Leucosolenia: Habitat, Structure and Reproduction

In this article we will discuss about Leucosolenia:- 1. Habit, Habitat and Distribution of Leucosolenia 2. Structure of Leucosolenia 3. Physiology 4. Reproduction 5. Development.

Habit, Habitat and Distribution of Leucosolenia:

Leucosolenia is a small, delicate branching, colonial marine sponge. It grows in shallow water below low tide mark on sea shore rocks where wave action is intense and is not found in calm water. Leucosolenia is found abundantly along the northern Atlantic coast and is supposed to be very sensitive to external conditions.

About 100 species of Leucosolenia have been reported from different seas all over the world. However, L. botryoides, L. complicata and L. variabilis are common species.

Structure of Leucosolenia:

The colony of Leucosolenia is whitish yellow in colour. In the simplest species of Leucosolenia, the colony consists of few simple vase-like, cylindrical individuals each terminating in an osculum and united at their bases by irregular horizontal tubes. Most species are more complicated, consisting of a confused network of branching tubes from which stand out a few larger erect cylinders bearing an osculum at their summit.

Finally, in the most complicated species, the outermost tubes fuse together forming a false surface or pseudoderm leaving a few large openings or pseudo pores so that the sponge appears solid and simulates a higher type of sponge; but sections show the network of ascon tubes in the interior.

Each tube of the colony may reach up to 25 mm in height and also produces a number of buds. Each main tube opens to the exterior by an aperture called osculum at the summit. The cavity of the tube is known as spongocoel or paragastric cavity.

Body Wall:

The body wall is thin and consists of an outer epidermis,the pinacoderm and an inner endodermis, the choanoderm separated by a jelly-like non-cellular layer of mesenchyme or mesogloea, enclosing a central cavity, the spongocoel. The wall of each tube is perforated by numerous pores through which water enters the spongocoel and passes out by osculum.

(i) Pinacoderm:

The outer epidermis consists of thin, scale-like, flattened cells, called pinacocytes (Gr., pinako = plank; kytes = cell) which lie with the edges touching and forming a single layer of cells, the pinacoderm. This layer forms the outer protective covering of the tube. A pinacocyte has thin highly contractile margins and contains a central bulging having nucleus.

(ii) Choanoderm:

The inner epithelium consists of a single layer of choanocyte (Gr., choane = funnel; kytos = cell) cells and forms the lining of the spongocoel and also referred to as gastrodermis. A choanocyte is an oval cell having a flagellum which arises from basal granule and is surrounded at its base by a contractile transparent protoplasmic collar.

The nuclei of choanocytes lie at the bases of cells. The beating of the flagella of choanocyte cells maintains a water current in the body of sponge.

(iii) Mesenchyme or Mesogloea:

Mesenchyme is the intermediate layer between the pinacoderm and choanoderm. Mesenchyme is a thin layer and is in the form of gel. It is secreted by the choanocytes and it holds the skeletal elements of CaCO₃ called spicules or sclerites in place. In the mesenchyme are some amoebocytes which are amoeboid cells, they wander about freely. These cells contain RNA and self- replicating in nature.

These cells can give rise to different types of cells, hence, totipotent in action; they may give rise to pinacocytes, choanocytes, collencytes, sclerocytes or scleroblasts and reproductive cells. The spicules are crystalline, needle-like which are monaxons, tetraxons (having four rays) or triaxons.

The spicules are scereted by scleroblast cells and remain embedded in the mesenchyme and some monaxon spicules project out of the pinacoderm. A few monaxon spicules form a scanty fringe around the osculum.

Ostia:

The wall of each tube is pierced or perforated by numerous pores called ostia or incurrent pores which pass through a space or lumen of cells called porocytes. The porocytes are supposed to be modified pinacocytes. Each ostium or incurrent pore is intracellular, i.e., it is a canal through a single, large, tubular and highly contractile porocyte cell communicating outside with the spongocoel.

Physiology of Leucosolenia:

No adult sponge is capable of locomotion, and some are quite devoid of contractile powers, except for changes in the porocytes. Most do have at least local contractile powers that appear to be restricted to within 3 or 4 mm of the point of strong stimulus. Reactions are most noticeable when stimuli are applied to the region of the osculum.

All reactions are slow and one to several minutes elapse before the responses become noticeable. Sponges are devoid of sensory or nerve cells, the contractile responses mentioned above are, therefore, direct reactions to stimuli.

Under normal conditions all the apertures (ostia and oscula) of a sponge are widely open and a current of water flows in through the incurrent openings or ostia and out through the osculum. The water currents are caused by the beating of the flagella of the choanocytes.

This flow brings food and provides opportunity for gaseous exchange and the elimination of metabolic wastes. The structure of sponge is such that the flow of water passes the choanocyte areas quite slowly.

The food comprises the plankton—microscopical animals and plants and bits of organic matter. It appears that food particles adhere to the collars or are caught between the collars of adjacent choanocytes. The food particles are ingested into the choanocytes or passed directly into underlying amoebocytes. Some observers think that food particles are also taken in through the epidermal cells.

Digestion is always intracellular, as in Protozoa, occurring in food vacuoles which are acidic at first and alkaline later. Un-digestible particles are ejected from the amoebocytes and find their way into the outgoing currents.

Several enzymes have been identified in sponge extracts: Protein-digesting enzymes similar to trypsin, pepsin, rennin and erepsin; lipase; invertase; and amylase in some cases.

Since sponges usually contain bacteria and other organisms, it is not clear that the enzymes in question really come from the cells of sponge. Digested food is stored in certain amoebocytes as glycogen, glycoproteins and lipoproteins. Respiration and elimination of metabolic wastes are accompanied by osmosis without the aid of special structures.

Reproduction of Leucosolenia:

Leucosolenia reproduces both asexually and sexually.

(i) Asexual Reproduction:

Leucosolenia reproduces asexually by budding. In budding, Leucosolenia sprouts new horizontal branches which grow over rocks and other substrata and give rise to erect vase-shaped individuals. When the upright branches attain sufficient size their tops break through as oscula.

Leucosolenia has also remarkable power of regeneration. Any piece of broken Leucosolenia is capable of growing into a complete individual. This process is slow, and months or even years may be required before size is attained.

(ii) Sexual Reproduction:

Sexual reproduction takes place by the formation of gametes, i.e., ova and sperms. Leucosolenia is hermaphrodite, because both the gametes are formed in the body of same individual, though gonads are altogether absent. The gametes are, however, formed by the differentiation (say, gametogenesis) of amoebocyte cells.

Cross-fertilisation occurs, which is internal. The sperms are drawn in the body of Leucosolenia with the water current which fertilize the ova. The development of Leucosolenia have been described by Metschnikoff (1879) and Minchin (1896).

Development of Leucosolenia:

The fertilised egg undergoes equal and holoblastic cleavage to form an oval hollow blastula, called coeloblastula. The coeloblastula is composed entirely of a narrow flagellated cells except at the posterior pole, where there is a group of rounded non-flagellated cells. These are believed to be archaeocytes which form all future archaeocytes of the sponge.

These together with adjacent flagellated cells (which thereupon lose their flagella) wander into the interior and fill it with a mass of cells. The resulting larva is, thus, a stereogastrula or parenchymula and ah inner mass of amoeboid cells.

The parenchymula swims freely for some hours. Then it attaches by the anterior pole and develops into a flat plate with an irregular outline. The amoeboid cells (interior cells) migrate to the external surface and form the epidermis (pinacoderm) and mesenchyme.

The flagellated cells are, thus, enclosed and become the choanocytes. A central spongocoel appears, an osculum breaks through, and spicules are secreted. After a few days of attachment, the larva is converted into the adult asconoid sponge.