Effects of biogenic concretions, epibionts, and endobionts on the relative growth of the clam Venus verrucosa in Bizerta Lagoon, Tunisia.
Article Type: Abstract
Subject: Clams (Observations)
Clams (Physiological aspects)
Authors: Menif, Najoua Trigui El-
Guezzi, Youssef
Lahbib, Youssef
Ramdani, Mohammed
Flower, Roger
Pub Date: 12/01/2008
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2008 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: Dec, 2008 Source Volume: 27 Source Issue: 5
Product: Product Code: 0913030 Clams NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish
Geographic: Geographic Scope: Tunisia Geographic Name: Bizerte, Tunisia Geographic Code: 6TUNI Tunisia
Accession Number: 191646293
Full Text: ABSTRACT Monthly monitoring of the clam Venus verrucosa was carried out in the channel of Bizerta Lagoon (Tunisia) from September 2002 to August 2003. Approximately 100 individuals were collected on each occasion from between 7- and 9-m depth. Individual shell sizes (measured as the antero-posterior shell length) varied from 13.7-59 mm. Some specimens showed shell abnormalities that indicated a heterospecific association. The proportion of specimens affected by these abnormalities was high (38%) and a comparison of the relative growth of healthy and abnormal clams was made. The relative growth of shell thickness and weight parameters, were compared with the antero-posterior valve length measurements. We calculated regression equations for annual and seasonal changes in valve length with shell thickness (E/L) and with shell weight (W/L). Annual and seasonal assessments of relative growth (as compared with the antero-posterior valve length) in the two clam populations showed that growth was negatively affected in the abnormal population. Affected individuals contained more intravalve water than normal individuals. The different growth metrics all indicated that affected clams were less well developed than unaffected clams. The organisms living in association with Venus verrucosa have a negative impact on shell thickness, on shell dry weight and on biomass (as dry flesh). This biometric growth study indicates that hetero-specific associations stressed the studied affected clam population and retarded somatic growth. It is suggested that clam emergence, probably caused by adverse conditions, encourages the formation of unfavorable heterotrophic associations by invertebrate species.

KEY WORDS: Venus verrucosa, hetero-specific association, relative growth, Bizerta Lagoon, Tunisia

INTRODUCTION

The clam Venus verrucosa is highly appreciated by European consumers. The mollusc lives on the East Atlantic coasts, from Norway to South Africa as well as in the Mediterranean Sea (Poppe & Goto 1993). On the Tunisian coasts, the species is common in the Gulf of Tunis and in the Gulf of Gabes. However, because of variations in the flesh taste of this species, it is collected from the Khneis Lagoon (in Northern Tunisia), from along the Eastern Tunisian coast and mainly from the Bizerta Channel. In the Bizerta Channel, we observed fishermen using scuba diving equipment to collect these clams at a rate of 3-4 kg/hour.

Most studies on V. verrucosa are related to ecology, reproduction, and parasitological aspects. The current state of ecological research regarding this species around the Tunisian coasts was described by Trigui El-Menif et al. (2005). Reproduction was studied by Valli and Cester (1980), Marano et al. (1980, 1982), Galinou-Mitsoudi et al. (1997), and the fishery by Bello (1985, 1986) and Del Piero (1994). Parasitology was investigated by Bartoli (1978), who described the impact of the metacercari Gymnofallus fossarurn on Venerupis aurea. Significant infestation by this digenous trematode is associated with the bivalve assuming an inverted position in the sediment. Furthermore, Venerupis aurea is readily predated by Haematopus ostralegus (Bartoli 1978).

Some specimens of Venus verrucosa are observed on the sediment surface and their emergence is usually caused by unsuitable environmental conditions or the presence of disturbing foreign organisms (Trigui El-Menif et al. 2005). Paillard et al. (1989) showed, in the clam Ruditapes philippinarum, that infestation by the bacterium, Vibrio tapetis, provoked individuals to emerge. Such a phenomenon was also observed in Cerastoderma edule infested by the trematode Curtuteria australis (Thomas et al. 1998) where the presence of larval cysts in the foot of the clam disturbs burrowing in the sediment.

Short siphon length means that V. verrucosa is a shallow burrowing clam (Tebble 1976), it is normally found 15-20 mm below the sediment surface (muddy sand substrate, or shell debris and gravel). Because of its roughness and thickness, the clam shell makes an attractive habitat for some invertebrates (Trigui El-Menif et al. 2005). Unusual emergence of this species is associated with the fixation of various ecto and endobiont organisms, notably in the posterior-dorsal region of the shell (Trigui El-Menif et al. 2005). These invertebrates can have a negative impact on this mollusc (Trigui El-Menif et al. 2005). This study aimed to compare the relative growth of normal Venus verrucosa individuals with and without attached organisms (associated invertebrates).

MATERIAL AND METHODS

From September 2003 to August 2004, scuba divers at a depth of 7-9 m collected randomly about 100 V. verrucosa clams monthly. Clams ranged in size from 13.7-58.2 mm (anteroposterior length of shell) and were all sampled near the bridge in the channel connecting Bizerta Lagoon to the Mediterranean Sea (37[degrees]11'N and 9[degrees]51'E, see Figure 1). This is an area characterized by strong hydrodynamic conditions and high biodiversity.

In the laboratory, samples were examined for specimens with shell disturbances caused by abnormal invertebrate associations. The proportion of individuals affected by these abnormalities was assessed. Shell length (L) and thickness (E) were measured in mm and the flesh was then separated from the shell for dry weight determination (after drying the specimens at 60[degrees]C for 48 h).

[FIGURE 1 OMITTED]

The relative growth of bivalve thickness, dry shell weight, and dry flesh weight was assessed relative to the antero-posterior length dimension (L) of each specimen. Annual and seasonal regression equations for the shell thickness-length and the weight-length relationships were calculated. The rate of growth was determined using the relation between the reference shell length and other morphometric parameters.

The relative growth was obtained by applying an allometric equation: Y = a [x.sup.b] (a: condition index; b: allometric coefficient; Y: the dependent variable presenting linear or weight parameters; x: the independent variable presenting the reference parameter). When linear measurements were used, the value of b was adjusted to 1. When weight growth was used, the slope value was adjusted to 3. We determined the nature of the allometry by comparing the value of the slope with the theoretical value by means of the Student test at the 5% level of probability.

To compare the growth of stressed and healthy populations of clam, we used the tests of slope (tpe) and position (tpo) (Mayrat 1959).

RESULTS

About 62% of the specimens collected possessed healthy shells (Fig. 2 A) whereas 38% of individuals showed shell disturbances in the form of brown spots on the internal and posterior sides of the valves (Fig. 2 B). All the individuais possessing a brown spot had a heterospecific association in the posterior part of the shell. This association could be direct, as is the case for some demosponges that attach to one or on both valves (Fig. 2 C-D), or the association could be endobiontic, living inside the shell. In the latter case, we observed the presence of a breach or perforation in the shell (Fig. 2 E-F), made by the bivalve lithophagous species, Gastrochaena dubia (Fig. 2 G). The breach can be covered with a soft calcareous layer produced by G. dubia (Fig. 2 E).

This attached species communicates to the outside through a calcareous tube located near the exit of the V. verrucosa siphons (Fig. 2 E-F). In other specimens, we observed small perforations (1-2 mm in diameter) that were located mainly on the latero-dorsal edge of the shell (Fig. 2 H). When opened, the clam showed tiny blackish spots visible through the transparent mantle (Fig. 2I). The removal of the mantle revealed more perforations of different sizes on the internal side of the shell (Fig. 2 J). These perforations are connected to an intravalve network of galleries (Fig. 2 K) in which Sipunculoidea, 10-12 mm long, were often present (Fig. 2 K).

Associations can also be superficial because the roughness and nature of the shell allows the formation of resistant deposits of hard biogenic concretions (Fig. 2 L) to accumulate. These concretions can shelter invertebrates such as the Annelid (Fig. 2 M-N) and other shellfish (Fig. 2 O).

This study of the relative linear weight growth of the clams is based on the development of shell thickness and the total weights of the dry flesh and shell with regard to the antero-posterior length reference. The correlation coefficients show a high degree of agreement between all parameters (Table 1).

Relation Length--Thickness

For the annual relative growth, the values of b >1 indicate that the growth of shell thickness, relative to length in both studied populations (healthy and stressed) is significantly greater for the normal population (Student t-test, Table 1). Between healthy and stressed populations, the slope (tpe = 0.93) shows that the growth of the thickness is the same for all the individuals. The value of tpo = 8.07 and indicates a difference at the level of the position. Table 2 shows that for any value of length, the thickness of the shell of healthy specimens is greater. Between seasons, the allometry always remains positive. The Student test also shows a difference of relative growth between both populations in autumn (slope = 2.81) and in spring (slope = 2.08), generally in favor of the healthy population (Table 1-2). In winter and in summer, the relative growth is the same and the difference is recorded only at the level of the position (Table 1-2).

Relation Weight--Length

Length--Total Weight

Values of b (significantly greater than 3, Table l) reveal that the allometry is positive for both populations. The value of the slope (tpe) shows that the relative growth is the same and the difference is in favor of the stressed population (tpo = 5.47). For any size, the weight of affected clams is greater than that of the healthy individuals (Table 2).

By considering seasonal differences, we also note a positive allometry, except during the summer seasons, when the length growth becomes relatively more than the weight growth. A comparison of relative growth in both populations shows that it is the same during winter (tpe = 0.96) and spring (tpe = 0.39). The test of position slope values reveals a significant difference in favor of the healthy population (Table 1-2). In autumn and summer, the relative growth of weight was different in the two groups but was in favor of the affected population for the small shell sizes (Table 2).

[FIGURE 2 OMITTED]

Length--Dry Flesh Weight

The weight increase was significantly greater than that of size (Student test, Table 1). Among populations, and considering annual and seasonal variations (autumn, spring, and summer), flesh growth was greater in the normal population (Table 2).

In winter, we recorded growth difference between each population (Table 1). The values derived from the weight of the dry flesh (Table 2) show that the small-sized specimens of the healthy population grow better. Beyond the size of 40 mm, the growth was in favor of the stressed population (Table 2).

Length--Dry Shell Weight

Considering annual variation, the allometric coefficient values were significantly greater than 3. This demonstrates significant growth of the dry shell weight with regard to length. The slope test values (tpe) showed that the rate of growth was similar between the two populations (Table 1). The test of position (tpo = 6.62) test as well as the values calculated for the weight of the dry shell, for certain antero-posterior length values, show that the healthy population grows better (Table 1-2).

During the autumn, spring, and winter we recorded at the healthy and stressed specimens, a relative growth of the shell dry weight significantly higher than that of the antero-posterior length. In summer, the relative growth of the dry shell was maintained only by the healthy population.

Between populations, the slope test revealed that growth was not significantly different; the only difference was recorded by the position test and this was for the winter, spring, and summer periods (Table 1).

DISCUSSION

Despite the abundance of the clam Venus verrucosa on Tunisian coasts (especially between 3 m and 12 m depth) and its important economic value in the Mediterranean area generally, this study is the first to be made on its relative growth in Tunisia.

The sampled clams, all from the Bizerta channel, revealed the presence of epibionts, endobionts, and biogenic concretions affecting more than a third of the sampled population. The affected individuals showed a brown stain on the internal posterior end of one or both valves that varied in size according to the extent of the disturbance.

Examination of relative growth in healthy and affected populations highlighted the impact of the various disturbances observed on shells. Various linear and weight parameters with regard to the antero-posterior length (taken as the reference for size) indicated negative growth effects on the affected specimens. Relative annual growth, as indicated by the morphometric and weight parameters (as referenced to the antero-posterior shell length), showed positive allometry for shell thickness growth, the total weight, the dry flesh weight, and the dry shell weight. Comparison between unaffected and affected populations showed similar relative growth except for the weight growth of dry flesh.

Except the total fresh weight, the position test values showed that shell thickness, shell dry weight, and flesh dry weight were relatively greater in normal individuals compared with affected specimens. This indicates that the affected individuals contained more intravalve water than did normal specimens.

The seasonal changes in relative growth in both populations revealed that growth of the healthy population was greater than the stressed population. Development of the various parameters is generally the same except for the winter and summer seasons for dry flesh weight. For the stressed population, shell length growth increased slightly more during winter.

Comparing the relative growth of the Tunisian clam with those in the natural harbor of Brest and Granville (Berthou 1983, Djabali & Yahiaoui 1978) indicates that in Venus verrucosa on the European coasts, shell thickness growth is generally more than that in the Tunisian populations. For example, clams of 30 mm length sampled from the Point of Chateau and to Keraliou (the natural harbor of Brest), have greater shell thickness than similar specimens collected in the channel of Bizerta. The mean values were, respectively, 21.7 mm, 17.57 mm, and 16.16 mm. On the other hand, in the cove of Poulmic (also a natural harbor of Brest), the shell thickness growth was similar to that of the Bizerta population. Overall, the presence of benthic organisms living in association with the clam seems to have a negative impact on the growth of shell thickness, of dry flesh and of shell size during most seasons.

If the associated species are filter feeders, as is the case for Gastrochaena dubia (living within a shell breach and using siphons at the posterior end of the clam), then both bivalves share the same food. The same observation can be made for some invertebrates such annelids and crustaceans living in the host shell concretions (Trigui El-Menif et al. 2005). Consequently, because of the competition phenomenon, this biometric study shows that these epibiont invertebrates can act negatively on the clam, notably on flesh growth. It is also likely that shell protrusions (on the posterior side), either by attachment of demosponge or by other epibionts, could disturb the valve opening and impede burrowing, so limiting food capture by the mollusc.

We consider that emergence of Venus verrucosa, in response to unfavorable environmental conditions, is promoted by formation of biogenic concretions and infestation by some invertebrate species. Based on biometric comparisons, evidence presented here suggests that the heterospecific associations exert growth (leading to negative growth) and behavioral (leading to emergence) stresses in affected clams. However, other mechanisms for affecting growth rate and possibly increased emergence could include environmental conditions such as water pollution.

LITERATURE CITED

Bartoli, P. 1978. Modification de la croissance et du comportement de Venerupis aurea parasite par Gymnophallus fossarum P. bartoli, 1965 (Trematoda, Digenea). Haliotis 7:23-28.

Bello, G. 1985. Analisi biometrica di Venus verrucosa L. (Bivalvia: Veneridae) dell'Adriatico meridionale, e considerazioni sulla sua pesca. Quad. Ist. Ric. Pesca Maritt., Ancona 4:173-182.

Bello, G. 1986. Porcentajes teoricos de retencion de Venus verrucosa L. (Bivalvia: Veneridae) con cedazos de varillas. Invest. Pesq. 50:167-177.

Berthou, P. 1983. Contribution a l'etude du stock de praire Venus verrucosa du golfe Normano-Breton. These de doctorat en biologie. Centre oceanologique de Bretagne. Universite De Bretagne occidentale. 153 pp.

Del Piero, D. 1994. The clam fishery in the Gulf of Trieste. In: Proceedings of the Conference, Coastal Zone'94 on Cooperation in the Coastal Zone. 4. pp. 1645-1660.

Djabali, F. & M. Yahiaoui. 1978. La praire (Venus verrucosa L.) en rade de Brest et en baie de Granville: Biologie, production, exploitation. These doctoral. University of Bretagne Occidental. 211 pp.

Galinou-Mitsoudi, S., A. I. Sinis & D. Petridis. 1997. Reproduction of Venus verrucosa in the Thessaloniki and Thermaikos Gulfs. In: Proceedings of the Fifth Hellenic Symposium on Oceanography and Fishery, Vol. 2. Kavala, Greece, pp. 107-110 (in Greek).

Marano, G., N. Casavola & C. Saracino. 1980. Indagine comparativa sulla riproduzione di Chamelea gallina (L.), Venus verrucosa (L.), Rudicardium tuberculatum (L.) nel basso Adriatico. Mem. Biol. Mar. Oceanogr. 10 (Suppl. 6):229-234.

Marano, G., N. Casavola, C. Saracino & E. Rizzi. 1982. Riproduzione e crescita di Chamelea gallina (L.) e Venus verrucosa (L.) (Bivalvia: Veneridae) nel basso Adriatico. Mem. Biol. Mar. Oceanogr. 12:97-114.

Mayrat, A. 1959. Nouvelle methodes pour l'etude comparee d'une croissance relative dans deux echantillons. Application a la carapace de Penaeus kerathurus. Bulletin de l'I.F.A.N., 21, ser. A, 1.9 pp.

Paillard, C., L. Percelay, M. le Pennec & D. le Picard. 1989. Origine pathogene de "l'anneau brun" chez Tapes philippinarum (Mollusque bivalve). Comptes Rendus de l'Academie des Sciences, Paris 309:235-241.

Poppe, G. T. & Y. Goto. 1993. European Seashells, Vol. 2. In: C. Hemmen, editor. Scaphopoda, Bivalvia, Cephalopoda. Wiesbaden. 221 pp.

Tebble, N. 1976. British Bivalve Seashells. Trustees of the British Museum Natural History, 2nd ed. Edinburgh: Her Majesty Stationery Office. 212 pp.

Thomas, F., F. Renaud, T. de Meeus & R. Poulin. 1998. Manipulation of host behaviour by parasites: Ecosystem engineering in the intertidal zone. Proc. R. Soc. Lond. B. Biol. Sci. 265:1091-1096.

Trigui El-Menif, N., Y. Guezzi, M. le Pennec, M. Boumaiza & G. le Pennec. 2005. Infestation of the clam Venus verrucosa by Sipunculoidea and the lithophagous bivalve, Gastrochaena dubia. Acta. Adriatica 46:83-90.

Valli, G. & P. Cester. 1980. Riproduzione e biometria di Venus verrucosa L. Nota preliminare. Nova Thalassia 4:193-194.

NAJOUA TRIGUI EL-MENIF, (1) * YOUSSEF GUEZZI, (1) YOUSSEF LAHBIB, (1) MOHAMMED RAMDANI (2) AND ROGER FLOWER (3)

(1) Faculty of Sciences Bizerte, Biology, Laboratoire de Biosurveillance de l'Environnement, Tunisia; (2) Institut Scientifique, OCEMAR, Av. Ibn Batouta BP 703 Rabat Agdal, Morocco; (3) Geography, ECRC, University College of London, 26 Bedford Way, London, United Kingdom

* Corresponding author. E-mail: Najoua.TriguiElMenif@fsb.rnu.tn
TABLE 1.
Values of equation parameters relating shell thickness (E), the
total weight (Wt), the dry flesh (Wd-f) and the weight of dry
shell (Wd-s) with shell length (L) in the two populations
(healthy and affected). Where, statistical a: condition index,
b: allometric coefficient, r: coefficient of correlation,
t: test of Student, tpe: test of slope, tpo: test of position,
(+ -): significance with the threshold of 5%.

                                    E/L Relationship

                    Healthy                       Affected
                    Population                    Population

Annual   Equation   E = 0.27645                   E = 0.257
                    [L.sup.1.1972]                [L.sup.1.2111]
         r           0.9765                        0.928
         t          33.57                         15.30
         tpe                          0.93 (-)
         tpo                          8.07 (+)
Autumn   Equation   E = 0.2729                    E = 0.2004
                    [L.sup.1.2058]                [L.sup.1.2846]
         r           0.9737                        0.9363
         t          15.89                         11.46
         tpe                          2.81 (+)
         tpo                             --
Winter   Equation   E = 0.26665                   E = 0.28754
                    [L.sup.1.2129]                [L.sup.1.1876]
         r           0.9879                        0.9691
         t          19.83                          9.61
         tpe                          1.135 (-)
         tpo                          2.62 (+)
Spring   Equation   E = 0.2268                    E = 0.2679
                    [L.sup.1.2529]                [L.sup.1.1995]
         r           0.9859                        0.9493
         t          28.21                          8.30
         tpe                          2.08 (+)
         tpo                             --
Summer   Equation   E = 0.3605                    E = 0.3126
                    [L.sup.1.1143]                [L.sup.1.1433]
         r           0.9657                        0.9164
         t          10.19                          5.20
         tpe                          0.97 (-)
         tpo                          2.92 (+)

                                   Wt/L Relationship

                    Healthy                       Affected
                    Population                    Population

Annual   Equation   Wt =                          Wt =
                    1.[10.sup.-4]                 1.[10.sup.-4]
                    [L.sup.3.2288]                [L.sup.3.2662]
         r           0.9763                        0.9516
         t          19.03                          8.76
         tpe                          1.14 (-)
         tpo                          5.47 (+)
Autumn   Equation   Wt =                          Wt =
                    2.[10.sup.-5]                 3.[10.sup.-5]
                    [L.sup.3.8003]                [L.sup.3.6568]
         r           0.9807                        0.9676
         t          27.95                         13.56
         tpe                          2.54(+)
         tpo                             --
Winter   Equation   Wt =                          Wt =
                    9.[10.sup.-5]                 1.[10.sup.-4]
                    [L.sup.3.3567]                [L.sup.3.2796]
         r           0.9878                        0.9746
         t          13.49                          5.47
         tpe                          0.96 (-)
         tpo                          6.65 (+)
Spring   Equation   Wt =                          Wt =
                    9.[10.sup.-5]                 8.[10.sup.-5]
                    [L.sup.3.3693]                [L.sup.3.3884]
         r           0.9894                        0.9770
         t          26.27                          8.41
         tpe                          0.39 (-)
         tpo                          4.40 (+)
Summer   Equation   Wt =                          Wt =
                    4.[10.sup.-4]                 8.[10.sup.-4]
                    [L.sup.2.2199]                [L.sup.2.7298]
         r           0.9632                        0.9192
         t           3.28                          4.25
         tpe                          2.79 (+)
         tpo                             --

                                  Wd-f /L Relationship

                    Healthy                       Affected
                    Population                    Population

Annual   Equation   Wd to f =                     Wd to f =
                    5.[10.sup.-6]                 4.[10.sup.-6]
                    [L.sup.3.2091]                [L.sup.3.2272]
         r            0.9583                        0.9177
         t          348.78                        174.44
         tpe                         12.68 (+)
         tpo                             --
Autumn   Equation   Wd-f =                        Wd-f =
                    [10.sup.-6]                   [10.sup.-6]
                    [L.sup.3.5494]                [L.sup.3.5134]
         r            0.9647                        0.957
         t          485.53                        321.10
         tpe                         18.28 (+)
         tpo                             --
Winter   Equation   Wd-f =                        Wd-f =
                    3.[10.sup.-6]                 2.[10.sup.-6]
                    [L.sup.3.3241]                [L.sup.3.4402]
         r            0.9253                        0.9529
         t          294.01                        186.08
         tpe                         44.44 (+)
         tpo                             --
Spring   Equation   Wd-f =                        Wd-f =
                    1.[10.sup.-6]                 2.[10.sup.-6]
                    [L.sup.3.648]                 [L.sup.3.437]
         r            0.981                         0.9805
         t          938.2                         190.15
         tpe                         44.27 (+)
         tpo                             --
Summer   Equation   Wd-f =                        Wd-f =
                    2.[10.sup.-5]                 2.[10.sup.-5]
                    [L.sup.2.9348]                [L.sup.2.7636]
         r           0.9529                         0.91
         t          55.62                         113.54
         tpe                         71.63 (+)
         tpo                             --

                                   Wd-s/L Relationship

                    Healthy                       Affected
                    Population                    Population

Annual   Equation   Wd to s =                     Wd to s =
                    7.[10.sup.-5]                 7.[10.sup.-5]
                    [L.sup.3.2972]                [L.sup.3.2699]
         r           0.9733                        0.9326
         t          35.87                         11.44
         tpe                          1.09 (-)
         tpo                          6.62 (+)
Autumn   Equation   Wd-s =                        Wd-s =
                    5.[10.sup.-5]                 7.[10.sup.-5]
                    [L.sup.3.4063]                [L.sup.3.3162]
         r          0.9841                        0.9463
         t          25.21                         7.36
         tpe                          1.96 (-)
         tpo                          1.72 (-)
Winter   Equation   Wd-s =                        Wd-s =
                    6.[10.sup.-5]                 4.[10.sup.-5]
                    [L.sup.3.3636]                [L.sup.3.54466]
         r           0.9832                        0.9613
         t          17.84                         10.47
         tpe                          1.75 (-)
         tpo                          6.55 (+)
Spring   Equation   Wd-s =                        Wd-s =
                    4.[10.sup.-5]                 5.[10.sup.-5]
                    [L.sup.3.4634]                [L.sup.3.3919]
         r           0.981                         0.9692
         t          47.01                         10.88
         tpe                          1.89 (-)
         tpo                          2.43 (+)
Summer   Equation   Wd-s =                        Wd-s =
                    2.[10.sup.-4]                 1.[10.sup.-4]
                    [L.sup.3.0346]                [L.sup.3.043]
         r           0.9699                        0.9282
         t           2.12                          1.16
         tpe                          0.21 (-)
         tpo                          7.08 (+)

TABLE 2.
Standard values of shell thickness (E), the total weight (Wt),
the dry flesh (Wd-f) and dry shell weight (Wd-s) of Venus
verrucosa starting from 10 mm shell length (Lmm). H: Healthy
Population, A: Affected Population.

Length
(mm)     Parameters       L10          L20         L30

Annual   E (H)           4.35         9.98        16.21
         E (A)           4.17         9.67        15.81
         Wt (H)          0.17         1.59         5.88
         Wt (A)          0.18         1.77         6.67
         Wd to f (H)     0.0081       0.075        0.27
         Wd to f (A)     0.0067       0.063        0.23
         Wd to s (H)     0.139        1.36         5.19
         Wd to s (A)     0.13         1.25         4.73
Autumn   E (H)           4.38        10.11        16.48
         E (A)           3.85         9.40        15.82
         Wt (H)          0.126        1.76         9.08
         Wt (A)          0.136        1.71         7.56
         Wd to f (H)     0.0035       0.0414       0.175
         Wd to f (A)     0.0033       0.037        0.155
         Wd to s (H)     0.127        1.35         5.37
         Wd to s (A)     0.144        1.44         5.45
Winter   E (H)           4.35        10.09        16.5
         E (A)           4.42        10.08        16.32
         Wt (H)          0.199        2.03         7.90
         Wt (A)          0.19         1.84         6.98
         Wd-f (H)        0.0063       0.063        0.249
         Wd-f (A)        0.0055       0.0598       0.24
         Wd-s (H)        0.138        1.42         5.57
         Wd-s (A)        0.112        1.22         4.93
Spring   E (H)           4.06         9.67        16.08
         E (A)           4.24         9.73        15.84
         Wt (H)          0.21         2.17         8.53
         Wt (A)          0.19         2.04         8.09
         Wd to f (H)     0.0047       0.060        0.245
         Wd to f (A)     0.0054       0.059        0.238
         Wd to s (H)     0.116        1.28         5.22
         Wd to s (A)     0.123        1.29         5.12
Summer   E (H)           4.69        10.15        15.95
         E (A)           4.34         9.60        15.26
         Wt (H)          0.33         2.51         8.22
         Wt (A)          0.42         2.84         8.62
         Wd to f (H)     0.017        0.132        0.43
         Wd to f (A)     0.012        0.079        0.24
         Wd to s (H)     0.21         1.77         6.07
         Wd to s (A)     0.11         0.92         3.18

Length
(mm)     Parameters       L40         L50         L60

Annual   E (H)           22.88       29.84        37.18
         E (A)           22.39       29.34        36.59
         Wt (H)          14.88       30.59        55.12
         Wt (A)          17.08       35.41        64.23
         Wd to f (H)      0.69        1.41         2.54
         Wd to f (A)      0.59        1.21         2.19
         Wd to s (H)     13.40       27.98        51.05
         Wd to s (A)     12.12       25.15        45.65
Autumn   E (H)           23.32       30.52        38.02
         E (A)           22.9        30.50        38.55
         Wt (H)          24.5        57.23       114.4
         Wt (A)          21.65       48.96        95.38
         Wd to f (H)     0.485        1.07         2.05
         Wd to f (A)     0.425        0.93         1.76
         Wd to s (H)     14.32       30.63        57
         Wd to s (A)     14.38       30.14        55.18
Winter   E (H)           23.39       30.66        38.25
         E (A)           22.97       29.94        37.18
         Wt (H)          20.69       43.67        80.38
         Wt (A)          17.95       37.32        67.86
         Wd-f (H)         0.63        1.33         2.44
         Wd-f (A)         0.649       1.39         2.62
         Wd-s (H)        14.68       31.10        57.43
         Wd-s (A)        13.29       28.68        53.78
Spring   E (H)           23.06       30.49        38.32
         E (A)           22.36       29.23        36.38
         Wt (H)          22.49       47.70        88.18
         Wt (A)          21.45       45.69        84.75
         Wd to f (H)      0.774       1.75         3.43
         Wd to f (A)      0.642       1.38         2.58
         Wd to s (H)     14.14       30.63        57.61
         Wd to s (A)     13.58       28.95        53.74
Summer   E (H)           21.98       28.18        34.54
         E (A)           21.21       27.37        33.72
         Wt (H)          19.05       36.54        62.24
         Wt (A)          18.89       34.74        57.16
         Wd to f (H)      1.006       1.94         3.3
         Wd to f (A)      0.53        0.99         1.64
         Wd to s (H)     14.54       28.62        49.77
         Wd to s (A)      7.65       15.09        26.32
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