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Effect of desiccation on Tridacna derasa seed: Implications for long distance transport
Aquaculture, 49 (1985) 363-367 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
363
Short Communication EFFECT OF DESICCATION ON TRIDACNA DERASA SEED: IMPLICATIONS FOR LONG DISTANCE TRANSPORT M.D.G. LOPEZ’ and G.A. HESLINGA’
’ Marine Science Institute, University of the Philippines, Diliman, Quezon City 3004 (The Philippines) 2Micronesian Mariculture Demonstration Center, Koror, Republic of Palau 96940 (Caroline Islands) (Accepted
20 August 1985)
ABSTRACT Lopez, M.D.G. and Heslinga, G.A., 1985. Effect of desiccation on Tridacna derasa seed: implication6 for long distance transport. Aquaculture, 49: 363-367. Saving6 in freight cost from removal of the by66us and adherent material from T. derasa seed may not offset man-hour6 required for pre-shipment preparation, and decreased seed viability.
INTRODUCTION
Tridacna gigas and T. derasu, the two larger members of the molluscan family Tridacnidae, have been classified by the IUCN (1983) as vulnerable species which may become endangered if present levels of exploitation continue. At the Micronesian Mariculture Demonstration Center in Palau, both species have been raised to maturity and seed clams are being produced to restock depauperate reefs (Heslinga and Perron, 1983). Shipments of seed clams ranging in size from 10 to 100 mm have been made to Guam, Yap, Hawaii, Pohnpei, California, Fiji, the Philippines and the Marshall Islands (Heslinga et al., 1984; Heslinga and Watson, in press). In preparation for shipment, these clams were wrapped in moist nylon netting and packed in plastic bottles or Styrofoam coolers. Byssal threads and any attached gravel substrate were not removed from the clams. For shipping large quantities of seed clams by air, it is obviously important to minimize weight. Because the gravel normally adhering to the tridacnid byssus adds significantly to shipping weight, we conducted a series of trials to determine how removal of the entire byssal apparatus would influence the survival of the seed clams under simulated shipping conditions. The results of these preliminary observations indicate that while removal of the byssal apparatus does decrease shipping weight, potential savings in freight cost would probably not be sufficient to offset 0044-8486/85/$03.30
0 1985 Ekvier
Science Publishers B.V.
1
Reimmersion time (h)
21
15
18
41
6
4
4
1
23
Emmersion time (h)
10
12
12
12
15
18
21
24
24
10-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50 lo-19 20-29 30-50 lo-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50
(mm)
Size class
1 2,18 I, 16 13 7,14 2, 16 10 4, 12 0, 15 5 75 57.5 65 52.5 45 50 40 37.5 25
Survivors
Survivors 19 20 20 20, 20, 19 20, 18, 20 20.20 19; 20 20,14 20 18, 20 20, 14 20 20,20 20, 20 20,19 18,20 19, 20 20, 20 17 20 20 14 12 4 14 14, 8 14
24-28°C %
regime
11°C
Temperature
97.5 85 100 95 -5 100 100 100 97.5 95 97.5 100 85 100 100 70 60 20 70 55 70
100
98.3 96.7
100
95 100
%
14 16 17 16, 16, 18 14, 17,19 16, 20 15,15 14,15 14 13,14 11,14 13 14,19 16, 20 19,18 10, 9 12, 8 16, 9 16 17 20 8 8 8 9 8 5
Survivors
33°C
70 80 85 83.3 83.3 90 75 72.5 70 67.5 62.5 65 82.5 90 92.5 47.5 50 62.5 80 85 100 40 40 40 45 40 25
%
Number of survivors per replicate (n = 20) and mean survival rate (%) per size class after variousemmersion periods at three temperature ranges
TABLE :
365
(1) a decrease in seed viability over long distances; and (2) an increase in time and labor required for pre-shipment preparation. Our preliminary observations on temperature effects indicate that artificial cooling of clam seed reduces viability, as does warming beyond an optimal range of approximately 24-28°C. In sum, presently available data suggest that giant clam seed survive best when shipped moist but not submerged, with the byssus intact, within a temperature range similar to that found in their natural habitat. METHOD
Five-month-old T. derasa in three size classes (10-19 mm, 20-29 mm, and 30-50 mm) from which adherent basalt chips had been pulled off were kept for at least 12 h in seawater tanks before use. Twenty clams in each size class were used per treatment, with replicates where the number of available clams allowed. The seed were packed as described above and stored for periods of 10, 12, 15, 18, 21 and 24 h in a refrigerator (11” C), an air-conditioned room (24-28” C), and at ambient temperatures (up to 33°C). After exposure, the clams were unpacked and placed in gravel-lined trays set in raceways. Survivors were counted after a minimum reimmersion period of 1 h. Subsequent mortalities were followed up to 41 h after reimmersion in clams subjected to 12 h emmersion. RESULTS
The number of survivors from all replicates and mean percentage survival for all treatments are shown in Table 1. Clams from which the byssus had been removed during sorting and immediately returned to the holding tanks exhibited no mortalities during the test period. Temperature
effects
Treatments at 11°C were discontinued after a 12-h test run resulted in survival rates below 50% for all size classes. For emmersion periods up to 24 h, mean survival rate was consistently higher at 24-28°C than at ambient temperatures for all size classes, except for the 21-h exposure of 30-50 mm clams, among which survival was 100% at both temperature ranges. Variation among replicates was considerable but overlaps in survivorship values between temperature ranges were small. In the 10-19 mm class, mean survival rate at ambient temperatures declined with increasing emmersion periods from a maximum of 83.3% at 12 h. With air-conditioning, survival in this size class fell below the maximum mean rate at ambient only after 21 h of exposure. Survival at ambient temperatures among 20-50 mm clams was erratic,
366
falling below 50% after 21 h. At 24-28”C, in most replicates exposed up to 21 h.
survival remained above 90%
Size effects
Clams below 20 mm appear to withstand refrigeration better than larger ones, but nonetheless, survivorship at 11°C was very low. At 24°C and above, clams over 20 mm in length exhibited better survival rates than smaller ones at exposures between 15 and 21 h at 24-28”C, and between 12 and 21 h at ambient temperatures. Variability among replicates was greater at ambient temperatures with exposures over 15 h. DISCUSSION
Wide variations among replicate treatments underscore the need for further tests conducted simultaneously with larger sample sizes. The present data were obtained over a period of 8 days, during which diurnal temperature ranges were not adequately monitored. Differences in these and in the duration of the recovery period after byssus removal probably account for the erratic survival rates relative to emmersion time observed in clams stored at ambient temperatures. It is clear, however, that byssus removal reduces the maximum emmersion period for 100% survival of 10-20 mm clams from at least 24 h (Heslinga et al., 1984) to less than 12 h. The byssus, which consists of a matrix of byssal threads and a thick jelly-like secretion, plugs the byssal orifice when the valves are shut, trapping water within the mantle cavity. Removal of seed clams from the settlement substrate frequently results in complete detachment of the byssus. Clams were not examined to determine damage, if any, to the foot and byssal gland. Clipping byssal threads to separate adherent material without removing the tissue at the level of the byssal orifice may improve survival during protracted emmersion. On the other hand, clipping would require more time and labor in the preparation of large quantities of seed than simply pulling off foreign material. Where emmersion is not expected to extend over 20 h, the latter operation would probably give satisfactory survival rates if large enough clams are maintained at sufficiently low temperatures without refrigeration. ACKNOWLEDGEMENTS
The study was conducted at the MMDC, Palau, during training of the first author under the Giant Clam Project of the Pacific Fisheries Development Foundation, with a travel grant from the United States Agency for International Development and the Philippine Council for Agriculture and Resources Research and Development.
367 REFERENCES Heslinga, G.A. and Perron, F.E., 1983. Palau giant clam hatchery. ICLARM Newsletter, 6(l): 5. Heslinga, G.A. and Watson, T.C. (in press). Recent advances in giant clam mariculture. Proc. Fifth International Coral Reef Congress, Tahiti, 1985, vol. 2. Heslinga, G.A., Perron, F.E. and Orak, O., 1984. Mass culture of giant clams (F. Tridacnidae) in Palau. Aquaculture, 39: 197-215. IUCN, 1983. The IUCN Invertebrate Red Data Book. IUCN, Gland, Switzerland, 632 pp.
363
Short Communication EFFECT OF DESICCATION ON TRIDACNA DERASA SEED: IMPLICATIONS FOR LONG DISTANCE TRANSPORT M.D.G. LOPEZ’ and G.A. HESLINGA’
’ Marine Science Institute, University of the Philippines, Diliman, Quezon City 3004 (The Philippines) 2Micronesian Mariculture Demonstration Center, Koror, Republic of Palau 96940 (Caroline Islands) (Accepted
20 August 1985)
ABSTRACT Lopez, M.D.G. and Heslinga, G.A., 1985. Effect of desiccation on Tridacna derasa seed: implication6 for long distance transport. Aquaculture, 49: 363-367. Saving6 in freight cost from removal of the by66us and adherent material from T. derasa seed may not offset man-hour6 required for pre-shipment preparation, and decreased seed viability.
INTRODUCTION
Tridacna gigas and T. derasu, the two larger members of the molluscan family Tridacnidae, have been classified by the IUCN (1983) as vulnerable species which may become endangered if present levels of exploitation continue. At the Micronesian Mariculture Demonstration Center in Palau, both species have been raised to maturity and seed clams are being produced to restock depauperate reefs (Heslinga and Perron, 1983). Shipments of seed clams ranging in size from 10 to 100 mm have been made to Guam, Yap, Hawaii, Pohnpei, California, Fiji, the Philippines and the Marshall Islands (Heslinga et al., 1984; Heslinga and Watson, in press). In preparation for shipment, these clams were wrapped in moist nylon netting and packed in plastic bottles or Styrofoam coolers. Byssal threads and any attached gravel substrate were not removed from the clams. For shipping large quantities of seed clams by air, it is obviously important to minimize weight. Because the gravel normally adhering to the tridacnid byssus adds significantly to shipping weight, we conducted a series of trials to determine how removal of the entire byssal apparatus would influence the survival of the seed clams under simulated shipping conditions. The results of these preliminary observations indicate that while removal of the byssal apparatus does decrease shipping weight, potential savings in freight cost would probably not be sufficient to offset 0044-8486/85/$03.30
0 1985 Ekvier
Science Publishers B.V.
1
Reimmersion time (h)
21
15
18
41
6
4
4
1
23
Emmersion time (h)
10
12
12
12
15
18
21
24
24
10-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50 lo-19 20-29 30-50 lo-19 20-29 30-50 10-19 20-29 30-50 10-19 20-29 30-50
(mm)
Size class
1 2,18 I, 16 13 7,14 2, 16 10 4, 12 0, 15 5 75 57.5 65 52.5 45 50 40 37.5 25
Survivors
Survivors 19 20 20 20, 20, 19 20, 18, 20 20.20 19; 20 20,14 20 18, 20 20, 14 20 20,20 20, 20 20,19 18,20 19, 20 20, 20 17 20 20 14 12 4 14 14, 8 14
24-28°C %
regime
11°C
Temperature
97.5 85 100 95 -5 100 100 100 97.5 95 97.5 100 85 100 100 70 60 20 70 55 70
100
98.3 96.7
100
95 100
%
14 16 17 16, 16, 18 14, 17,19 16, 20 15,15 14,15 14 13,14 11,14 13 14,19 16, 20 19,18 10, 9 12, 8 16, 9 16 17 20 8 8 8 9 8 5
Survivors
33°C
70 80 85 83.3 83.3 90 75 72.5 70 67.5 62.5 65 82.5 90 92.5 47.5 50 62.5 80 85 100 40 40 40 45 40 25
%
Number of survivors per replicate (n = 20) and mean survival rate (%) per size class after variousemmersion periods at three temperature ranges
TABLE :
365
(1) a decrease in seed viability over long distances; and (2) an increase in time and labor required for pre-shipment preparation. Our preliminary observations on temperature effects indicate that artificial cooling of clam seed reduces viability, as does warming beyond an optimal range of approximately 24-28°C. In sum, presently available data suggest that giant clam seed survive best when shipped moist but not submerged, with the byssus intact, within a temperature range similar to that found in their natural habitat. METHOD
Five-month-old T. derasa in three size classes (10-19 mm, 20-29 mm, and 30-50 mm) from which adherent basalt chips had been pulled off were kept for at least 12 h in seawater tanks before use. Twenty clams in each size class were used per treatment, with replicates where the number of available clams allowed. The seed were packed as described above and stored for periods of 10, 12, 15, 18, 21 and 24 h in a refrigerator (11” C), an air-conditioned room (24-28” C), and at ambient temperatures (up to 33°C). After exposure, the clams were unpacked and placed in gravel-lined trays set in raceways. Survivors were counted after a minimum reimmersion period of 1 h. Subsequent mortalities were followed up to 41 h after reimmersion in clams subjected to 12 h emmersion. RESULTS
The number of survivors from all replicates and mean percentage survival for all treatments are shown in Table 1. Clams from which the byssus had been removed during sorting and immediately returned to the holding tanks exhibited no mortalities during the test period. Temperature
effects
Treatments at 11°C were discontinued after a 12-h test run resulted in survival rates below 50% for all size classes. For emmersion periods up to 24 h, mean survival rate was consistently higher at 24-28°C than at ambient temperatures for all size classes, except for the 21-h exposure of 30-50 mm clams, among which survival was 100% at both temperature ranges. Variation among replicates was considerable but overlaps in survivorship values between temperature ranges were small. In the 10-19 mm class, mean survival rate at ambient temperatures declined with increasing emmersion periods from a maximum of 83.3% at 12 h. With air-conditioning, survival in this size class fell below the maximum mean rate at ambient only after 21 h of exposure. Survival at ambient temperatures among 20-50 mm clams was erratic,
366
falling below 50% after 21 h. At 24-28”C, in most replicates exposed up to 21 h.
survival remained above 90%
Size effects
Clams below 20 mm appear to withstand refrigeration better than larger ones, but nonetheless, survivorship at 11°C was very low. At 24°C and above, clams over 20 mm in length exhibited better survival rates than smaller ones at exposures between 15 and 21 h at 24-28”C, and between 12 and 21 h at ambient temperatures. Variability among replicates was greater at ambient temperatures with exposures over 15 h. DISCUSSION
Wide variations among replicate treatments underscore the need for further tests conducted simultaneously with larger sample sizes. The present data were obtained over a period of 8 days, during which diurnal temperature ranges were not adequately monitored. Differences in these and in the duration of the recovery period after byssus removal probably account for the erratic survival rates relative to emmersion time observed in clams stored at ambient temperatures. It is clear, however, that byssus removal reduces the maximum emmersion period for 100% survival of 10-20 mm clams from at least 24 h (Heslinga et al., 1984) to less than 12 h. The byssus, which consists of a matrix of byssal threads and a thick jelly-like secretion, plugs the byssal orifice when the valves are shut, trapping water within the mantle cavity. Removal of seed clams from the settlement substrate frequently results in complete detachment of the byssus. Clams were not examined to determine damage, if any, to the foot and byssal gland. Clipping byssal threads to separate adherent material without removing the tissue at the level of the byssal orifice may improve survival during protracted emmersion. On the other hand, clipping would require more time and labor in the preparation of large quantities of seed than simply pulling off foreign material. Where emmersion is not expected to extend over 20 h, the latter operation would probably give satisfactory survival rates if large enough clams are maintained at sufficiently low temperatures without refrigeration. ACKNOWLEDGEMENTS
The study was conducted at the MMDC, Palau, during training of the first author under the Giant Clam Project of the Pacific Fisheries Development Foundation, with a travel grant from the United States Agency for International Development and the Philippine Council for Agriculture and Resources Research and Development.
367 REFERENCES Heslinga, G.A. and Perron, F.E., 1983. Palau giant clam hatchery. ICLARM Newsletter, 6(l): 5. Heslinga, G.A. and Watson, T.C. (in press). Recent advances in giant clam mariculture. Proc. Fifth International Coral Reef Congress, Tahiti, 1985, vol. 2. Heslinga, G.A., Perron, F.E. and Orak, O., 1984. Mass culture of giant clams (F. Tridacnidae) in Palau. Aquaculture, 39: 197-215. IUCN, 1983. The IUCN Invertebrate Red Data Book. IUCN, Gland, Switzerland, 632 pp.