. The structure of the flagellar apparatus has phylogenetic significance, but sponge choanocytes are poorly studied in this respect, and we still do not know what the ancestral kinetid of Porifera looks like. The kinetid structure of choanocytes in Sycon sp. is investigated here for the first time, and a 3D reconstruction of the kinetid provided. It is composed of a flagellar kinetosome with a nuclear fibrillar root and a basal foot with a few microtubules; the accessory centriole lies orthogonal to and just below the foot of the kinetosome, and a dictyosome is near the centriole. This kinetid is similar to that of the choanocyte of Corticium candelabrum (Homoscleromorpha) and is considered to be the ancestral type for the whole branch Calcarea + Homoscleromorpha.     

Spicule and flagellated chamber formation in a growth zone of Aphrocallistes vastus (Porifera,Hexactinellida)

Three species of glass sponges (Class Hexactinellida) form massive deep‐water reefs by growing on the skeletons of past generations, with new growth largely vertical and away from sediment that buries the lower portions. Growth is therefore essential for reef health, but how glass sponges produce new skeleton or tissue is not known. We used fluorescence, light, and electron microscopy to study skeletal and tissue growth in the reef‐forming glass sponge Aphrocallistes vastus. The sponge consists of a single large tube (the osculum), usually with several side branches, each of which can function as an effective excurrent vent. New tissue forms at the tips of each of these extensions, but how this occurs in a syncytial animal, and how the tubes expand laterally as the sponge gets larger, are both unknown. The fluorescent dye PDMPO labeled more spicule types in the tips of the sponge than elsewhere, indicating growth that was concentrated at the edge of the osculum. New tissue production was tracked using the thymidine analog EdU. EdU‐labeled nuclei were found predominantly at the edge or lip of the osculum. In that region new flagellated chambers were formed from clusters of choanoblasts that spread out around the enlarging chamber. In cellular sponges clusters of choanocytes form flagellated chambers through several rounds of mitotic divisions, and also by immigration of mesohyl cells, to expand the chamber to full size. By contrast, chambers in glass sponges expand as choanoblasts produce enucleate collar bodies to fill them out. Growing chambers with enucleate structures may be an adaptation to life in the deep sea if chambers with cells, and therefore more nuclei, are costly to build.     

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.     

Hydroxyurea: An Inhibitor of the Differentiation of Choanocytes in Fresh-Water Sponges and a Possible Agent for the Isolation of Embryonic Cells

During the development of a fresh-water sponge from its gemmules, most cell types originate from the undifferentiated archaeocytes through a few divisions, whereas each choanocyte chamber, composed of several tens of choanocytes, arises from a single archaeocyte through repeated mitoses.
This process was studied on gemmules incubated in various concentrations of hydroxyurea.
A concentration of 100 μg/ml postponed the hatching by about two days, and blocked the differentiation of the choanocytes and the morphogenesis of the aquiferous system. The resulting organism was a hollow dome of pinacoderm, stretched on spicules, the bottom of which was strewn with embryonic archaeocytes. After washing and incubation in mineral medium, the sponge differentiated its choanocytes and achieved normal development.
The incorporation of ³H-thymidine into DNA was compared throughout the development of normal and hydroxyurea-treated gemmules. Hydroxyurea delayed the first peaks of incorporation and abolished the large peak that normally occurs around 90 h, just before the formation of choanocyte chambers.
When added after 96 h incubation, hydroxyurea did not affect the differentiation of the choanocytes.
These results suggest that the differentiation of the choanocytes and the further morphogenesis of the aquiferous system depend on the repetitive divisions of the archaeocytes that normally occur around 90 h.
Furthermore, hydroxyurea-blocked sponges provide a suitable source for the isolation of pure populations of embryonic archaeocytes.     

A new type of central cell in the choanocyte chambers of Pellina fistulosa (Porifera,Demospongiae)

Central cells of a hitherto unknown type, forming a continuous, perforated layer at the level of the distal collar ends in each choanocyte chamber, have been found in the choanocyte chambers of Pellina fistulosa. The collars project through the pores of the perforated central cell layer. The spaces between the collar ends and between the collars and the cone cell ring in the apopyle region are sealed by the central cell cytoplasm. The latter represents an impermeable barrier for particulate material as well as for water and thus enhances the filtration efficiency by preventing a bypass of water and particles between the collar apices.     

Hsp70 sequences indicate that choanoflagellates are closely related to animals.

Over 130 years ago, James-Clark noted a remarkable structural similarity between the feeding cells of sponges (choanocytes) and a group of free-living protists, the choanoflagellates. Both cell types possess a single flagellum surrounded by a collar of fine tentacles. The similarity led to the hypothesis that sponges, and, by implication, other animals, evolved from choanoflagellate-like ancestors. Phylogenetic analysis of ribosomal DNA neither supports nor refutes this hypothesis. Here, we report the sequence of an hsp70 gene and pseudogene from the freshwater choanoflagellate Monosiga ovata. These represent the first nuclear-encoded protein-coding sequences reported for any choanoflagellate. We find that Monosiga and most bilaterian hsp70 genes have high GC contents that may distort phylogenetic tree construction; therefore, protein sequences were used for phylogenetic reconstruction. Our analyses indicate that Monosiga is more closely related to animals than to fungi. We infer that animals and at least some choanoflagellates are part of a clade that excludes the fungi. This is consistent with the origin of animals from a choanoflagellate-like ancestor.     

The aquiferous system of two <Emphasis Type="Italic">Oceanapia</Emphasis> species (Porifera,Demospongiae) studied by corrosion casts

The aquiferous systems of two Indopacific Oceanapia species (Oceanapiidae) were studied by corrosion casts: O. amboinensis living in shallow lagoons and O. fistulosa living at the base of the reef slope. Both species show a massive, entirely buried body, emerging from the sediment only by long, completely close fistules. Particularly in O. fistulosa the corrosion casts revealed a complex, grape-like structure of the choanosome organised in anatomical and functional units composed by an incurrent web whose anastomosed meshes are crossed by a central excurrent canal. A system of thin canals connects the two systems giving rise to an area of choanocyte chambers. The corrosion casts revealed that in both species incurrent water penetrates into the sponge body by the fistules and that it is expelled through specialised structures buried in the sediment. This observation is in accordance with field experiments performed on O. fistulosa. In some specimens of this species, a solution of china ink injected into plastic bags enveloping the external fistules was observed, after waiting for a while, to flow through the buried structures.