Porifera: Sponzen en hun Biologie

Introduction to Porifera and General Characteristics

Porifera, commonly known as sponges, represent the most primitive group within the Metazoa. There are approximately $5000$ recognized species. The group is primarily categorized into four major taxa: Calcarea (calcareous sponges), Demospongiae (siliceous sponges), Hexactinellida (glass sponges), and the smaller taxon Sclerospongiae. Structurally, sponges are composed of two types of epithelial-like tissues. The first is the pinacoderm, which consists of flat pinacocytes that border the exterior and the internal canals. The second is the choanoderm, which is always located internally and is composed of choanocytes; these cells bear a strong resemblance to choanoflagellates. Between these two epithelial-like layers lies the mesohyl, a space containing various cell types and skeletal elements. Sponges lack sensory cells, muscle tissue, and nervous tissue. The connections between cells are relatively loose, with weak forces holding the pinacoderm and choanoderm together, and no desmosome structures are found. Due to this loose organization, cells that have been mechanically separated can regroup to form new sponges. Metabolism occurs at the cellular level, with food uptake happening via phagocytosis by individual cells. In the cells of freshwater sponges, a contractile (pulsating) vacuole is present.

External characteristics show that sponges range in size from a few millimeters to $2\,m$. While they often form irregular lumps, many species have a constant aspect, appearing bag-like or cup-shaped. Common colors include red or yellow, while a green color is usually caused by a symbiotic relationship with zoochlorellae.

General Body Plans and Organization

Sponges are organized into three primary structural types based on their complexity: the Ascon type, the Sycon type, and the Leucon type.

The Ascon type is the simplest structure, resembling a basic sac. Its anatomy includes a central cavity called the atrium or spongocoel, which opens at the top via the osculum. The atrium is lined with choanoderm, while the exterior consists of pinacoderm. The beating of the flagella on the choanocytes creates a water current; water enters through numerous small openings (ostia) in the wall and exits through the osculum. Because choanocytes can only move a limited volume of water, Ascon-type sponges remain small, typically reaching a maximum size of $1\,cm$.

The Sycon type features a folded wall. Here, choanocytes are located in tube-shaped, radially arranged chambers around a central atrium that is lined with pinacoderm. A canal system exists where incoming channels lie between these radial chambers; water reaches the choanocyte chambers from the outside via ostia. In cases where the mesohyl increases between the radial canals, the supply channels branch out, a variant known as the Sycon type with a cortex. These sponges can reach a maximum size of $10\,cm$.

The Leucon type is the most complex. Choanocytes are housed in small chambers with a diameter of approximately $2\,mm$. Water is supplied via supply channels that contact the outside world through ostia. Water reaches the chambers through narrow openings called prosopyles and leaves through wide openings called apopyles into discharge channels. These discharge channels merge and exit through one or more oscula. Sometimes supply channels are absent, and the flow proceeds entirely through the mesohyl. The pressure drop between the prosopyles and apopyles ensures a strong flow through the chambers. There is no restriction on total volume for this type. The surface is often formed by a dermal membrane with ostia giving access to a vestibulum, from which the supply channels originate.

The Unique Structure of Hexactinellida

Hexactinellida (glass sponges) are usually cup- or plate-shaped. Their exterior consists of a dermal membrane formed from a single syncytium. Beneath this is a syncytial trabecular system characterized by large open spaces. The choanoderm lines thimble-shaped chambers that have prosopyles on the side of the trabecular system and wide apopyles facing the inside of the vase-shaped body. In this group, both the choanoderm and the pinacoderm are syncytial, meaning the cells are fused into a continuous mass of cytoplasm.

Specialized Cell Types in Porifera

Sponges possess several specialized cell types that perform distinct physiological and structural roles. Choanocytes, or collar cells, line the spongocoel in Ascon types, radial canals in Sycon types, or specific chambers in Leucon types. In Hexactinellida, they form a syncytium. These cells possess a collar made of microvilli with a central cilium that lacks a ciliary root and sometimes features two lateral "wings." The beating of the cilium creates the water current; these cells filter food particles from the water and partially digest them in food vacuoles before passing them to archaeocytes.

Pinacocytes are polygonal, flattened cells that form the pinacoderm. They are divided into exopinacocytes (outer lining, often T-shaped with a sunken nucleus) and endopinacocytes (inner lining, pavement-shaped). Porocytes contain an opening (ostium) surrounded by a single cell with slightly contractile fibers around the pore. In Ascon types, they connect the outside world to the spongocoel, while in others, they are situated between exopinacocytes. However, in most species, ostia are simply openings between pinacocytes.

Archaeocytes are located in the mesohyl, which is the space between the epithelial layers containing fibers, skeletal elements, and various cells. Archaeocytes are large, amoeboid, totipotent cells capable of phagocytosis. They can differentiate into any other cell type, including gonocytes. They capture partially digested food from choanocytes or directly from the water current for further digestion and distribution, typically staying close to the choanocytes.

Supporting cells within the mesohyl include collencytes, which are fixed in a $3D$ network and produce fine collagen fibers capable of slight contractions; lophocytes, which produce thicker collagen fibers to which the cells remain attached; spongocytes, which produce thick spongin fibers (a specialized type of collagen); and scleroblasts, which produce the skeletal elements.

Contractile and specialized cells include myocytes, which are spindle-shaped cells containing microfilaments found around the osculum or canals. They contract very slowly and, unlike true muscles, are insensitive to electrical stimuli. Central cells occur in freshwater sponges like Spongilla; they regulate water flow by narrowing the apopyle and perform supplementary phagocytosis. Mucus and adhesive cells produce substances for attachment to substrates. Finally, amoeboid "cleaners" crawl on the outside of the exopinacoderm to keep the sponge surface clean.

Skeletal Elements: Spicula and Collagen

Collagen is the fundamental structural element found in all sponges. In Demospongiae, short collagen molecules polymerize into thick, dense fibers known as spongin. Most sponges possess spicula, which are mineral skeletal elements. Calcareous spicula ($CaCO_3$) are made of calcium in the form of calcite or aragonite and are formed extracellularly through the cooperation of multiple scleroblasts. Siliceous spicula ($SiO_3$) consist of silicic acid deposited on an organic base and are formed intracellularly within a single scleroblast.

Spicula are classified by their axes and rays. Axes options include monaxon ($1$ axis), diaxon ($2$), triaxon ($3$), tetraxon ($4$), or polyaxon (multiple). Rays are denoted by terms such as monactine or triactine. In Hexactinellida and Demospongiae, a distinction is made based on size: megascleres are large skeletal needles often connected by collagen to provide rigidity, while microscleres are small sclerites scattered throughout the mesohyl.

Asexual Reproduction and Survival Mechanisms

Sponges have a high capacity for regeneration; any fragment that detaches naturally or artificially can grow into a complete new individual. Freshwater sponges (family Spongillidae) survive unfavorable periods via gemmulae. These are spherical structures with a thick spongin wall reinforced with sclerites, filled with archaeocytes rich in reserve substances. During formation, archaeocytes gather, divide, and store reserves by phagocytosing trophocytes. When the parent sponge dies, gemmulae remain; when conditions improve, the archaeocytes exit through an opening called the micropyle to form a new sponge. Some saltwater sponges form reduction bodies, which are accumulations of archaeocytes with thinner walls. Other species form buds that detach from the mother sponge and are dispersed by currents.

Sexual Reproduction and Gametogenesis

Sponges can be gonochoristic (separate sexes) or hermaphroditic. Hermaphroditism may be proterandric (first male) or protogynous (first female). An individual can change sex one or more times, and long pauses between phases can sometimes create a false impression of gonochorism. Spermatozoa develop from choanocytes that migrate to the mesohyl and are surrounded by follicle cells. Oocytes develop from archaeocytes, storing reserves by phagocytosing trophocytes, and are also surrounded by follicle cells.

The Fertilization Process

The fertilization process involves three main steps. 1. Emission: All spermatozoa from an individual are expelled simultaneously through the osculum. 2. Intake and Transport: Sperm enters another individual via the water current and is captured by choanocytes. The choanocyte does not digest the sperm; instead, it loses its collar and flagellum to become amoeboid. This carrier cell migrates through the mesohyl to deliver the spermatozoon to the oocyte. 3. Species Recognition: Sponges only recognize sperm from their own species. Spermatozoa from other species are digested as food by the choanocytes.

Embryonic Development and Larval Stages

Cleavage in Porifera is typically equal, leading to a multicellular embryo. In many Calcarea, this results in a coeloblastula, a hollow vesicle of monociliary cells. These cells can enter the cavity via unipolar or multipolar immigration. If cells lose their flagella and enter the cavity as amoeboids, the coeloblastula becomes a massive cell mass called a stereoblastula. Larvae may be released into the plankton as either a coeloblastula larva or a stereoblastula larva. After a planktonic phase, the larva becomes epibenthic (creeps on the bottom) and develops into a young sponge with a simple asconoid structure called an olynthus.

Some Calcarea follow a "reversal" path. At the $16$-cell stage (placula), there are $8$ large macromeres and $8$ small micromeres. The micromeres divide rapidly and develop cilia pointing into the cavity, forming a stomoblastula. The vesicle then turns inside out, creating an amphiblastula larva where micromeres (with cilia now outside) are at one pole and macromeres at the other. The amphiblastula swims, eventually attaching with the ciliated micromeres facing downward. These micromeres invaginate to form the internal choanoderm. An osculum develops on the side away from the substrate, and the resulting young sponge is also called an olynthus.

In Demospongiae, the process is more direct. A stereoblastula forms immediately without a hollow stage, resulting in a parenchymula larva. Upon attachment, it loses its cilia. Cavities and canals form in the central cell mass to create the water system. Cells differentiate into choanocytes or mesohyl elements. The resulting young sponge in most Demospongiae is called a rhagon.

Phylogeny and Modern Taxonomy

Autapomorphies of Porifera include being sessile as adults, having a filtering feeding method via pores and canals, having the exterior and canals lined by pinacoderm, having internally located and grouped choanocytes for water flow, and the presence of archaeocytes in the mesohyl. While Calcarea, Hexactinellida, and Demospongiae are the three recognized monophyletic groups, two morphological hypotheses exist regarding their relationships. Hypothesis $1$ (Silicea) groups Hexactinellida and Demospongiae based on the intracellular formation of siliceous needles by a single scleroblast, with Calcarea as the sister group. Hypothesis $2$ (Cellularia vs. Symplasma) groups Demospongiae and Calcarea into Cellularia based on the presence of porocytes, placing Hexactinellida in Symplasma (characterized by syncytia); this suggests siliceous spicula were an ancestral trait lost in Calcarea.

Recent research confirms the monophyly of four main groups. Homoscleromorpha (a small taxon with siliceous spicula and aragonite deposits, phylogenetically close to Calcarea) is the sister group to Calcarea. Demospongiae and Hexactinellida together form the monophyletic taxon Silicea. Calcarea is the only group containing all three body types (ascon, sycon, leucon) and is exclusively marine, limited to depths around $80\,m$. Demospongiae are mostly marine but include the only two freshwater families. Hexactinellida are unique for their syncytial tissue and six-rayed (hexactine) siliceous spicula.