Excavating sponges (by Marco Angelozzi)

Into the sea there are some microscopic organisms, "relatives" to the soft natural sponges (Spongia officinalis) with which our grandparent usually washed, that however have lot of differences..
They spend their life excavating, grinding and breaking up all that is made of carbonate, in trials that can last years and cause the death of organisms as Molluscs and madrepore.. we are speaking about the EXCAVATING SPONGES… 

The excavating sponges are organisms, belonging to the phylum of the Porifera, that have the ability to puncture the carbonatic substrata, mineral and biogenic, present into the sea. The substratum can be constituted, over that from rocky backdrops, also from shells of Molluscs, algas corallinacea and calcareous skeletons of madrepore and corals. 

The perforation happens through sour secretions that allow the sponge to create a complex net of rooms and galleries inside which the development happens.
After having fixed to the calcareous substratum, the larva of sponge begins to dig a series of galleries, of varying dimensions and forms, in the different kinds, very strongly sticking with its fabrics to the calcareous walls. This excavating stadium is called alpha. 

The body of the sponge is sustained by a whole fibers of "collagene", the "SPONGINA" and from inorganic elements, the "SPICOLES".  The calcareous spicoles are the most primitive, while those made of silisium are the most frequent and both the types, with dimensions and characteristic forms for every kind, are fundamental elements for the systematic recognition of the sponges.

Excavating sponges: Cliona orientalis, galleries into a madrepora, staircase 4 mm.

The excavating sponges of some kinds, under favorable conditions, not finding the space anymore to dig new galleries, continue their development to the outside assuming the typical encrusting form.
Finally, if the sponge subsequently grows in free final form, come to break up completely the native substratum!!
The action of the excavating sponges, whose more important family is that of the "Clionidae", diffused in the Mediterranean sea, represents therefore an important element in the erosion processes and in the production of sediments. 

In the coral barriers the growth of the madrepores is hindered, producing a weakening of the colonies and an acceleration of the demolitive action of the waves.  The shells of the Molluscs result particularly fragile, weaken and they have the tendency to break themselves with serious economic results in the areas where the breeding is practised. 

To have information about the spicoles, the fabric of the sponge is dissolved in hot nitric acid and what remains is seen through an electronic microscope.
In this moment the man can observe another of the innumerable magics of the nature.. 

Very beautiful forms, elegant, eccentric, a fabulous micro-architecture that remembers that some crystals of snow for shine and unbelievable sense of brittleness. (even if in reality they are not fragile, considering that they result uninjured from the action of the nitric acid).

Spicoles: dicotrieni and microrabdi

There are a lot the names that have been given to the spicoles of various form and dimensions (the dimensions of the spicoles in image have gone since 5 to the 50 millesimis of mm!). In order of the greatness we can distinguish them in "megasclere" and "microsclere" and here following are described some types.
The megascleres can have pointed extremity (oxea), a pointed extremity and one truncates (stylus), a pointed extremity and one bulgy (tilostilo), both the extremities tronche (strongilo), etc...  Besides this the spicole can have some thorns (acantoxea, acantostilo, acantostrongilo), to be triraggiate (triassoni) and tetraraggiate (tetrassoni), etc.. 

The microscleres can have starry form "pluriraggiata" (aster); the rays are born from a more narrow central part (euaster, sferaster) or from an axle more or less folded up (metaster, spiraster), etc.. 


Spicoles: oxee, microrabdi tilostilo, anfiaster,

The Spongia officinalis the "natural sponge" used especially in past in our bathrooms, has an elastic and soft consistence, without spicoles, alone "spongina" and it is for this motive that the contact with the skin doesn't create abrasions or wounds.

The body of the excavating sponges is constituted by two layers, not organized in true fabrics, with mediate an intermediary layer; the external wall appears all pierced and crossed by channels that allow the passage of the water from the outside in an inside hollow, the "spongocele", from which water escapes through an opening called osculo.


Spicoles: oxiaster lisci

Effect of the action of the excavating sponges

The excavation of the carbonate from the boring sponges is an important biological trial in the ecology of the sea. In the tropical and subtropical waters the numerous kinds of excavating sponges are among the principal agents (together with bacterias, algas, molluscs, echinoderma, fishes, etc.) in the processes of erosion of calcareous substrata; particularly in the coral barriers the sponges, balancing or at times overcoming the rates of calcification of the corals, develop a key function in the evolutionary processes. 

The active erosion determines the production of great quantities of end calcareous sediment that will go to constitute the coral carbonatic beaches. 

The action of weakening of the coral structures, caused by the rooms and by the galleries of perforation, reflects him in the ecological relationships among the corals of the barriers, modifying them: the most fragile colonies (because of attach) can easily be broken and detached by the atmospheric agents and perhaps replaced by other kinds of coral; the corroded superficial parts can be colonized by numerous organisms. 

Also in the moderate seas as the Mediterranean the bioerosio of the excavating sponges is important in the rehash and in the evolution of the coast, and it produces meaningful quantity of sediment in the form of fragments to elegant nuisance.  The activity of perforation of these kinds can have almost catastrophic effects with remarkable economic lapels in those areas where the breeding is practised (of bivalve, oysters particularly) and the fishing to the red coral.  The attached oysters result particularly fragile and they have the tendency to break themselves when they are handled, both during the consumption, and during the phases of the seeding in the breedings of manufacturing oysters of pearls. 

Shell (Trochus niloticus) attached by excavating sponges

E' assolutamente vietata la riproduzione, anche parziale, del testo e delle
 foto presenti in questo articolo, senza il consenso dell'autore