Wood‐borers Dr. L. N. Santhakumaran Scientist‐SG {Retd.}, Institute of Wood Science and Technology Bangalore angroves constitute the most important habitat among various types of wetlands. They are highly specialized and unique forest ecosystems found along sheltered coasts and estuaries in the tropical and subtropical regions of the world. They are of great ecological and economic benefits to mankind, in addition to providing the basic life supporting mechanisms for a wide range of coastal fauna. Mangrove flora are well adapted to thrive under salt and brackish water conditions, tolerating the stresses of continuous flooding and high salinity. The fauna comprise such animals which can withstand abrupt changes in water conditions, particularly salinity. M Perhaps, one of the major aspects of mangrove‐related investigations, which have received considerable thrust in the recent past, is the biodeterioration of vegetation by various insect pests and marine borers. Of these, the entomofauna of mangroves along Indian coasts has not been studied so far in detail, except that of Andamans. However, the marine wood‐borers, inhabiting the mangrove ecosystems, are rather well‐ documented and several published records are available on the subject (Santhakumaran, 1994, 1996; Santhakumaran and Surekha Sawant, 1998). It may be mentioned in this connection that, endowed with several highly specialized adaptations for leading a sedentary life within the wood, marine wood‐boring organisms cause extensive damage to underwater timber structures and are, therefore, economically a very important group. They are also capable of extending their destructive activities to mangroves as well. This paper gives an overview of the subject giving details on systematics and all other available information on these organisms pertaining to the mangrove ecosystem. Collection and Preservation of Marine Wood‐borers Borers can be collected from destroyed trees and trash‐wood in the mangrove ecosystem, and in fact, from any sort of infested under‐water woody materials, such as submerged structures, hulls of fishing boats, 426 Wood-borers
jetty‐piles etc. Ecological and biological aspects, such as faunal composition, seasonal intensity and breeding period, growth‐rate and vertical distribution can be studied by exposing series of short‐term and long‐term panels for overlapping periods so as to get a continuous picture of borer activities throughout the year. The molluscan borers are to be extracted by carefully cutting open the damaged wood samples and best specimens are obtained when the borers are dissected out as soon as the wood is removed from the water. If this is not possible, the destroyed wood pieces should be submerged in 70% alcohol for a few days and then can be transported to the laboratory wrapped in cotton and cloth soaked alcohol. Specimens removed from the wood should be preserved in a mixture of four parts 70% alcohol and one part and one part glycerine. This way the periostracal margins of the pallets can be kept soft and pliable and in case the alcohol evaporates, the glycerine will keep the pallets moist for some more time. While cutting open the timber is required to collect the molluscan borers, the crustaceans can be collected by keeping the infested wood samples in a trough of diluted see water or by adding a little formalin. The change in the milieu and the traces of formalin will force the borers out of their burrows and specimens can be easily collected with a fine brush or foreceps. The crustacean borers also are to be preserved in alcohol, as formalin will slowly dissolve the tubercles on the dorsal surface of the animals, in the same way as it damages the periostracal covering of pallets of teredinids, rendering species identification difficult. Characters of Taxonomic Value for Identifying Marine Wood ‐borers Shipworms (Teredinidae): For a long time, classification of teredinid species (shipworms) was based entirely on shells and pallets, although variations in specimens of the same species were confusing. It was Turner (1966) who brought out the importance of the anatomy of shipworms in the systematics of this group, particularly in generic classification. Characters of systematics value for species identification are nature of the shell valves, tubes (internal lining of the borrows which sometimes gets thickened as a tube particularly at the posterior end), pallets (a pair of calcareous organ situated at the posterior end of the animal which is used to plug the entry hole during adverse conditions or when the borer is disturbed) and siphons. Of these, the morphological variations exhibited by the pallets are remarkable and almost all the species can be identified L. N. Santhakumaran 427
from their pallets (Turner, 1966, 1971). Other characters are of limited help but, when considered with more important characters, may prove useful in separating closely related species. Piddocks (Pholadidae): The piddocks or members of the family Pholadidae are classified based on the shape of shell valves, nature and arrangement of accessory plates (protoplax, mesoplax, metaplax and hypoplax), presence or absence of callum in the adult stage, presence or absence of apophysis, and on the morphology of the siphons. In some members (Sub‐Family Martesiinae), the young and adult are different morphologically, the former having an anteriorly beaked and widely gaping shell and the latter having this gape closed by a calcareous deposit, the callum (Turner, 1971). The nature of the chitinous lamellae on the posterior slope of the shell, when present, also helps in species separation. Nomenclature of parts in members of Teredinidae and Pholadidae is shown in Fig.1‐A to G. Fig. 1. Hypothetical, composite drawing of a wood‐boring teredinid and holad giving nomenclature of parts. A: Entire shipworm; B: External view of right value of shipworm; C: Internal view of right value of shipworm; D: Pallet of Teredo; E: Hypothetical pallet of Bankia; F: Hypothetical pholad; G: Young phlad. 428 Wood-borers
1. shell; 2. foot; 3. cephalic hood; 4. mantle collar; 5. excurrent siphon; 6. incurrent siphon; 7. pallet; 8. anterior slope; 9. disc; 10. posterior slope; 11. umbonal ventral sulcus; 12. umbonal reflection; 13. dorsal condyle; 14. chondrophore; 15. apophysis; 16. imbonal‐ventral ridge; 17. ventral condyle; 18. blade; 19. stalk; 20. calcareous base; 21. periostracal cap; 22. outer margin; 23. inner margin; 24. cone; 25. calcareous portion; 26. coarse serrations; 27. periostracal portion; 28. comb‐like serrations; 29. serrated long awn; 30. fine serrations; 31. web; 32. non‐serrated long awn; 33. broad short awn; 34. beak; 35. callum; 36. protoplax; 37. mesoplax; 38. metaplax; 39. siphonoplax; 40. hypoplax; 41. periostracal lamellae; 42. flange. Pill ‐ Bugs (Sphaeromatidae): Characters of taxonomic value in species of Sphaeromatidae are the number and disposition of large tubercles on the dorsal posterior part of body, posterior part of the telson (Fig.2.) and shape of the epistome. Of these, the arrangement of the large tubercles is strikingly different and shows variations characteristic of each species (Pillai, 1961). Fig. 2. Pleotelson of Sphaeroma spp. Showing the arrangement of tubercles A: S. terebrans; B: S. annandalei, C: S. annandalei travancorensis; D: S. triste Nature of burrows produced: Burrows produced by each of the above four types of borers are also characteristic of its occupant. Shipworms bore deep into the wood making long tunnels almost parallel to the grain (Fig. 3A), whereas burrows of pholads are pear‐shaped, superficial and nearly at right angle to the grain (Fig. 3B). Pill‐bugs produce cylindrical burrows on the wood surface at right angle to the grain (Fig. 3C). Sometimes the juveniles start working from the main parent tunnel leaving side branches (Fig. 3D). L. N. Santhakumaran 429
Fig. 3. Burrows produced by (A) Shipworms; (B) Pholad; (c) Pill bugs; (D) Pill bugs with juveniles Systematics of Marine Wood‐borers Types of Wood‐borers There are two groups of wood‐borers in the sea. One group, closely related to oysters and clams (Mollusca : Bivalvia), is called ʹshipwormsʹ (Family : Teredinidae) and ʹpiddocksʹ (Family : Pholadidae). The other group, distant cousins of prawns and crabs (Arthropoda : Crustacea) is known as ʺpill‐bugsʺ (Family : Sphaeromatidae) and ʺgribbleʺ (Family : Limnoriidae) (Limnoriids have not so far been reported from mangroves in India and members of a third family of amphipods, called Cheluridae, are not active along Indian coasts and hence both are not important). Sixty‐three species of wood‐borers have so far been identified from Indian seas, of which 27 occur in different mangroves along Indian coast. Phylum : Mollusca Class : Bivalvia (Pelecypoda) Order : Myoida (Pholadacea ) Sub‐order : Pholadina Family : Teredinidae Sub‐family : Bankiinae 1.Bankia bipennata (Turton); 2. Bankia campanellata Moll & Roch; 3. Bankia carinata (Gray); 4. Bankia fimbriatula Moll & Roch; 5. Bankia nordi Moll; 6. Bankia rochi Moll; 7. Nausitora dunlopei Wright; 8. Nausitora fusticula (Jeffreys); 9. Nausitora hedleyi Schepman; 10. Nototeredo edax (Hedley); 11. Nototeredo knoxi (Bartsch); Sub‐family : Teredininae 430 Wood-borers
12. Bactronophorus thoracites (Gould); 13. Dicyathifer manni (Wright); 14. Lyrodus massa (Lamy); 15. Lyrodus pedicellatus (Quatrefages); 16. Teredo clappi Bartsch; 17. Teredo furcifera von Martens; 18. Teredora princesae (Sivickis); 19. Uperotus rehderi (Nair); Family : Pholadidae Sub‐family : Martesiinae 20. Martesia (Martesia) striata (Linnaeus); 21. Martesia (Particoma) nairi Turner & Santhakumaran; 22. Lignopholas fluminalis (Blanford); Sub‐family : Pholadinae 23. Barnea birmanica Philippi; Phylum : Arthropoda Class : Crustacea Malacostraca Sub‐class : Order : Isopoda Sub‐order : Flabellifera Family : Sphaeromatidae 24. Sphaeroma terebrans Bate; 25. Sphaeroma triste Heller; 26. Sphaeroma annandalei Stebbing; 27. Sphaeroma annandalei travancorensis Pillai (Identification of B. fimbriatula, N. fusticula and N. knoxi needs confirmation). Nature of pallets of shipworms and entire specimens of piddocks and crustacean borers are shown in Figs. 4 to 8. Fig.4. Pallets of shipworms collected from mangroves A: Bactronophorus thoracites; B: Dicyathifer manni; C: Lyrodus pediclellatus; D: Bankia rochi; E: Bankia campanellata; F: Nausitora hedleyi; G: Nausitora dunlopei. I: Outer face; II: Inner face L. N. Santhakumaran 431
Fig. 5. Pallets of shipworms collected from mangroves J: Bankia fimbriatula; K: Bankia bipennata; L: Nausitora fusticula; M: Uperotus rehderi; N: Teredo furcifera. I: Outer face; II: Inner face Fig. 6. Pallets of shipworms collected from mangroves. O: Nototeredo edex; P: Notoeredo knoxi; Q: Teredora princesae; R: Bankia nordi; S: Teredo clappi; T: Bankia carinata; U: Lyrodus massa; I: Outer face; I: Inner face; Si: posterior end of excurrent siphon 432 Wood-borers
Fig. 7. Four species of Pholadidae from mangroves. A, B, C, D : Martesia striata; E, F: Martesia nairi; G, H, I, J : Lignopholas fluminalis; K: Barnea birmanica; A, C, F, H & I : dorsal view; B, D, E, G & J: ventral view; L: lateral view Fig. 8. Four species of Sphaeromatidae from mangroves. A: Sphaeroma terebrans; B: Sphaeroma triste; C: Sphaeroma annandalei; D: Sphaeroma annandalei travancorensis; E: Mandible of Sphaeroma Significance of Marine Wood‐borers in Mangroves The role played by marine wood‐infesting organisms (borers and foulers) in the elimination of mangroves has not received the attention it deserves. L. N. Santhakumaran 433
In India, marine wood‐borers are economically a very important problem because of the extensive damage they cause to underwater timber structures. Settlement by marine fouling organisms ‐ the sedentary animals such as barnacles, oysters, serpulids and even gastropods and algae ‐ is also a serious menace with several undesirable effects on the fishing and shipping operations, on coastal industrial installations using sea water for cooling purposes and on mangrove seedlings. During routine survey on distribution of marine wood‐borers along Indian coast, it has come to light that they are capable of extending their depredations to living mangrove vegetation as well. A perusal of the Annotated Bibliography on marine wood‐borers and mangrove biodeterioration (Santhakumaran,1994; Santhakumaran and Sawant, 1991) will show that publications on borer activities in mangrove areas are of a preliminary nature and that too limited to studies on faunistic composition of borers restricted to few mangrove areas. Species identification in some of these studies has not been properly confirmed resulting in confusing and erroneous records. Although most of these authors have highlighted the need for continued studies to ascertain the extent of damage, the role played by marine borers in the ultimate elimination of this important ecosystem is either over‐
looked or under‐estimated subsequently. Same is the case with the harmful effects of marine fouling organisms as well. No concerted efforts have so far been made for an in‐depth quantitative assessment of the damage to living trees with a view to understanding the ecological aspects of the problem. While their destruction and the consequent degradation of mangrove areas continue, no attempt to evolve ways and means to minimise the damage was ever made. Subsequent detailed observations made along Goa coast revealed that in some localities as much as 40% of the living trees have been infested and even healthy trees of Excoecaria agallocha (a highly poisonous plant) have been attacked and destroyed (Santhakumaran and Surekha Sawant, 1998). The borers concentrate their attack near the mud level and due to intensity of riddling, the tree eventually gets up‐rooted even during a mild gale. On the positive side, it may be mentioned that marine wood borers in a mangrove ecosystem do play a significant useful role in cleaning the habitat with their unusual ability to disintegrate wood. It can be safely said that but for the presence of these organisms, our estuaries and river mouths could have been clogged with uprooted trees, broken branches and other trash woody materials. Woody tissues (roots, branches and trunks), channelled into litter in a mangrove environment is a long‐lived component with life‐span and 434 Wood-borers
residence‐time (period during which the biomass is available to herbivores) higher than the leaves. Their break‐down to products easily transportable by tides and currents depends on the activities of wood‐
degrading organisms, especially marine borers. The rate of turn‐over of dead woody materials, which constitute a high proportion of total biomass is mangrove forests, thus, plays an important role in replenishing nutrients and energy along the coasts and off‐shore through currents and food‐webs (Cragg, 1993). Rapidity with which samples of different species of timbers were destroyed and recycled by marine borers, indicates the significant ecological role played by them in the ecosystem. Tunnels produced by the borers greatly increase the surface area available for fungal and bacterial decay process, which, in turn, accelerate the disintegration of wood into smaller pieces. The pathway of wood‐derived energy fluxes through mangrove‐
related food‐webs has not so far been studied in any detail. Christensen (1978) reported that in a 15‐year old tall stand of R. apiculata in Thailand, the above‐ground biomass contained only 5% leaves and 0.2% reproductive parts, the remaining component being woody tissue. According to Cragg (1993), at least 50% of wood litter biomass is converted entirely by teredinids in a Rhizophora‐dominated forest in Papua New Guinea. Considering the quickness with which timber panels were destroyed in Goa mangroves (Fig. 12), this appears to be a very conservative estimate. Thus, the service rendered by teredinid community in the process of wood disintegration thereby cleansing the mangrove ecosystem from unwanted trash‐wood is very significant. Nevertheless, when this phenomenon occurs on living vegetation, their role tends to be negative and unwelcome and they are to be viewed as pests that warrant control measures. Occurrence of marine wood–borers in mangroves has other significance as well. Firstly, monitoring borer and fouler distribution and abundance in mangroves is essential because of their vulnerability in the event of sea level rise. These forests are expected to recede down in places or in some places migrate inland and hence bench‐marks can be established against which such changes can be measured keeping these organisms as indicator species. Secondly, natural immune mechanisms or stress‐induced responses of mangrove plants may provide opportunities for evolving methods of control of boring and fouling organisms. Such an outcome from any investigations will be a boon to the fishing and shipping industry and to the Navy, which are all plagued by biofouling and biodeterioration problems. Thirdly, there is yet another economic L. N. Santhakumaran 435
significance to studies on mangrove biodeterioration. The habitat, with easy availability of large quantities of substratum in the form of dead stumps, dried branches and twigs and other trash‐wood, offer an ideal habitat and breeding ground to wood‐borers and thus form a perennial source of borer larvae for fresh infestation on marine structures in the neighbourhood. This will not only result in continued destruction of the vegetation, but also in accelerating deterioration of other marine structures in the vicinity, thereby further contributing to the already complex problem of marine wood biodeterioration. The outcome of the present investigation might be useful to combat the biodeterioration processes from the above angle as well. Such approaches will have far‐reaching implications in mangrove ecosystem restoration, environmental conservation and management. Ecological Requirements of Marine Wood‐borers of Mangrove Habitat Survey on the marine wood‐borers of the mangroves along Indian coast revealed that the problem is very acute as the lignicolous fauna is rich both in number of species and intensity of attack. The environmental conditions of the habitat are also conducive to the breeding, larval growth, infestation and destruction of the woody material including the living vegetation. As mentioned earlier, some of the borers like B. thoracites, D. manni, M. nairi and sphaeromatids are specific to this habitat and all of them, except D. manni, were rarely found in test panels and timber structures in the adjacent areas outside mangrove habitat. The reason for this habitat specificity of borers can probably be attributed to the interrelationship that may exist between microorganisms and marine borers. The microflora that colonise timber substratum are known to influence and facilitate larval settlement of sedentary marine organisms (Ralph Mitchell and David Kirchman, 1984) and it may be that the above‐
mentioned borer species might require certain specific types of microflora to induce settlement, attachment and metamorphosis. Investigations on the biodiversity of fungi species colonising wood panels exposed within and outside mangrove ecosystems have revealed considerable variations in the faunal composition (Santhakumaran and Sawant, 1998). Destruction of timber panels within the ecosystems is extremely severe. Remedial Measures Against Biodeterioration of Mangroves by Marine Wood‐borers A perusal of the literature on the subject reveals the intense activity of marine borers in mangroves, sometimes to the extent of damaging live mangrove vegetation. The problem is by itself very complex not only due 436 Wood-borers
to the presence of a diverse group of boring organisms, but also by virtue of their nutritional requirements and ecological adjustments for sharing the same substratum ‐ timber. It is difficult to totally prevent borer damage to living vegetation, as normal wood preservation methods cannot be applied on a tree. In the extreme upper reaches of the estuary, because of the inhibitive action of fresh water conditions, activities of destructive organisms is very much restricted and rate of regeneration of vegetation is always higher than its deterioration, while near the estuary and up to places, where sea water percolates, destruction is faster. The following suggestions are made to minimise, if not totally control, biodeterioration of mangrove vegetation. a. Constant monitoring of the incidence of borers very much like the one for insect pests and defoliators on land. This is very essential up to areas which are under the influence of sea water, which will enable the larvae to migrate upstream. This effort will be beneficial in two ways (1) To get an overall idea of the intensity of the problem and (2) To enable to remove the source of infestation at the very onset. b. Method of ʺgood house keepingʺ to be practised. Infested stumps and other trash wood lying in the habitat should be removed, as they provide a perennial source for the supply of borer larvae for fresh attack. c. To study the natural durability of Derris sp. While marine borers have been reported to attack various mangroves, no infestation was observed on Derris during the present survey. While the possibility of the occurrence of this tree upstream with protection afforded by fresh water conditions cannot be precluded, it may still prove useful to study its natural bioresistance in a truly marine environment, as this tree is reported to contain substances with poisonous properties. The results from such studies may have wider applications. d. Detailed investigations on the efficacy of specific fungal metabolites to control incidence of borers. In view of the fungi‐
borer interrelationships and specificity, detailed investigations on effect of fungal metabolite on larval settlement are worth pursuing. e. Systematics and occurrence of mangrove plant species. The absence of literature giving authentic accounts on the systematics and distribution of mangrove plant species was significantly felt while recording the incidence of marine organisms. Erroneous and L. N. Santhakumaran 437
confusing details are available in published records. It is therefore suggested that a comprehensive account on systematics and distribution of mangrove vegetation, delineating true mangroves and associate species properly may be brought out to facilitate future research on mangroves and on their deterioration by marine organisms. References Christensen, B. (1978). Biomass and primary production of Rhizophora apiculata Bl. in a mangrove in southern Thailand. Aquatic Botany, 4 : 43‐52. Cragg, S.M. (1993). Wood break‐down in mangrove ecosystems : a review. Papua New Guinea Journal of Agriculture, Forestry & Fisheries, 36 (1): 30‐39. Pillai, N.K. (1961). Monograph, wood – boring Crustacea of India. 61 pp. Manager of Publications, Govt. of India. Press, New Delhi Ralph Mitchell and David Kirchman, (1984). The microbial ecology of marine surfaces. Ibid. pp. 49‐56. Roonwal, M.L. (1954). The marine borer, Bactronophorus thoracites (Gould) (Mollusca, Eulamellibranchiata, Teredinidae), as a pest of living trees in the mangrove forest of the Sunderbans, Bengal, India. Proc. Zool. Soc. (Calcutta), 7 (2) : 91 – 105. Santhakumaran, L.N. (1994). Marine wood‐borers of India ‐ an annotated bibliography. 262 pp. Institute of Wood Science & Technology, Bangalore. Santhakumaran, L.N. (1996). Marine wood‐borers from mangroves along Indian coasts. J. Indian Acad. Wood Sci., 26&27 (1&2) : 1 – 14. Santhakumaran, L.N. and Surekha G. Sawant, (1991). Biodeterioration of mangrove vegetation by marine organisms along Indian coast ‐ an annotated bibliography. 48 pp. Wood Biodegradation Division (Marine), C/o National Institute of Oceanography, Dona Paula, Goa ‐ 403004. Santhakumaran, L.N. and Sawant, S.G. (1998). Marine Wood – infesting organisms in the destruction of living mangrove vegetation along Goa coast. Indian Forest Bulletin. 67 pp. 40 figs, ICFRE, Debradan. Turner, R.D. (1966). The identification of molluscan borers. Report to the Governemnt of India. FAO Report No. TA. 2155 : 1‐30. Turner, R.D. (1971). Identification of marine wood – boring molluscs. In : Jones E.B.G. and Eltringham, S.K., (Ed.) Marine borers, fungi and fouling organisms of wood, OECD, Paris : 17‐64.