in mangrove habitats in Panay - Springer Link

Wetlands Ecology and Management 10: 363–370, 2002.  2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Collection of the clam Anodontia edentula in mangrove habitats in Panay and Guimaras, central Philippines J.H. Primavera*, M.J.H.L. Lebata, L.F. Gustilo and J.P. Altamirano Aquaculture Department, SEAFDEC, Tigbauan, Iloilo 5021, Philippines; * Author for correspondence (email: jhprima@ aqd.seafdec.org.ph; phone: (63 – 33) 3351009 /3362965 /3362937; fax: (63 – 33) 3351008) Received 29 February 2001; accepted in revised form 23 January 2002

Key words: Bacterial symbionts, Food security, Gleaning, Lucinid bivalve, Mangrove clam

Abstract The mangrove clam Anodontia edentula is highly prized in the Philippines for its flavor and large size. Because this infaunal species is found down to one meter deep in mangrove areas, harvesting the clam reportedly damages mangrove stands. To evaluate such reports, a survey of collection methods was undertaken in Panay and Guimaras, central Philippines in August 1997–December 1999. Host to chemosynthetic bacterial symbionts that utilize sulfide as energy source, A. edentula are strategically situated in sulfide-rich anoxic substrates but also gain access to oxygenated seawater through a ventilation burrow or tube. By locating the opening of this burrow, collectors can detect the presence of a buried clam and harvest it nondestructively with a blade or bare hands. In contrast, the indiscriminate tilling of wide mangrove areas can damage mangrove plants. Most collectors were 40–45 years old with 22–30 years collection experience, married with 5–7 children, and had low educational attainment. They sold clams directly in the local markets or through middlemen (to restaurants and beach resorts); sales provided from 10% to 100% of daily family income. Collectors complained of decreasing clam sizes and numbers and the physically strenuous work of collecting.

Introduction Mangrove ecosystems have provided a variety of goods and services to coastal communities and the larger society over the centuries. Fisheries products from mangroves are important for food security and income, and may be harvested offsite or onsite. Popular among the latter are crustaceans and bivalve mollusks such as the mangrove clam Anodontia edentula Linne 1758 (Figure 1), known as imbao in the Philippines. A. edentula commands a high price because of its large size of 9 cm maximum shell length and 210 g body weight (Lebata 2000), high flesh content of 25–30% (J.H. Primavera, pers. observation) and reputed aphrodisiac properties. An infaunal species, it possesses bacterial symbionts (Janssen 1992) and belongs to Class Bivalvia, Order Veneroida and Family Lucinidae, together with most eulamellibranchs known to harbor symbiotic bacteria

(Schweimanns and Felbeck 1985). Evidence of this symbiotic relationship in A. edentula include a reduced digestive system, thick dark-colored gills with coccoid or spherical bacteria, and high levels of elemental sulfur in the gills (Lebata 2000; Lebata and Primavera 2001). These features have also been observed in other bivalves that possess chemoautotrophic bacterial symbionts (Vetter 1985; Reid 1990; Distel and Felbeck 1987). A. edentula is found throughout the Philippine archipelago (Poutiers 1998) and widespread harvesting of this clam has reportedly damaged mangrove stands (Walters (1995) who mistakenly identifies the species as Phacoides philippinarum). A survey was therefore conducted a) to document various aspects of A. edentula collection in the islands of Panay and Guimaras, and verify the destructiveness of the collecting methods, and b) to relate survey results to available information on mangrove clam biology.

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Figure 1. The mangrove clam Anodontia edentula collected from Estancia, Iloilo, central Philippines.

Materials and methods A. edentula is regularly collected in Estancia, Iloilo (118279N, 1238109E) and Batan, Aklan (118359N, 1228359E) in Panay Island, and Nueva Valencia in Guimaras Island (108259N, 1228339E) in central Philippines. Clam collectors in these sites were identified and interviewed following a prepared survey questionnaire that covered personal information, collecting place and time, and collection methods (Appendix A). Interview period and number of respondents were Estancia: August 1997–March 1998 (n 5 4), Batan: July 1998–November 1999 (n 5 4), and Nueva Valencia: December 1999 (n 5 8).

Results Collectors (Table 1) – The respondents were all male in Estancia, all female in Batan (Figure 3a), and mostly female in Nueva Valencia (Figure 4a). They were all married, except two in Nueva Valencia, with 5–7 children on the average. Their highest educational attainment was Grade 2 to Grade 6 of primary school; one respondent from Estancia had no formal education.

At the time of interview, their average age was 40–45 years. They had been collecting mangrove clams for 30 years starting at 9–11 years of age in Estancia and Batan (Figure 3a), and for a shorter period of 22 years starting at 23 years old on the average in Nueva Valencia. They learned how to collect clams by observing family members, particularly their mothers (Figure 3a) and grandmothers in Estancia and Batan, and other collectors in Batan and Nueva Valencia. Collection site and time (Table 2) – Collection sites cover nine villages and sitios in Nueva Valencia, five in Estancia and two in Batan. The sites are located in the mangrove forest itself (Figure 3a, 3b) or in adjacent tidal flats (Figure 4a) where the substrate is muddy or sandy-muddy. Mean collecting depth is only 25–50 cm in Batan where the substrate is more sandy, compared to 30– 75 cm in Estancia and 30–90 cm in Nueva Valencia where the substrate is muddy. Collecting methods (Table 2) – Only one collecting method is used in Estancia and Batan—the presence of a clam is de-

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Figure 2. Map showing collecting sites for the mangrove clam Anodontia edentula in Panay and Guimaras islands in the Philippines (inset).

tected by looking for a 2–5 mm diameter hole in the substrate (locally called mata) which is the opening of its ventilation burrow. The collector starts to dig from this hole in a downward direction using bare hands or a long flat iron blade or tagad (Figure 3b). Aside from this method, Nueva Valencia collectors locate the clam by poking an iron rod, or walking barefoot in knee-deep water, on the substrate. They also use blades (Figure 4b) to dig a wide area without prior location of individual clams (Figure 4a). Clams can be collected only during daylight (for 2–6 hours) at low tide. Although gleaning is possible year-round, collections are infrequent during the rainy season when the burrow openings are not readily visible.

resorts). Retail price per piece is Phil. Peso (P)1.50– 5.00 in San Miguel and Estancia and P2.50 in Batan (US$1:PhP40.00). Collectors earn P80–250, P110– 140, and P15–150 representing 10–90%, 10–90%, and 40–100% of daily income in Estancia, Batan and Nueva Valencia, respectively. Although many other bivalves and gastropods were also collected, these were mostly for domestic consumption. Estancia and Nueva Valencia collectors complain about the decreasing clam sizes and numbers, and the physically strenuous work of collecting. Collectors in Nueva Valencia also point to competition among collectors.

Markets

Discussion

(Table 3) – Regular collections are sold directly in the local markets in San Miguel, Guimaras and through middlemen in Estancia, Iloilo and Batan, Aklan. Collections are also made when there are orders (e.g., for Iloilo City restaurants or Kalibo, Aklan tourist

The mangrove clam Anodontia edentula has been reported from many mangrove areas of the Philippines including Silut Bay in Cebu (Miller 1973), Cogtong Bay in Bohol (Janiola 1996; Katon et al. 1998), Bais Bay in Negros (Luchavez and Abrenica

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Figure 3. Mangrove clam Anodontia edentula collection by locating the inhalant tube opening: a) Mother and daughter in a mixed Avicennia marina-Sonneratia alba stand in Batan, Aklan, Panay Island, and b) male collector among Rhizophora stylosa mangroves in Panubolon Is., Nueva Valencia, Guimaras.

1997) and other islands (Sotto and von Cosel 1982), but not from non-mangrove habitats. Like other lucinid bivalves, A. edentula harbor chemoautotrophic bacteria in their gills (Lebata 2000). Sulfide, which is abundant in the muddy substrate of man-

groves, is oxidized to elemental sulfur (Lebata 2000), providing energy for the bacterial symbionts to fix carbon dioxide into organic compounds that become available to the host (Distel and Felbeck 1987). Elemental sulfur is stored in the gills and imparts the unique (delicious, to many) taste of steamed, baked and especially raw clams. A. edentula has a reduced gut (Lebata and Primavera 2001) hence bacterial chemosynthesis is vital to its nutrition. Located down to one meter deep in the mud substrate, the clam obtains needed oxygen from seawater that passes through a ventilation burrow or inhalant tube. This tube or burrow is constructed by the flexible foot that can extend several times the length of the clam (Lebata and Primavera 2001). Most collectors locate the infaunal clams by looking for the opening of the inhalant tube in the mud substrate. Digging follows only after a clam is located, therefore these methods are not harmful to mangroves. However, the practice of tilling or excavating wide tidal areas without prior location of clams as in Guimaras (this study) and Bais Bay, Negros Oriental (Walters 1995) damages mangrove roots and may kill seedlings, saplings and young trees. Aside from applying nondestructive methods, collectors do not harvest clams below 3 cm shell length, the minimum size noted to have ripe gonads (J.H. Primavera, pers. observation). Despite this size selection, smaller clam sizes (and declining numbers) have been observed by the collectors. According to Garcia (1986), A. edentula and other Philippine bivalves with higher market value have become less available. Mollusks and other sedimentary animals in the intertidal zone are gleaned both for home consumption and market sale. In a mangrove reforestation area in Banacon, Bohol, yearly per capita consumption and sales of mollusks was 212 liters and 276 liters, respec-

Table 1. Personal data on collectors of mangrove clam Anodontia edentula in Panay, and Guimaras central Philippines.

Interview period Sex: Male Female Civil status: Single Married Mean no. of children (range) Educational attainment Mean age (range)* Age when collecting started (range) Mean no. of years collecting (range)* Taught by whom *

As of interview date

Estancia, Iloilo (n 5 4)

Batan, Aklan (n 5 4)

Nva. Valencia, Guimaras (n 5 8)

Aug 97–Mar 98 4 0 0 4 7 (6–9) None to Grade 5 42 yr (38–49) 11 (8–15) 29.8 (27–34) mother, grandmother

Jul 98–Nov 99 0 4 0 4 5 (3–8) Grade 3 to 5 40 yr (28–58) 9 (7–10) 30.5 (19–51) mother, observing other collectors

Dec 99 3 5 2 6 7 (5–9) Grade 2 to 6 45 yr (20–68) 23 (10–40) 21.8 (3–50) family, observing other collectors

during low tide, daytime only 2–5 low catch during rainy season

Tidal / diel phase

No. collection hours / day Season

depends on tide

30 (25–30) cm 75 (50–100) cm locate opening of burrow (mata), then dig with tagad or long blade

mud / tidal flats inside and near mangroves,

Frequency

Mean depth (range): min max Method

Habitat

Estancia, Iloilo (n 5 4)

2–6 year-round

a) depends on tides b) when there are orders during low tide, daytime

25 (20–25) cm 50 (30–60) cm Locate opening then dig with blade or bare hands.

sandy-muddy substrate in mangroves

Batan, Aklan (n 5 4)

Table 2. Data on collection of mangrove clam Anodontia edentula in Panay and Guimaras, central Philippines.

3–6 year-round

30 (20–30) cm 90 (90–120) cm a) locate opening then dig with blade, b) locate clam with bare feet then dig with bare hands, c) locate clam by poking iron rod in mud, d) use blade to till wide area on mudflat then gather clams a) depends on tide & weather, b) 3–4 days before market day (Sun) during low tide, daytime

muddy areas in and near mangroves

Nva. Valencia, Guimaras (n 5 8)

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Figure 4. Collection of mangrove clam Anodontia edentula by indiscriminate digging in Panubolon Is., Nueva Valencia, Guimaras: a) male and female collectors, and b) long flattened blades used for digging.

tively (Tura and Melana 1993). Unlike other shells that are marketed in batches, A. edentula is sold by piece because of the high prices commanded by its large size, delicious flavor and alleged aphrodisiac properties. Hence clam collection can provide all of the family income for the day. Gleaning is usually done by children and women in many coastal communities but various factors have driven menfolk to activities traditionally reserved for women in the Philippines. These factors include a decline in most small-scale or artisanal fisheries (McManus 1989), the physical demands of collecting (especially where the clams lie deep below the surface), and lucrative prices for A. edentula.

Moreover, the dependence of the respondents in this study on imbao collecting despite the physically strenuous work may be due to their low educational attainment (primary level).

E. Conclusions and Recommendations Most collecting methods of A. edentula in Panay and Guimaras, central Philippines are non-damaging to mangroves, but other problems remain. The following recommendations are made to address these problems: 1. Clam habitats should be conserved by protecting

Table 3. Market data on mangrove clam Anodontia edentula in Panay, central Philippines. Estancia, Iloilo (n 5 4)

Batan, Aklan (n 5 4)

Nva.Valencia, Guimaras (n 5 8)

Location Price a

Estancia, Iloilo P1.50–5.00 / pc accdg to size

Income / day %income from imbao collection Preparation of imbao Problems

P80–250 10–90% raw, broiled, steamed, boiled a) burrow opening disappears during rains, b) decreasing number and size, c) hard to collect

Batan, Aklan P0.90 / pc ex-collection P2.50 / pc ex-market P110–140 10–90% boiled, soup, baked No data

San Miguel (Alibhon), Guimaras P1.50–3.00 / pc: small, medium P4.00–5.00 / pc: large P15–150 40–100% raw, broiled, boiled, steamed a) stiff competition, b) decreasing number and size, c) physically strenuous job

a

US$1:Phil.Peso 40.00

Appendix A

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Figure 5. IMBAO SURVEY QUESTIONNAIRE

370 remaining mangroves and rehabilitating degraded areas, to improve declining stocks over the long term. 2. Ongoing trials to rear A. edentula larvae at the Iloilo-based Aquaculture Department of the Southeast Asian Fisheries Development Center can contribute towards recovery of natural populations by restocking of hatchery-produced juveniles. 3. Local municipal and village governments can also regulate overexploitation by limiting the number of collectors through issuance of licenses and setting of minimum clam size at harvest. Toward this end, studies are needed to establish minimum size at first maturation. 4. The wide variety of mollusks gleaned should be inventoried for a possible multi-species approach to sustainable collection and conservation.

Acknowledgements We would like to thank R. Buendia for the blackwhite photo conversions, and to dediicate this paper to Kalbo, our collector from Estancia whose humanity managed to touch us despite a life all too nasty, brutish and short.

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