Sycon

Sycon is a marine sponge widely distributed in shallow water. Typically, it is found attached to shells, rocks and is sedentary. Predominantly, they harbor in temperate regions.

Systematic Position of Sycon

Phylum: Porifera

Class: Calcarea

Order: Heterocoela

Family: Sycettidae

Genus: Sycon

Phylum Porifera

• The phylum includes nearly 5000 species that are known
• Typically characterized by the presence of pores all through the body justifying the name Porifera, the ones who bear pores and are also referred to as sponges
• Generally, they are represented by Sycon, Leucosolenia, Euplectella, Hyalmonema, etc

Based on the nature of the skeleton, Phylum Porifera is classified into three classes:

Sycons are typically small, they range from 2.5cm to 7.5cm in height. These tube-shaped structures resemble water vessels. In some instances, they are also referred to as crown sponges because of the spicules they possess, arranged in the form of a crown while in some other instances, they are referred to as Q-tip or “Pineapple” sponges based on their resemblance. Typically, Sycons are delicate in appearance, organized in an asconoid system wherein the body has a simple tube with a lack of foldings of the outer body wall.

Habit and Habitat

Sycons lead a solitary/colonial life. These marine entities are found attached to the solid substrate such as shells and rocks of corals and molluscs. Various species of sponges under this genus do not have the tendency to thrive deeper in the ocean. Sycons seem to thrive well in slow-wave action and low tides. They are found in the underside of rocks in comparatively protected regions amongst hydroids, bryozoans, and other entities. Some also grow as seaweeds.

Structure

There is a great diversity observed in the form of sponges ranging from simple to complicated entities. However, they still can be categorized as intermediate when it comes to structural diversities. It has branched cylinders which are connected at the basal end, intact to the substratum. The body is flexible though it is secured firmly. A closer look at these entities reveals the presence of many tiny Ostia or inhalant pores. The free end of every cylindrical branch has an opening at the summit known as osculum.

They are fringed with calcareous monaxon spicules or oscular fringes which check the entry of any foreign substance. The body below the osculum is narrow, forming the collar region. The body surface has elevations emerging as spicules (oxeotes) rendering the bristle appearance.

Canal System

Sycons exhibit a canal system which is an anatomical peculiarity, unique to them. It penetrates the body with watcher channels. The canal type precisely seen in Sycon is Syncoid Stage I type, more advanced than asconoid canal system. The Ostia opens into a central cavity through a system of canals, the central cavity is known as spongocoel.

The body is organized into a complex network of canals and pores, which goes on to form the aquiferous or the canal system. The body wall is composed of Choanoderm, Mesenchyme and Pinacoderm.

Composition of the canal system:

• Ostia – these dermal pores are lined by thin membranes. These possess two openings, one for entry and one for the exit. The closing and opening of Ostia for water flow is regulated by myocytes
• Sponogocoel/Paragastric or gastral cavity – osculum leads to a large central canal known as spongocoel, named variously. Its wall is lined by pinacocytes, which are ectodermal flattened cells. Osculum is engirdled by a layer of cells known as myocytes acting as sphincters, contractile in nature. Spongocoel appears to open to the outside through the osculum
• Radial canals – evaginations of the body wall. The body wall lining of the spongocoel is shoved periodically as finger-like projections known as radial canals whose walls are lined with choanocytes. The outer end of the radial canal is closed while the inner end is openly mediating with the spongocoel via the excurrent canal
• Incurrent canal – this tubular structure is found between two successive radial canals, hence are alternatively arranged. Invaginated folds of the body wall. The inner end of the incurrent canal is blind whose walls are lined by pinacocytes (flat and ectodermal). Gastral cortex, a thickened mesoglea is found between the radial and the incurrent canal. Ostia is located on the pore membrane engirdled by contractile myocytes acting as sphincters, checking the openings
• Prosopyles – these minute pores are found between the radial and incurrent canals. Each of these is an intercellular channel in Sycons, through which the incurrent canals open into the radial canals. Porocytes, to which prosopyles open into, are cylindrical and thick-walled, possessing a nucleus in the cytoplasm towards one end. It regulates the inflow of water and is highly contractile
• Excurrent canals: these canals are used by radial canals to mediate with the spongocoel and are wide and short canals whose walls are lined by pinacocytes
• Apopyles – these are engirdled by contractile myocytes and are the openings of the radial canals in spongocoel

Current Of Water

Water circulation in Sycon follows this route. Water races into the body through innumerable Ostia along the external surface, each of which leads to the incurrent canal. From here, water flows into the radial canal through prosopyle. From the radial canal, water gushes into the spongocoel through the apopyles. Hence, spongocoel is the shared chamber in which all the radial canals of the body open into.

Finally, the spongocoel opens to the outside through the osculum, an aperture. Hence, water enters through numerous Ostia but exits through the osculum, which is a single opening. In the radial canals, the planar beating of the flagella of choanocytes generates a current that simultaneously draws in water and gushes it out too.

Microscopic Organization

A microscopic view shows the presence of a single layer of cells covering the outer surface, which is designated as the dermal layer. It is from here that the spicules seem to emerge. The layer possesses large cells known as pinacocytes.

Spongocoel possesses a lining of flattened endodermal cells while the radial canal is lined by unique collar cells, each possessing flagellum (a long whip-like structure). These are collar cells or choanocytes. Each choanocyte is oval or round shaped exhibiting vacuoles in their cytoplasm. The open end of the cell body has a relatively lengthier flagellum whose base is engirdled by contractile transparent collar-like projections of the cytoplasm.

Flagellum emerges from the basal granule associated with the rhizoplast. The collar-like projections consist of cytoplasmic tentacles, whose number ranges from 20-30. The skeleton of Sycon is made up of spicules that arise from the scleroblasts and are organized to safeguard the delicate parts. A common observance is the presence of tetraradiate or triradiate spicules. The mesenchyme is composed of a gelatinous proteinaceous matrix containing spicules and many amoeboid cells.

There are many types of amoeboids such as:

• Collenocytes
• Myocytes
• Thesocytes
• Archaeocytes
• Gland cells
• Chromocytes

Respiration

• Simple diffusion causes exchange of gases between sponge cells and the flowing water
• Dissolved oxygen in the water, diffuses into the cells causing the oxidation of protoplasmic molecules, along with the release of energy captured in ATP
• ATP is critical in supplying energy to metabolizing cells
• Contractile vacuoles in the amoeboid cells of freshwater sponges are said to take part in osmoregulation and excretion

Nutrition

• Water current brings along microbes on which the sponges feed. They enter through the numerous Ostia
• They are filter feeders, subsisting on planktons and other organic substances
• The choanocytes engulf these and transfer them to amoeboid cells present under the choanocytes wherein digestion occurs. Here the products of assimilation are passed to different body parts
• Digestion is intracellular and nutrition here is holozoic. Some of the amoeboid cells have chlorophyll performing autotrophic nutrition
• In the flagellated chambers beating of flagella or collar cells, leads water to pass through collar causing food substances to cling to them
• Food particles are trapped with the help of microvilli of collars, serving as filters
• As a result, the engulfed food particles are taken to food vacuoles after the pseudopodial action of choanocytes at the bed of their collars
• Initially, the phase is acidic in food vacuoles and then alkaline
• Food here undergoes digestion partially, which is then transferred to amoebocytes present in the mesenchyme, which are in constant motion
• Amoebocytes cause complete food digestion, the residue is pushed through outgoing water current. In addition to this, amoebocytes also dispense digested food to all cells while some store it, to be used later
• Many enzymes are isolated from them including fat-digesting, starch and protein enzymes

Reproduction

Both sexual and asexual modes of reproduction is observed. Asexually, they produce a bud, in some other instances they produce special structures that resemble gemmules of freshwater sponges. Both ova and sperms in sexual reproduction are produced from archaeocytes found in mesoglea. Sperm cells possess long tails, swimming freely in the water current while the ova are amoeboid, moving through the mesoglea.

Sperm cells do not enter directly into the ovum. When the sperm cells enter the radial canal, the nearest choanocytes to the egg captures it. Choanocyte that absorbs the sperm is the carrier cell, it ejects its flagellum and collar approaching the egg. With time, the sperm loses its tail and enters the egg. The earliest development stages are inside the mother sponge. Once the developmental stage is complete, the larva gushes into the radial canal and then to the outside.

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