Length - biomass and energy relationships of terrestrial gastropods in northern forest ecosystems

J.W. Hawkins, MmWm Lankester, R.A. Lautenschlager, and FmWm Bell

Abstract: Length-biomass models are a convenient and time-efficient method of estimating the biomass of invertebrates. Our purpose was to develop such a model for terrestrial gastropods that adequately predicted gastropod dry tissue biomass (Y) on the basis of animal length (X). The power equation Y = 0. 172X1 hX8 (r2 = 0.85) proved to be the best model for this purpose. Gastropod dry tissue biomass was 6.52 + 1.58 mg (mean + SE) and, based on gastropod densities ranging frok 2 to 38/m2 reported in the literature, snails and slugs active on the surface of the forest floor accounted for 2.5 and 6 % of the total animal biomass and energy, respectively, of boreal forest ecosystems. However, because densities of gastropods in both the litter and underlying soil can reach 1607/m2, our results suggest that published values for total animal biomass (4.9 g/m2) and caloric energy (lo4 cal/m2) in boreal forest ecosystems are underestimated.

R6sum6 : Les modttles longueur-biomasse constituent une methode commode et expeditive d'estimation de la biomasse chez les invertebres. Nous avons cherche a elaborer un tel modttle pour predire la biomasse sttche des tissus (Y) a partir de la longueur des animaux (X) chez les gastropodes terrestres. L'equation exponentielle Y = 0 , 172X',h8n (r2 = 0,85) est le meilleur modttle dans ce cas. La biomasse moyenne de tissu sechk de gastropode (moyenne ecart type) a ete estimee a 6,52 + 1,58 mg et, d'aprtts les densites rapportees dans la litterature, soit 2-38 gastropodes/m2, les escargots et limaces actifs a la surface du sol forestier composent 2.5% de la biomasse animale totale et 6 % de l'energie animale totale dans les ecosystttmes de fortt boreale. Cependant, comme les densites de gastropodes dans la litittre et le sol sous-jacent peuvent atteindre 1607/m2, nos risultats laissent croire que les valeurs deja publiees sur la biomasse animale totale (4,9 g/m2) et I'energie calorifique (lo4 cal/m2) dans les ecosystttmes forestiers boreaux sont des sous-estimations des valeurs reelles. [Traduit par la Redaction]

Introduction studies have estimated the density of terrestrial gastropods in

Terrestrial gastropods play an important role in the ecology of the forest floor. They provide food for a variety of soil arthropods (Newel1 197 1 ; Brandmay r and Brandmay r 1986; Digweed 1993), ground-foraging birds (South 1980), and small mammals (Hamilton 194 1 ; Whitaker 1966; Rudge 1968; Whitaker and Mumford 1972; Churchfield 1984), and act as intermediate hosts for a number of helminth parasites affecting vertebrate wildlife (Lankester and Anderson 1968; Gleich et al. 1977; Rowley et al. 1987; Raskevitz et al. 1991). Snails and slugs also play a role in litter decomposi- tion (Mason 1970) and nutrient cycling of forest ecosystems (Richter 1979).

To quantify the contribution of terrestrial gastropods to different ecological processes, it is necessary to estimate the biomass of snails and slugs present within an area. Numerous

different habitat types (Kearney and Gilbert 1978; ~ o a ~ and Wishart 1982; Kralka 1986; Hawkins et al. 1996~ ; Lankester and Peterson 1996), but few have attempted to quantify the biomass represented by this component of an ecosystem.

Length-biomass models provide an effective, time- efficient method of determining the biomass of invertebrates (Rogers et al. 1977). In addition, they allow biomass esti- mates to be achieved without destructive sampling (Rogers et al. 1976).

The purpose of this study was to develop a general length - biomass model for terrestrial gastropods on the basis of dry tissue mass. Gastropod density estimates and tissue caloric values from the literature were used to evaluate the available biomass and potential trophic importance of snails and slugs in boreal forest ecosystems.

Received July 1 1, 1996. Accepted October 9, 1996.

J.W. Hawkins and M.W. Lankester. Department of' Biology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada. R.A. Lautenschlager and F.W. Bell. Ontario Forest Research Institute, 1235 Queen Street East, P.O. Box 969, Sault Ste. Marie, ON P6A 5N5, Canada.

Materials and methods

Terrestrial gastropods were collected from a 70-year-old mixed- wood forest located in Fraleigh Township (49"08'N, 87"49'W), approxinlately 60 km southwest of Thunder Bay, Ontario. Speci- mens were refrigerated at approximately 4°C for 2 -4 days before being measured and weighed. The length, width, and wet and dry masses of 10 specimens were determined for each of seven gastropod

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Table 1. Shell and body measurements, total mass, and tissue wet and dry masses for seven species of terrestrial gastropod.

Length Width Total wet mass Tissue wet mass Tissue dry mass (mm) (mm) (mg) (mg) (mg)

Deroceras laeve 14.3 1 f 1.68 (6.88-22.86)

Zonitoides arboreus 3.39f 0.27 (1.9 -4.52)

Discus cronkhitei 4.04f 0.12 (3.54-4.71)

Euconulus fulvus 2.5 f 0.09 (1.85-2.95)

Vitrina limpida 3.29f 0.13 (2.4-3.79)

Succinea ovalis 10.31 k 1.28 (4.75 - 16.86)

Zoogenetes harpa 2.08 k0.18 '(1.12-2.88)

Total Snails only (n = 60) 4.2 f0.43

(1 .O1 - 16.86) Overall (n = 70) 5.64k0.61

(1 .Ol -22.86)

Note: Values are given as the mean + SE, with the range in parentheses. *Equal to total wet mass.

species including Deroceras laeve, Zonitoides arboreus, Discus cronkhitei, Euconulus fulvus, Vitrina limpida, Succinea ovalis, and Zoogenetes harpa. For each species, specimens encompassed a range of sizes typical of those found in boreal environments.

The length and width of the shell were measured, to the nearest 0.02 mm, with digital calipers (Mitutoyo Model CD-6"BS). Shell length was measured from the apex of the spire to the bottom of the outer lip in species with globose shells, and across the diameter of the shell, at its widest point, for species with depressed shells (Burch 1962). Shell width was measured as the dimension perpen- dicular to length in each case. For the slug D. laeve, body length and width were measured when the animal was moving and most elongated. Immediately after they were measured, the total live wet mass (tissue + shell) of individual gastropods was determined, to the nearest 0.0001 g, on a Mettler AE200 electronic balance. Gastropods were then placed on a previously weighed glass cover slip (22 x 40 mm, No. 1) and the tissue was carefully removed from the shell beneath a 1 6 ~ dissecting microscope. Tissue separa- tion was facilitated by breaking open the shell and using a dissecting probe and scalpel to free the tissue from the shell; shell fragments were placed on a separate preweighed glass cover slip. Fibre-optic lighting was used during dissections to reduce the heat to which specimens were exposed, thereby diminishing water loss from body tissues. Immediately after dissection, the tissue and shell were weighed separately. Both were then placed in a drying oven at 70°C for 5 days and their dry masses determined.

Using length, width, and dry tissue mass, linear and polynomial regression analyses (Neter et al. 1989) were used to construct a general length - biomass model for terrestrial gastropods. Four different models were tested. These included the simple linear model (eq. 1) using length as the independent variable, the multiple regression model (eq. 2) using length and width as independent variables, and the power model (eq. 3) and quadratic polynomial model (eq. 4), each using length as the independent variable.

where the regression coefficients, a and b , , in eq. 3 were esti- mated by applying a logarithmic transformation to produce the linear relationship In(Y) = In(a) + b, . In(X,)

In each of the four equations Y is dry tissue mass, XI is length, X2 is width, and a , b , , and b2 are empirically derived regression coefficients. The dependent variable (Y) was subject to logarithmic transformation (In) in the case of eqs. 1 , 2, and 4 in order to meet the assumptions of the linear model. The most appropriate model was then validated by randomly splitting the data and using the cross-validation procedure (Neter et al. 1989).

Results

The six species of terrestrial snail had a shell length of 4.20 f 0.43 mm (mean f SE), width of 2.60 f 0.24 mm, and wet mass (tissue + shell) of 45.84 f 14.71 mg, com- posed of 32.92 + 1 1.39 mg wet tissue mass and 7.90 f 2.47 mg wet shell mass (Table 1). Following drying at 70°C, tissue dry mass was 5.97 f 1.80 mg, indicating a tissue water content of 82%. The slug D. laeve had a length of 14.31 1.68 mm and a width of 2.14 f 0.19 mm when elongated (Table 1). Wet and dry tissue masses for D. laeve were 62.45 f 17.2 1 and 9.82 f 2.40 mg, respectively, indicating a tissue water content of 84%. Snails and slugs combined had an overall length of 5.64 f 0.61 mm, width of2.54 f 0.20 mm, and dry tissue mass of 6.52 f 1.58 mg. These data were used to calculate the four length-biomass models tested.

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Notes

Table 2. Coefficients and statistics for the regression models tested.

Model n a b , k SE b2 f SE* r2 F

[I] Y = a + b l X l 70 -0.479 0.226k0.018 na 0.70 156.7 [2] Y = a + b l X l + b2X2 70 -0.994 0.166f0.016 0.335f0.048 0.82 156.8 [3] Y = ax:] 70 0.172+ 1.688k0.088 na 0.85 370.2 [4] Y = a + b l X l + b2XI2 70 -1.418 0.565f0.054 -0.017+0.003 0.81 147.4 [V] Y = aXFl 38 0.206' 1.555 k0.113 na 0.84 190.4 [V] Y = a*] 32 0.127' 1.935k0.138 na 0.87 197.2

Note: "V" indicates a validation model based on an approximately 50% random split of the raw data. * "na" denotes "not applicable," since the model contains only one independent variable. +Corrected for bias following Sprugel (1983).

Each of the four length-biomass models revealed a sig- nificant relationship between length (length and width in eq. 2) and gastropod dry tissue biomass (P < 0.0005) (Table 2). However, the power model Y = 0. 172X1 688

(Fig. I), where Y is dry tissue mass and X is length, accounted for the greatest amount of variation in the data (r2 = 0.85, F1 ,681 = 370.17). and met the assumptions of the model most rigorously. The scaling coefficient of this model was corrected for bias, introduced by the logarithmic transformation (Baskerville 1972; LaBarbera 1989), using the procedure of Sprugel (1983). The results of the cross- validation procedure are presented in Table 2.

Discussion

Ecological studies generally use dry tissue mass as the stan- dard measure of animal biomass (Paine 197 1). Estimating the dry tissue mass of small terrestrial gastropods, however, is a time-consuming and tedious procedure. Much time can be saved by using an appropriate length -biomass equation.

The power equation proved to be the most adequate for describing the general length -biomass relationship for terrestrial gastropods on the basis of dry tissue mass. This model not only accounted for the greatest amount of varia- tion in the data (r2 = 0.85), but also met, with greater rigour, the model assumption of normally distributed, homo- scedastic residuals. Although the multiple (r2 = 0.82) and quadratic polynomial (r2 = 0.81) regression equations also accounted for similarly large amounts of variation, these models violated the assumption of homoscedastic residuals even after transformation of the dependent variable.

The power equation has been used to describe the majority of allometric relationships (Peters 1983; LaBarbera 1989) and, in most instances, it has been observed that animal mass is proportional to the cube of length (i.e., a scaling exponent of 3) (Peters 1983; Niklas 1994). This is generally attributed to volume increasing proportionally with the cube of linear size (Begon et al. 1990). The allometric power equation developed here has a scaling exponent equal to 1.7, possibly because of the " phyletic effect" (Niklas 1994). Taxonomic similarity between the species used in developing allometric models can cause coefficients to deviate from those of models involving a more taxonomically diverse range of species. Additionally, the scaling exponent reported herein is based on dry tissue mass, whereas those reported by Peters (1983) and Niklas (1994) were based on tissue fresh mass.

Fig. 1. Regression relationship between terrestrial gastropod dry tissue mass and length as described by the power equation Y =

0 . 1 7 2 ~ l . ~ ~ ~ (log transformed: In(Y) = 0.199 + 1.688 . In(X)).

Ln(length) (mm)

may be partially attributed to tissue water content. As well, Niklas (1994) used reduced major axis regressions to derive his scaling coefficients. This method consistently produces greater values for the scaling coefficient than ordinary least- squares regression (LaBarbera 1989).

Collins ( 1 992) described a linear model for predicting gastropod biomass on the basis of length, and accounted for 72% of the variation in his data. However, the model was constructed using the whole body mass (including the shell) of specimens that had been preserved in glycerin alcohol. The shell represents a dietary source of calcium to some vertebrates (Graveland 1996) but makes no energy contribu- tion in trophic relationships (Mason 1970). Many gastropod predators feed only on the fleshy tissue present within the shell (Drewes and Roth 198 1 ; Digweed 1993). As well, bio- logical preservatives have been demonstrated to reduce the mass of invertebrate specimens (Howmiller 1972). There-

Therefore, the observed difference in the scaling exponents fore, the power model-developed here, based on dry tissue

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mass, may provide more ecologically meaningful infor- mat ion.

Dry tissue mass for individual gastropods was 6.52 f 1.58 mg (mean f SE). Mean densities of snails and slugs, active on the surface of the boreal forest floor, range between 2 and 38/m2 (Kearney and Gilbert 1978; Hawkins et al. 1996a; Lankester and Peterson 1996). Considering a median density of 18/m2, gastropod biomass on the surface of the litter would equal 0.12 g/m2. Tissue caloric values for terrestrial snails and slugs range from 472 1 to 5971 callg dry mass (Golley 1961 ; Cummings and Wuycheck 197 1). Assuming a mean caloric value of 5000 cal/g dry tissue mass, and using the mean biomass estimated here (0.12 g/m2), terrestrial snails and slugs on the surface of the forest floor represent a potential contribution of approximately 600 cal/m2 of available energy.

Kimmins (1987) estimated the entire animal biomass of boreal forests at 4.9 g/m2, representing lo4 cal/m2 of energy. Values presented here for gastropods active on the forest floor account for 2.5 and 6% of these estimates, respec- tively. However, numbers of terrestrial gastropods are up to 50 times greater within the litter and underlying soil of the boreal forest than on the surface of the forest floor (Hawkins et al. 1996b). Mean gastropod densities were 80/m2 in the upper 5 cm of soil in the boreal forest of Alberta (Kralka 1986) and as high as 1607/m2 in the upper 10 cm of soil of a regenerating spruce plantation in northwestern Ontario (Hawkins et al. 1996b). Even at the lower end of this range of densities, snails and slugs would account for a very large proportion of the animal biomass and energy present within the boreal forest. Considering the large biomass represented by these and other soil invertebrates, including earthworms and nematodes (Petersen and Luxton 1982; Blair et al. 1994; Maxwell and Coleman 1995), Kimmins' (1 987) total animal biomass value, 4.9 g/m2, may be an underestimate.

Terrestrial gastropods clearly make a significant contri- bution to the animal biomass and energy in boreal forest ecosystems. Values presented here may provide greater insight into their role in ecological processes and their impor- tance in trophic relationships. The power model, based on length, provides a convenient method of estimating gastro- pod dry tissue biomass and facilitates a more critical assess- ment of the role of gastropod communities in terrestrial ecosystems.

Acknowledgments

We gratefully acknowledge funding provided for this work by the Vegetation Management Alternatives Program through the Agricultural Research Institute of Ontario under the Sustainable Forestry Initiative, Ontario Ministry of Natural Resources, Sault Ste. Marie. We also thank Karen Watt for assistance with the preparation of the manuscript.

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Glochidial metamorphosis of the freshwater mussel Lampsilis cardium (Bivalvia: Unionidae) on larval tiger salamanders, Ambystoma tigrinum ssp. (Amphibia: Ambystomidae)

G. Thomas Watters

Abstract: Larval tiger salamanders (Ambystoma tigrinum ssp.) were infected with glochidia of the freshwater mussel Lampsilis cardium in laboratory experiments. At 20-21 "C, metamorphosis occurred from 9 to 39 days, primarily between 9 and 17 days. The percentage of attached glochidia that metamorphosed varied from 0.27 to 15.7%. Metamorphosis on the salamanders occurred more quickly than on a known piscine host, largemouth bass (Micropterus salmoides), but a smaller percentage of the total attached glochidia metamorphosed. The role of amphibians as hosts of freshwater mussels in North America has not been addressed. Recognizing such a relationship could have important consequences for our understanding of mussel zoogeography.

RCsumC : Des larves de la salamandre Ambystomu tigrinum ssp. ont Cte infectees de glochidies de la moule d'eau douce Lampsilis cardium dans des experiences de laboratoire. A 20-21°C, la metamorphose s'est produite entre les jours 9 et 39, principalement entre les jours 9 et 17. La metamorphose des moules s'est produite plus tat chez la salamandre que chez un poisson hate, I'Achigan h grande bouche (Micropterus sulmoidc~.r), mais un pourcentage moins eleve des glochidies fixees sont parvenues h la metamorphose. Le r61e des amphibiens comme h6tes de la moule d'eau douce en Amerique du Nord n'a jamais CtC Ctudiee. L'aprofondissement de cette relation pourrait contribuer a augmenter notre comprehension de la zoogeographie des moules. [Traduit par la Redaction]

Introduction

G.T. Watters. Ohio Biological Survey and Aquatic Ecology With rare exceptions (Kondo 1990), freshwater mussels are Laboratory, Ohio State University, 13 15 Kinnear Road, obligate vertebrate parasites as larvae. Of the approximately Columbus, OH 43212, U.S.A. 300 species in North America, only one, the salamander (e-mail : gwatters@magnus.acs.ohio-state.edu) . mussel, Simpsonaias ambigua, was known to use a non-

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