1 (Vizhinjam Research Centre of C.M.F.R.I., Vizhinjam - 695 521)
Introduction
The part played by sponges in the destruction of coral reefs and gregarious molluscs has been well documented in the past by several workers, and as understood at present the sponge constitutes a major group among 12 different taxa of marine plants and animals that cause considerable damage and ecological disturbance in the marine ecosystems. Various borers resort to various methods in gaining entry into calcareous substrata, in some it may be by chemical means, in others by mechanical means or by a combination of both.
Since Calcium carbonate forms the hard skeleton in different marine organisms such as coral, molluscs, barnacles, calcareous algae etc. the deterioration caused to them by other marine organisms has been a topic of interest to both marine biologists and geologists alike. The biological, chemical and geological changes that the boring organisms would bring about in the marine environment are, by no way, insignificant as they cause bio-erosion, influence calcium balance in the sea and control the structure of calcium carbonate secreting communities in the sea.
How Sponges bore into hard Calcareous skeleton in Corals?
Sponge bore into the hard calcareous substrata through tunnels and cavities formed by the etching of hard calcium carbonate in the form of microchips of almost the same dimensions (0.056 x 0.047 x 0.032 mm average), and hence the interior of these cavities, when viewed under high magnification, may have a pitted appearance. These chips are of uniform size, shape and ornamentation and are etched out by the filopodial structures borne by peculiar cells of archeocytic origin. These filopodial structures cut hard calcareous particles by the help of a chemical agent (probably carbonic anhydrous) and then the chip is pulled out from the site by the contraction of filopodial basket (mechanical means). Chips, thus formed are drained into the canal system of sponge and thence to the exterior through the excurrent stream of water (again mechanical means). Since boring sponges etch out microscopic calcareous particles of uniform size and shape this phenomenon may be termed micro-erosion or bio-erosion.
A bio-erosion at microlevel
The quantity of calcareous chips dislodged from any coral substrate by the activity of sponge varies considerably both in time and space. Rutzler (1975) has shown experimentally that 1 mg (dry weight) of Cliona lampa can dislodge 16 mg of calcium chips over a period of one year, and based on this it is calculated that, in Bermuda reefs, as much as 250 mg/sq.m/yr are removed from coral and this corresponds to a layer of 0.1 mm thick every year from an area of 1 sq.m. But this may go up to 3 kg/sq.m/year (or 1 mm thick) in areas where sponge concentration is high. The above figure is in addition to 2 to 3 per cent calcium carbonate which goes in solution in chipping process, and hence the problem of altering the calcium balance of the sea water in the adjoining realms is not severe.
Goreau and Hartman (1963) have shown that such chips contribute much to the mud fraction of the sediment generated in the reef environment in Jamaica, and constitute 2 to 3 per cent of the total sediment load in the Adriatic Sea and the Persian Gulf, while it is to the tune of about 30 per cent in Fanning Island.
Factors influencing bio-erosion
Rutzler (1975) has shown that when a coral piece infected with boring sponge is cut into two the resultant bits show accelerated boring activity. Hence intervention by man or by boring organisms by way of cutting channel across the reef or by burrowing into it may accelerate boring activity of all existing species of boring sponges along the cut ends of the reef. Calcite blocks infested by boring sponges, when illuminated by low voltage microscope lamp may produce chips at an accelerated rate (0.5 mg/sq. cm/day). Clarity of sea water is another factor. Euphonic zone, in Lakshadweep area, is upto 90 m. and this may also give some clue to the accelerated activity of boring sponges in this area.
Sponge infection in Lakshadweep atolls-a case study
The Central Marine Fisheries Research Institute, Cochin, initiated a project to investigate the living resources of Lakshadweep in 1887, and 12 atolls were surveyed in detail and the results were published in the form of Bulletin in 1987 (Bulletin No. 43). An interesting feature noticed with regard to the sponge fauna, both morphozone-wise and transect-wise, is the dominance of boring sponges in relation to non-boring ones. The total percentage of boring species in each atoll may be given as follows: Kavaratti - 46.4 per cent, Kalpeni - 36.1 per cent, Suheli - 48.3 per cent, Androth - 66.6 per cent, Minicoy - 50.0 per cent, Amini - 38.4 per cent, Kiltan - 20.0 per cent and Kadamath - 58.3 per cent. The abundance of coral skeleton may be the main cause of the richness of boring sponges in the various atolls investigated.
Atoll-wise data on sponges were collected on a transect-morphozone basis, and the abundance of boring sponges was calculated. Mortality of coral due to sponge attack was calculated for an area of 25m2. It was seen that in some places the mortality was as high as 80 per cent.
It is found that sponges can bore into both dead and living corals. Since boring sponges obtain their food from sources other than the host, the death of a coral will never affect the sponge adversely. And as such the chipping of calcium carbonate matter can go incessantly even after the death of the coral. The galleries formed inside the coral by the removal of calcareous particles weaken the entire reef frame-work making it more susceptible to the wear and tear caused by waves. Further, such a weakened substratum may also accelerate the activity of secondary borers such as polychaete, molluscs, sipunculids etc.
When the inroads of boring sponge reaches the climax stage, the interior of coral becomes practically hollow except for a few 'pillars' of calcium carbonate stretching across these hollow cavities inside. The outer surface of the coral remains untouched except for a few microscopic pores through which the incurrent and excurrent papillae protrude for taking in and expelling water respectively. Hence it is difficult to assess the damage caused to a coral mearly by external examination. Though the outer layer of coral appears normal slightest pressure would make the entire outer layer cave in as the interior is practically hollow. Massive corals examined from various atolls of Lakshadweep showed this character extensively.
Damage caused to corals
In order to evaluate the damage caused to corals by boring sponges data were collected separately for branching the massive corals. For branching forms the damage caused to branches as well as for stalk portion was collected, and it could be noticed that in branching corals the tip of branches just above the point of penetration of the sponge larvae usually die out as the sponge grows up through the interior of branch. Some filamentous algae of black colour usually colonise these dead branches giving a black furry appearance to such branches underwater. The sponge makes chamber and tunnels inside the branches initially, but later these cavities fuse together and form a single continuous tunnel running through the centre of each branch. It is noticed that the sponge Cliona celata prefer to grow upwards (negatively geotropic), i.e. towards the distal end of each branch, and hence the portion below the penetration point of the larvae (of boring sponge) remains unaffected. Such dead distal portion of coral branches may later break off due to the action of secondary borers or waves.
Stalk of branching coral is also analysed for various boring sponges/boring groups, and it could be seen that nearly 40 per cent of the area of stalk, in cross section, is being damaged by an array of boring organisms drawn up from different phyla of the animal kingdom. In any branching colony, that too when it occupies the reef front zone, a partially disintegrated stalk can result in the sliding away of the entire colony into deeper areas where it will be buried by sediments.
Massive corals are also equally infested by boring sponges, and here also the boring sponges show a tendency to grow upwards (negatively geotropic) from the point of penetration of sponge larvae. When many larvae penetrate a massive coral at different points and all of them grow upward they will ultimately reach the summit or the distal part of the coral causing damage to living polyps. Side by side boring algae or other borers may settle at this damaged part and establish themselves. It could also be seen that in some cases the coral colony compensates the situation by accelerating the growth along the periphery producing a circular rim around the zone of dead polyps. This outer rim, in due course, may curl in and completely cover the central area of dead polyps. If the damage is extensive and when this type of a growth is not possible, other sedentary forms may attach and grow luxuriently on the damaged summit of any massive coral.
Biodiversity of boring sponges
Extensive survey on sponges in Lakshadweep revealed the presence of a total of 91 species referable to 30 families and 66 genera (Thomas, 1989). Abundanance of boring sponge specimens over non-boring ones was seen not only with regard to various atolls investigated, but with each collection site also. In some sites only boring sponges were represented. The abundance of boring sponges in these atolls may be attributed to the easy availability of calcareous matter in the form of coral skeleton.
The total number of boring species represented in the various atolls of Lakshadweep is 18, and these are referable to 4 orders, 5 families and 9 genera of the Class Demonspongiae of Phylum porifera.
The various genera of boring sponges and the number of species under each genus are as follows: 1. Genus Rhabderemia (1); 2. Genus Spirastrela (4); 3. Genus Amorphinopsis (1); 4. Genus Aka (2); 5. Genus Cliona (6); 6. Genus Thoosa (1); 7. Genus Jaspis (1); 8. Genus Halina (1) and 9. Genus Samus (1). Of these, species falling under the genus Cliona are widely distributed in the various atolls of Lakshadweep followed by those of the genus Spirastrella.
In order to get a clearer picture of the abundance of various species of boring sponges, the total incidence and the species wise incidence in the various atolls were calculated for 5 atolls. It could be seen that Cliona celata Grant was the dominant species in two atolls (Kavaratti and Suheli); C. ensifera Sollas in Kalpeni and C. mucronata Sollas in Androth. In Minicoy atoll both C. celata and C. vastifica Hancock dominated equally. The other species of Cliona viz. C. viridis (Schmidt) and C. carpenteri Hancock, had stray occurrence.
Out of 4 species of the Genus Spirastrella, two species viz., S. inconstans (Dendy) and S. aurivilli Lindgren are somewhat well distributed. Of these the former show a tendency to overgrow the substratum in an advanced stage of growth. Asexual buds formed from the tip of tubular branches later get nipped off, and these later get anchored to the substratum and form new colonies; this character is widespread for S. inconstans.
The other boring species are of no significance at present from the point of coral destruction; but it is possible that any species can cause an outburst and dominate over others.
While analysing the boring sponges of the Indian seas it could be seen that 32 species infest the various calcium carbonate secreting organisms such as coral, mollusc etc. (Thomas, 1979). This number (32) is very large when compared to any part Of the world seas, but all species were equally dominant during the time of the above study. During the survey of Lakshadweep atolls also a few species were dominating. But here also a number as high as 18, with a few dominating, indicates that the coral reefs of Lakshadweep group of islands are under severe threat of sponge infection.
Out of the total of 91 species recorded from Lakshadweep atolls (Thomas, 1989) only 18 were boring species while the others (73 species) may be considered harmless associates. Subsequent studies made on some of these 73 species indicate that at least some may elaborate peculiar chemical compounds with biodynamic properties, and many of these compounds could be used in synthesising 'wonder drugs' which will fight many a dreadful disease in man. Hence it is time to think in this line, and the vast sponge resource available in these atolls may be judiciously exploited for initiating R & D programmes on marine organism based drug industry.
Recommendations
During the present survey of Lakshadweep atolls (Thomas, 1989) only 5 atolls were studies in detail, and based on the information collected it is possible to say that the boring sponge infection was quite widespread and the mortality due to the same was very high in some transects. Studies on boring sponges of the Seychelles Bank (mainly Mahe Island) also indicated the presence of 15 species of boring sponges infecting the corals of that area (Thomas, 1973). It is not known whether the other reefs in the Indian Ocean are infected in the same pattern, and how much mortality is effected through sponge infection. A detailed study in this line is worth undertaking in all coral formations in the Indian Ocean.
An exhaustive study made on the boring sponges infecting the economically important shells of the Indian seas (Thomas, 1979) revealed that the frequency of occurrence of various boring species can vary from time to time and any one or more species can at any time cause an outburst creating severe havoc and then disappear into a quiescent stage, while some others may continue to exhibit moderate incidence without causing any such deleterious effects. This shows that the composition of the boring sponge fauna of Lakshadweep islands is likely to get altered in future, and such changes, if any, may be well affirmed by comparing with the observations presently reported. Hence regular surveys, at least after every five years, may be attempted in these atolls to study the changing patterns in the boring sponge populations.
Since boring sponges etch out microscopic particles of calcareous matter making the hard substrata susceptible to the vagaries of nature, the problem of bio-erosion is of considerable importance to both marine biologists and geologists alike, and any study, in this line, should be undertaken as an inter-disciplinary one. In India this aspect is now covered by biologists Only, and geologists have not yet taken it seriously.
It may often be difficult to control the attack of borers on coral or molluscs in the marine environment effectively, but the changing patterns in the their abundance, both in time and space, when monitored on a long term basis, will be of considerable value in the management of calcium secreting animals.
Acknowledgements
The author is thankful to Dr. M. Devaraj, Director, C.M.F.R.I., Cochin-14, for the permission granted to him in submitting the paper.
References
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K. Rutzler. 1975. The role of burrowing sponges in bio-erosion. Oecologia. 19: 203-216.
K. Rutzler and G. Rieger. 1973. Sponge Burrowing: Fine structure of Cliona lampa penetrating Calcareous substrata. Mar. Biolo., 21: 144-162.
P.A. Thomas. 1973. Marine Demospongiae of Mahe Island in the Seychelles Bank (Indian Ocean). Mus. Roy. L'Afr.Cent. No. 203: 1-98.
P.A. Thomas. 1979. Boring sponges destructive to economically important mulluscan beds and coral reefs in the Indian seas. Indian J. Fish., 26: (1 & 2): 163-200.
P.A. Thomas. 1989. Sponge fauna of Lakshadweep. Bull. CMFRI., No. 43: 150-161.