Life-history characteristics of a stream population of the freshwater clam Sphaerium striatinum...

Life-history characteristics of a stream population of the freshwater clam Sphaerium sfriatinurn Lamarck (Bivalvia: ~isidiidae)'

DANIEL J. HORNBACH~ AND THOMAS E. WISSING Department of Zoology, Miami University, Oxford, OH, U.S.A. 45056

AND

ALBERT J. BURKY Department of Biology, University of Dayton, Dayton, OH, U.S.A ,45469

Received June 1 1, 198 1

HORNBACH, D. J., T. E. WISSING, and A. J. BURKY. 1982. Life-history characteristics of a stream population of the freshwater clam Sphaerium striatinum Lsunarck (Bivalvia: Pisidiidae). Can. J. Zool. 60: 249-260.

Life-history characteristics of a population of Spkaeriurn striatinurn in Little Four Mile Creek, OH, were studied from August 1977 to April 1980. Two generations of clams were produced each year, with major recruitment occurring in April - early July and August-October. Individuals born during either birth period lived for about 1 year. Minimum size at birth was 4.0 mm, and only adults with a shell length > 10.0 mm contributed to recruitment in the population. Examination of seasonal size distributions of embryos revealed an average embryonic development rate of 0.32 mm-week-' during April-October; the rate was lower during other times of the year. The annual selection ratio (average number of young born f&r adult) was 10.49:l (7.68:1, April-July; 3.8 1 : 1, August-October); embryonic mortality was 97.5%. These results are compwwith published values for a lake population of S. striatinum and indicate that the differences observed in life history characteristics for these populations apparentIy do not conform to expectations based on r- and K- selection theory. Instead the differences can best be explained by a "bet-hedging" theory dependent on stochastic rather than deterministic processes. Life-history characteristics of other pisidiid clams are compared with results from this study and discussed in relation to the adaptive plasticity of freshwater molluscs.

HORNBACH, D. J., T. E. WISING et A. J. BURKY. 1982. Life-history characteristics of a stream population of the freshwater clam Spkaeriurn striatinurn Lamarck (Bivalvia: Pisidiidae). Can. J. Zool. 60: 249-260.

Les caract65ristiques du cycle biologique ont kt65 ktudibes chez une population de Spkaeriurn striatinurn de Little Four Mile Creek, OH, d'aolft 1977 h avril1980. On compte deux gknkrations par annke et le recrutement atteint un sommet d'avril au debut de juillet et une autre d'aoilt tt octobre. Les individus vivent environ un an, quel que soit le moment de leur naissance. La taille minimale la naissana est de 4'0 mm et seuls les adultes dont la coquille mesure plus de 10,O mm contribuent au recrutement de la population. La 1-6 artition saisonni&re des embryons selon leur taille dv&le que le taux de d6veloppement embryomaire est de -P 0,32 mrn.semaine d'avril B octobre et qu'il est plus faible en d'autres temps de l'annke. Le rapport annuel de sklection (nombre moyen de jeunes nks par adulte) est de 10,49: 1 (7,68: 1 d'avril A juillet et 3,8 1 : 1 d9aoQt B octobre); la msrtalitk embryonnaire est de 97'5%. La cornparaison de ces dsultats avec les valeurs trouvkes dans la litt6ratm-e sur une population lacustre de S. striatinurn indique que les diffbrences entre ces populations quant aux caract6ristiques de leur cycle biologique ne semblent pas se conformer aux pdvisions blabodes B partir de la thborie des stratbgies dbmographiques r et K. Les diffkrences semblent mieux s'expliquer par une thbrie de "pari pour et contre" bade sur des mkcanismes stochslstiques plutdt que d6tenninistes. Les rksultats de cette etude sont comparbs aux caractkristiques du cycle biologique d'autres pisidiidks et la comparaison pennet de discuter de la plasticit65 de l'adaptation de ces mollusques dul~aquicoles.

[Traduit par le journal]

Introduction Pisidiid clams are cosmopolitan and ubiquitous in

their distribution and are often the dominant (in terms of numbers and (or) biomass) benthic organisms in certain freshwater ecosystems (Cam and Hiltunen 1965; Gale 1975; Avolizi 1976; Healey 1977; Eckblad et al. 1977). They can also serve as important prey items for fish (Eyerdam 1968; Jude 1968, 1973), waterfowl (Thomp-

'Based on a portion of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

'present address: Department of Biology, University of Virginia, Charlottesville, VA, U.S .A. 22901.

son 1973), and aquatic insects (Foote 1976). Pisidiids are filter feeders and can be classified functionally as collectors (Cummins 1974). Organisms with this mode of feeding often have a key role in the dynamics of nutrient and energy cycling in streams (Cummins 1974; Wallace et al. 1977). Despite this potential importance in aquatic environments, relatively little is known of the life histories of the pisidiid clams. Waters (1979) summarized a recent symposium on current and future needs in freshwater benthic invertebrate life histories by stating that ". . . there is a need for the development of a conceptual framework for benthic ecology within which life history may play the key role."

There are three major genera of pisidiid clams in

0088-4301 /82/020249- 12$01 .OOIO. 81982 National Research Council of CanadaIConseil national de recherches du Canada

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250 CAN. J. ZOOL. VOL. 60, 1982

TABLE 1 . Annual means of physical-chemical and biological characteristics of Little Four Mile Creek, OH, October 1978 - September 1979. (All values in meqIL unless specified;

N = number of dates on which determinations made)

Standard Mean Deviation Range N

pH Oxygen (mgIL)

Conductivity (~mhoslcm) ca2+

Filterable residue (mgIL)

Ash-free filterable residue (mgIL)

Nonfilterable residue (mgIL)

Nonfilterable residue as carbon (mg CIL)

Periphyton production (mg chlorophyll a.m-2.week-')

North America: Musculium, Pisidium, and Sphaerium. Species of Pisidium contain the smallest individuals, whereas those of Sphaerium include the largqst. Life- history studies of the three genera have been carried, out and include Muscuiium (Thomas 1963; Mitropolskij 1965; Mackie et al. 1976a, 1976b; Gale 1977; Mackie 1979; Hornbach, Way, and Burky 1980; Way et al. 1980); Pisidium (Heard 1965; Gillespie 1969; Ladle and Baron 1969; Thut 1969; Mitropolskij 1969, 1970; Meier-Brook 1970; Had1 1972; Danneel and Hinz 1976; Holopainen 1978; Mackie 1979; Burky et al. 1981); and Sphuerium (Monk 1 928; Foster 1 932; Mitropolski j 1966; Alimov 1967; Avolizi 1976; Heard 1977; Mackie 1979). Herrington (1962) noted that S. striatinum is the most common species of Sphaerium in North America, where it inhabits creeks, rivers, and large and small lakes. The studies of Monk (1 928), Foster (1932), and Avolizi (1 976) dealt with the life history of S. striatinum in standing water systems; only one stream population of this species has been studied (Heard 1977). In this study, the dynamics of the growth and reproduction of S. striatinum were studied in a small stream in south- western Ohio.

Study area Clams were collected from Little Four Mile Creek near

College Comer, OH, (Preble Co.; United States Geological Survey Map Quadrangle College Comer, OH; 39'43.8' N, 84'48.2' W; voucher specimens on deposit at the University of Michigan Museum of Zoology, voucher No. UMMZ 250047). This stream has a maximum width of approximately 10m and a maximum nonflooding depth of approximately

0.7 m. Current velocities of approximately 0.7 m-min-' were recorded near the water-sediment interface with a pygmy current meter (Gurley No. 625) at six sites on 10 dates in 1979. It is bordered throughout most of its length by deciduous trees and receives runoff from agricultural lands.

Various physical-chemical and biological characteristics were measured throughout the year. These included pH (Sar- gent-Welch model PBL pH meter), conductivity (Hach conductivity meter), nitrite and nitrate (Hach chemical test with a Bausch and t o m b Spectronic 20 spectrophotometer), hardness (EDTA titration), alkalinity (mixed bromcresol, green-methyl, red indicator method), oxygen (azide-modified Winkler method), filterable and nonfilterable residue (as dry weight; Whatrnan GFIB filters), periphyton production (as chlorophyll a; American Public Health Association 1975), and nonfilterable residue as organic carbon (Strickland and Parsons 1965; Russell-Hunter et al. 1968). The annual means, standard deviations, and ranges for these characteristics (for the period October 1978 - September 1979) are presented in Table 1.

In general, Little Four Mile Creek contains water which is slightly basic (pH > 7.0), well oxygenated (> 64% saturated), and very hard (high levels of c a 2 + and M~~~ ). The fairly high levels of nitrite and nitrate are probably indicative of runoff from agricultural lands, yet there is little organic pollution, as indicated by normal levels of filterable and nonfilterable residue. Periphyton production in the stream reaches maxi- mum levels in early spring and late fall. These are periods when water temperatures and light intensities are fairly high. Periphyton production decreases during the summer, when the canopy closes over the stream.

Aside from S. striatinum the benthic macroinvertebrate community of the stream includes midge larvae (Chironom- idae), oligochates (Lumbriculidae, Aelosomidae), water mites (Diplodontidae) , horsehair worms ( Gordius sp . ) , gastropods

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HORNBACH ET AL. 25 1

(Goniobasis sp.), insect larvae (Elmidae, Ceratopogonidae, An estimate of the annual selection ratio (mean number of Stratiomyiidae), and other bivalves (Pisidium sp., Anodonta young born per average adult) was made by dividing the annual SP.>. number of young born per square metre by the weighted annual

average density of adults (i.e., those capable of producing Materials and methods newborn individuals). An estimate of embryonic mortality was

Fifty samples of Sphaerium striatinum (usually > 100 individuals per sample; n = 9 13 1) were taken from Little Four Mile Creek during August 1977 - April 1980. Individuals were removed from the stream sediment with a sieve (1 .O-mm opening) and either fixed in the field in 12% neutral formalin or returned live to the laboratory and fixed within 4-5 h. Shell length (anterior-posterior dimension) was measured to the nearest 0.1 mm with a dial caliper. General patterns of growth in the population can be determined by seasonal changes in shell-length distributions over time, by examination of cumu- lative frequency distributions plotted on probability paper (Harding 1949; Cassie 1950, 1954), and with data on reproductive periods (see below).

To assess population density, six grabs were taken with a ponar grab (bite = 0.026 m2) on 30 dates from August 1977 to December 1978. Collection sites were chosen which appeared to be representative of the extremes of sediment type and depth characteristic of the stream. Consequently, density measures are representative of the entire stream rather than of just those areas where clams are normally found. The density estimates were used to calculate the degree of aggregation in the population by Taylor's Power Law (Taylor 196 1).

Adult clams that had been collected on specific dates in 1978 and 1979 were dissected to analyze the dynamics of reproduc- tion in the population. The methodology used in this analysis followed that discussed in Hornbach, Way, and Burky (1980). Pisidiid clams are ovoviviparous, that is, they brood their young in marsupial sacs formed as outgrowths of gill filaments (Mackie 1978). The embryos (all developmental stages found in adults) from 236 clams taken on 17 dates in 1978 were removed and measured to the nearest 0.1 mm under a dissection microscope with a stage micrometer. An additional 189 adults from 1978 were also dissected, but only embryos > 0.4 mm were counted and measured. Another 139 adults, from 16 collections made in 1979, were dissected (only embryos > 0.4 mm measured) to confirm the general pattern of reproduc- tive activity. For purposes of analysis, embryos were grouped into 10 size classes: 0.2-mm class = embryos 0.0-0.4 mm; 0.7-mm class = embryos 0.5-0.9 mm; 1.2-mm class = embryos 1.0-1.4mm; ... 4.2-mm class = embryos 4.0-4.4 mm; and 4.7-mm class = embryos 1 4.5 mm.

Developmental rates of embryos were estimated by examin- ing seasonal shifts in the size distribution of embryos in adults of similar size. Once the minimum birth size was established, birth rates for specific time periods were estimated. The average developmental rate (rnillimetres per week) was multi- plied by the number of weeks in the time interval under consideration, and the result (i.e., millimetres of growth in the time interval) was subtracted from the minimum birth size, giving the minimum size of embryos which could be born over the interval. The mean number of embryos equal to or larger than this minimum size per adult (capable of bearing embryos of this size) times the average number of adults per square metre over the interval yielded the number of young born per square metre.

derived from the difference in the density (based on the annual average of each size class per square metre) of 0.2-mm-class and 3.7-mm-class embryos. Embryos of the 0.2-mm class are in very early developmental stages, whereas those of the 3.7-mm class are just slightly less than minimum birth size. The difference between the annual number of young born per square metre and the weighted annual average density of adults large enough to produce newborn individuals yielded an estimate of mortality between birth and the age at which the adults could contribute to reproduction in the population.

Results Shell length distributions of S. striatinum collected in

1978 and 1979 are shown in Fig. 1. Periods of extreme flooding or low water and the deviation of precipitation from monthly averages (data from the Oxford Station of the Miami Conservancy District, Dayton, OH) are also shown. Comparison of shell-length distributions from January 1978, January 1979, and December 1979 showed that the age structure of the population changed over the 2-year period. The shell length of the median- size animal from each collection date in 1978 and 1979 is given in Fig. 2. A linear regression of median shell length (in millimetres) on day yielded the following equation: median shell length = 7.703 + 0.004 (day) (3 = 0.49, F = 36.92, probability F < 0.0001). This relationship indicates that the median shell length of the population was greater in 1979 than in 1978, again suggesting that population structure changed over the 2-year period. The median shell length ranged from 7.6 to 9.0 mm in the 1 1 samples taken from August through December 1977 and 9.6 to 1 1.4 mm in the collections taken from January through April 1980.

The densities (clams per square metre) of clams in the stream are given in Table 2. The standard errors of the means are fairly large; however, they agree well with predictions of the expected standard error based on the size of sampler, the number of samples taken, and the mean density of clams (Downing 1979). When all the density data were grouped, the standard error was 36.2% of the mean; the expected standard error was 27.8% (Downing 1979). The probable cause of the high standard errors that were observed was the great degree of aggregation of S. striatinum in in the study area. Regression of log, (s2) on log, (x) (i.e., Taylor's Power Law) yielded the equation s2 = 2 0 . 5 8 ~ ' . ~ ~ (3 = 0.53). t-Tests showed that the exponent was not significantly (95% level) different from 1 .O; however, the constant, 20.58, was significantly (99% level) greater than 1 .O, which indicates that the S. striatinum population from Little Four Mile Creek had a clumped distribution. The

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CAN. 1. ZOOL. VOL. 60, 1982

. .. h

. r + F F F L L L L L L F F

FIG. 1 . Shell lengths of Sphaerium striatinum, field temperatures, and deviation from the average monthly precipitation, 1978-1979. Histograms of shell-length distributions are given for specific dates; sample sizes are given below each distribution; lines give the probable patterns of growth (-, fall recruitment; ---, early peak of spring-summer recruitment; -.---, late peak of spring-summer recruitment). Stippled areas indicate periods of ice cover. The line for zero deviation from average monthly precipitation is indicated; letters F and L indicate periods of flooding and low water, respectively.

patchy nature of the stream bottom was probably responsible for much of the observed clumping.

Reproduction in the population was assessed by examining seasonal changes in brood composition. It was found that 2.12% of the individuals examined were parasitized by trematodes of either the family Allocrea- diidae or the family Gorgoderidae. These parasitized animals contained few embryos and thus were not included in subsequent analyses of the data on reproduc- tion. The mean number of embryos per adult was 8.22 (SD = 4.71, range = 1-25, n = 187) for adults, from which all size classes of embryos were removed and measured. Clams of shell length less than 6.0mm contained no embryos. Only those individuals with a shell length greater than 8.0mm contained embryos larger than 0.9 mm, whereas those of shell lengths greater than 10.0mm contained embryos larger than 4.0mrn. The smallest clam in the population samples was usually 4.0 mm, suggesting that this was the normal minimum birth size (Fig. 1).

The mean number of embryos in the 0.2-mm class (length = 0.0-0.4 mm) was greater tban zero for all sizes of adults in all seasons (Fig. 3). This suggests that

adults were reproductively active (i.e., producing new embryos) throughout the year or that embryos had periods of little or no growth.The data for Little Four Mile Creek were inconclusive regarding these two possibilities, though embryonic growth rates apparently declined from late fall through the winter (Fig. 4).

Length-frequency diagrams are given in Fig. 4 for embryos greater than 0.4 mm in length from adults with shell lengths between 8.0 and 10.0 mm and for adults of shell length greater than 10.0 mm. Embryos greater than 4.0mm (minimum birth size) were found from April through October. However, there was a decline in number of embryos greater than 4.0 mm in July, and during late July no embryos of this size were found. This suggests that there were two major periods (May - early July and August - October) of reproduction in S. striatinurn during 1978. Data from animals collected in 1979 showed a similar pattern of reproduction.

Approximations of the probable patterns of embry- onic growth are shown as arrows in Fig. 4. Embryonic growth rates (values in italics) were apparently highest during the spring and summer. Average developmental rate during the spring-summer was 0.32 mmnweek-'.

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HORNBACH ET AL.

FIG. 2. Median shell length of Sphaerium sfriatinum from Little Four Mile Creek, OH, 1978- 1979. Solid dots indicate median length, and vertical bars indicate one standard error of the median. The slanted line shows the relationship between median length and date based on the equation given (median shell length in millimetres; January 1, 1978 = day 0, December 3 1, 1979 = day 730), and indicates that the median shell lengths of samples taken in 1979 were greater than those taken in 1978.

This rate was used to compute the number of births per square metre over specific time intervals (Table 3). Summing these values over the year gave 444.4 young born per square metre during 1978. Of these, 244.4 and 200.0 were born from April to July and August to November, respectively. The annual selection ratio (mean number of young born per average adult) was 10.49: 1 (7.68: 1, April-July; 3.81: 1, August-Novem- ber). Though the annual selection ratio differed between the two birth periods, the number of young born per square metre was similar (244.4 vs. 200.0). This similarity was due to the greater density of adults during the fall (Table 3). Since there were 444.4 births per square metre and the annual weighted average of adults of shell length greater than 10.0 mm (i.e., those capable of giving birth) was 4 1.6, only 9.9% of the clams born reached sufficient size to contribute to reproduction in the population.

The annual average numbers of embryos per square metre of size classes 0.2, 0.7, 1.2, 1.7, 2.2, 2.7, 3.2, 3.7, 4.2, and 4.7 mm were 511.8, 102.9, 65.2, 42.7, 33.2, 32.0, 17.3, 13.0, 8.5, and 3.5, respectively. Embryonic mortality (percentage of embryos dying before attaining a size of 4.0 mm, the minimum birth size) was estimated as 97.5%, with the greatest mortal- ity occurring between the 0.2- and 0.7-mm size classes.

The general pattern of growth and reproduction in the S. striatinum population was determined by examina- tion of the time series of shell-length frequencies for

FIG. 3. Mean number of 0.2-mm-class embryos (length 0.0-0.5 mm) in adult Sphaerium striatinum, 1978. Means (solid dots) and ranges (vertical lines) are given for small (shell length < 8.0 mm), medium (8.0 I shell length < 10.0 mm), and large (shell length 1 10.0 mm) adults.

1978 and 1979 (Fig. 1) and the data on reproduction. There were two main periods of reproduction: spring- summer (May-July) and summer-fall (August-No- vember). Individuals born during the May-July period grew, and some (probably those born in the spring) were of sufficient size to contribute to the fall period of reproduction. The individuals born in spring-summer overwintered as adults and contributed to the spring-

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CAN. J . ZOOL. VOL. 60, 1982

TABLE 2. Population density of Sphaerium striatinum in Little Four Mile Creek, OH, August 1977 - December 1978

Mean density Range Standard error Date (clams/m2) (clams/m2) (% of mean)

Aug. 10, 1977 Aug. 27, 1977 Sep. 10, 1977 Sep. 27, 1977 Oct. 8,1977 Oct. 22, 1977 Nov. 6, 1977 Nov. 18, 1977 Dec. 16, 1977 Jan. 1, 1978 Feb. 14, 1978 Feb. 25, 1978 Mar. 11, 1978 Apr. 8, 1978 Apr. 22, 1978 May 6, 1978 May 22, 1978 Jun. 9, 1978 Jun. 17, 1978 Jun. 28, 1978 Jul. 15,1978 Aug. 17, 1978 Sep. 11, 1978 Sep. 27, 1978 Oct. 7, 1978 Oct. 21, 1978 Nov. 4, 1978 Nov. 18, 1978 Dec. 2, 1978 Dec. 12, 1978

summer reproduction in the following year. Large (> 10.0 mm), overwintering adults were probably the major contributors to the early pulse of spring-summer reproduction (May, Fig. 1). Smaller (8 .O- 10.0 mm), overwintering adults, which grew in the spring, were probably the major contributors to the later (June) pulse of spring-summer births, though the larger adults may also have added to this birth period by producing a second brood. Smaller (8.0- 10.0 mm) overwintering adults may have also produced young in the fall. Individuals born in the fall appeared to overwinter at sizes less than 8.0 mm and grew rapidly in the following spring. Some of these overwintering individuals may have reached sufficient size to produce young during the later (June) peak of spring-summer reproduction and were probably the major contributors to fall birth.

The overlap in size distributions of adults from the various generations is large and makes the estimation of life-span difficult. However, these animals probably lived for 1 year.

Discussion Few studies of the population dynamics of pisidiid

clams are available in the ecological literature. This is particularly true for Sphaeriurn striatinurn, the most cosmopolitan Sphaeriurn species in North America (Herrington 1962). Foster (1932) reported that S. solidulurn (= S. striatinurn) had a life-span of approxi- mately 1 year in an oxbow pond in Illinois, whereas Avolizi ( 1976) reported a life-span of 18-24 months for S. striatinurn in a mesotrophic lake in New York. In Little Four Mile Creek, S. striatinurn born in the spring-summer or fall probably live for 1 year. The major periods of reproduction (spring-summer and fall) reported for S. striatinurn by Avolizi (1976) correspond with those observed in this study. Peaks in reproduction have also been observed in summer and winter (Foster 1932) and during the summer in a stream population of this species (Heard 1977).

Though births in the Little Four Mile Creek popula- tion were usually confined to two major periods they

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HORNBACH ET AL.

FIG. 4 . Brood development in adult Sphaerium striatinum from Little Four Mile Creek, OH, 1978. Size distributions of embryos are given for medium (8.0 5 shell length < 10.0 mm) and large (shell length 2 10.0 mm) adults. Histograms indicate the percentage of embryos at each size class ( 2 0.7-mm-class embryo). Numbers in parentheses represent the number of adults dissected and the mean number of embryos ( 2 0.7-mm-class embryo) per adult, respectively. Horizontal broken lines indicate the minimum birth size (4.0 mm). Arrows show the kinetics of probable embryonic development; numbers on arrows are embryonic development rates (millimetres per week). Development rates during the major periods of birth are given in italics.

were not synchronous, that is, adults of similar size did not necessarily reproduce at the same time. If births were synchronous, and adverse environmental condi- tions developed just after birth, all young could be lost. This asynchrony in reproduction is probably advanta- geous in freshwater habitats with fluctuating conditions. Heard (1965, 1977) has discussed the occurrence of synchronous and asynchronous reproductive patterns in the Sphaeriidae (= Pisidiidae) . Variation in life-span and reproductive pattern is not uncommon in freshwater molluscs and suggests that these forms are well adapted to many environments (Russell-Hunter 1978).

Intrappulational variation in life-history characteris- tics has also been reported in pisidiid clams. Studies of Musculium securis (Mackie et a1 1976a, 1976b) and M. partumeium (Way et al. 1980) showed that individuals from ephemeral ponds were normally univoltine, but in the odd years, when these ponds remained full, they became bivoltine. Extreme variation in life cycle was not noted in the Little Four Mile Creek population. However, there were shifts, in shell-length distributions over the period August 1977 through April 1980. The

increase in the median shell length of S. striatinum over this period suggested that growth rates were higher in 1979 than in 1977 or 1978. The underlying causes of these apparent differences in growth rate are unclear, but there are at least two factors which may have contributed to them. The winter of 1977- 1978 was more severe than that of 1978-1979. The stream was covered by ice for less than 1 month in the winter of 1978-1979, but for longer than 2 months in the winter of 1977-1978. In addition, rainfall in 1978 was generally lower than in 1979 (Fig. 1; the respective precipitation levels in 1978 and 1979 were 0.5 1 and 23.35 cm above annual aver- ages for southwestern Ohio). The lower levels of rainfall in 1978 resulted in an extended period of time during which stream level was low. This period of low water (July-October) probably subjected the clams to temper- ature extremes higher than normally experienced by the organisms. For example, water temperature exceeded 22°C for about 6 weeks in 1978 as compared to about 3 weeks in 1979. Metabolic studies of S. striatinum showed that, during the summer, the increases in rate of chemical reaction for each 10°C increase in temperature

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256 CAN. J. ZOOL. VOL. 60, 1982

TABLE 3. Reproductive output per square metre in 1978 for Sphaerium striatinum from Little Four Mile Creek, OH. Births over specific time intervals were based on an embryonic development rate of 0.32 mmtweek

Minimum size class Mean number

of embryos of embryos Number of Number of to reach 1 size in adults of Average of Number of weeks in 4.0 mm in column 3 per SL 1 10 mm column 5 births per square

time time adult of per square over time metre over time Date interval interval SL 1 10 mm metre interval interval

Apr. 14

Apr. 22

May 6

May 22

Jun. 9

Jun. 17

Jun. 28

Jul. 15

Jul. 28

Aug. 17

Sep. 11

Sep. 27

Oct. 7

Oct. 21

Nov. 18

NOTE: SL, shell length.

(Qlo) were generally less than one for small clams (Hornbach 1980). This indicates that as temperature increased, the metabolic rates of small clams decreased. A general decrease in metabolic rate at higher tempera- tures may have been responsible for the lower growth rates observed in 1978. These in turn could have resulted in a lower median shell length for clams in 1978, as compared with those taken in 1979. In 1977, rainfall was even lower than in 1978 (2.74 cm below and 0.51 cm above annual average rainfall for 1977 and 1978, respectively). This may have accounted for the smaller median shell lengths observed in the fall of 1977, as compared with those of animals taken in the fall of 1978. Though periods of higher than normal tempera- tures probably reduced growth rates, they apparently did not affect population densities. Densities of S. stria-

tinum were higher in the fall of 1977 than in the fall of 1978 (Table 2). The decrease in density in 1978 was probably due to spring flooding, which severely altered the characteristics of the stream bottom.

Despite the harsh conditions which are often experi- enced by pisidiid clams in various freshwater habitats, the reproductive potential of these forms is surprisingly low. In this study, the annual selection ratio for S. striatinum was 10.49:l (7.68:l and 3.81:l for the spring-summer and fall birth periods, respectively). Other published annual selection ratios for pisidiids are 12:l for S. striatinum and 6:l for S. simile (Avolizi 1976), 5: 1-8: 1 for populations of Musculium securis (Mackie et al. 1976a, 1976b), and 25:l-136:l for populations of M. partumeium (Hornbach, Way, and Burky 1980). These ratios are much lower than those

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reported for marine molluscs, many freshwater gastro- pods (Browne and Russell-Hunter 1978), and other freshwater bivalves (approximately 75 000: 1 for Cor- bicula sp. (Aldridge and McMahon 1978); > 500 000: 1 for unionids (Ellis 1978)). These differences can be explained in part by the ovoviviparous nature of pisidiid clams. Most marine molluscs produce planktonic larvae which must grow and metamorphose before reaching adult size. J. E. Morton (1979) has described three ecological types of molluscan larvae: Planktotrophic larvae with a larval life-span of 2-3 months; plankto- trophic larvae with a short swimming life of less than a week; and, lecithotrophic larvae which hatch from yolky eggs. The first type of larvae feeds is distributed in the plankton and is characteristic of lamellibranch bivalves and prosobranch gastropods. The plankto- trophic larvae with short larval life-spans serve mainly for dispersal of the species and are found among the nudibranch gastropods. The lecithotrophic larvae swim little, are carried passively in the plankton, and take no food from the plankton. They are normal for the Polyplacophora, Aplacophora, Scaphopoda, and proto- branch bivalves. The survival of lecithotrophic larvae is relatively independent of adverse conditions and species with these larval forms show very constant numbers from year to year. In nonmarine molluscs (and some marine molluscs, cf. Scheltema (1 972)), selection has generally been for fewer eggs of larger size (corbiculids and unionids being the exceptions) and for reductions in free-living larval stages (gastropods (Russell-Hunter and Apley 1966; Apley et al. 1967); Corbicula sp. (B. Morton 1979); unionids (Ellis 1978)). Again this pre- sumably allows larval survival to be relatively inde- pendent of the adverse environmental conditions char- acteristic of many freshwater systems. Pisidiid clams have maximized this strategy of reduction in the number of larval stages by giving birth to miniature adults. The relatively large size and complete development of newborn pisidiid clams also allow births to occur shortly before the onset of winter. In molluscs which produce free-living, larval forms, birth must occur well in advance of winter to allow sufficient time for growth and development.

There is also considerable intra- and inter-popula- tional variation in the reproductive output of pisidiid clams (Mackie et al. 1976a, 1976b; Hornbach, Way, and Burky 1980). Though a statistical comparison could not be made, the difference between the annual selection ratio computed in this study (10.49:l) and that reported by Avolizi (1976) for a lake population in upstate New York (12: 1) suggests that the reproductive output of S. striatinum may vary with habitat. Other aspects of the reproductive cycles in these populations may also be different. Avolizi (1976) reported that the smallest adult capable of giving birth was 9.2mm, the mean size of

young at birth was 3.6 mm, the maximum birth size was 4.4mm, and embryonic mortality was 90%. In this study, S. striatinum had a larger minimum size at first reproduction (1 0.0 mm), produced larger young (mini- mum birth size = 4.0mm), and were capable of retaining embryos as large as 5.5mm. Embryonic mortality (97.5%) was also higher. The smaller size at first reproduction and at birth, and the lower embryonic mortality in the lake population of S. striatinum may account for the higher reproductive output (12: 1) of this population, when compared to that (10.49: 1) of the Little Four Mile Creek population.

Current theories of r- and K-selection predict that populations which inhabit stable environments should produce few young of larger size (K-strategy), whereas those inhabiting more variable environments should produce many young of smaller size (r-strategy) (Pianka 1970). If it is assumed that lake ecosystems are more stable than streams, mollusc populations inhabiting lakes should exhibit K-selection. Yet, the lake popula- tion of S. striatinum studied by Avolizi (1976) showed a greater tendency toward r-selection . There were large numbers of smaller young in the lake, whereas the stream produced fewer newborn of larger size.

In the stream environment, variations in physical- chemical factors are often large, and fluctuations can occur during periods of reproduction, thereby affecting juvenile survival. For example, if the stream is flooding during periods of reproduction, newborn clams may be transported to poor-quality substrates or buried under sediment. Both of these events could affect survival rates (Rogers 1976). On the other hand, if water levels are unusually low during reproduction, the greater extremes of temperature to which the young clams are exposed could inhibit growth and affect their ability to survive the winter. Consequently, juvenile mortality in the stream is probably quite variable and dependent on physical-chemical conditions during the reproductive cycle. Changes in the physical-chemical characteristics of a lake are usually of less magnitude and more predictable than those of a stream. Thus, the mortality rates of newborn clams in the lake environment are probably less variable than those of juveniles in a stream. However, other abiotic (e.g., low oxygen tensions during stratification) or biotic (e.g., greater predation) factors may result in greater absolute mortality of young clams in lake habitats. The tendency for newborn individuals to be of a larger size in the stream may increase survival rates and allow the popula- tion to hedge against the highly variable juvenile mortality that results from the fluctuating physical- chemical environment.

The apparent reversal of expectations based on r- and K-selection theory observed in this study has been described as "bet-hedging," and is probably dependent

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258 CAN. J . ZOOL. VOL. 60, 1982

on stochastic rather than deterministic processes (Steams 1976, 1977). This "bet-hedging" strategy occurs in populations of the pisidiid clam Musculium partumeium that inhabit ephemeral ponds (Hornbach, Way, and Burky 1980; McLeod et al. 1981). Holo- painen and Hanski (1979) also noted that the differences in life-history traits observed for two Pisidium species did not strictly conform to expectations based on r- and K-selection theory. However, Mackie et al. (1 978) reported that in M. securis populations the balance between the number and size of offspring produced can be explained in terms of r- and K-strategies.

The results of this study and others suggest that S. striatinum exhibits considerable plasticity in its re- sponse to environmental conditions. This plasticity is probably reflected in both the phenotype and genotype of the organism. Hornbach, McLeod, and Guttman (1980), Hornbach, McLeod, Guttman, and Seilkop (1980), and McLeod et al. (1981) showed that there is little genetic variability in these forms; thus, much of the flexibility of response to changing environmental factors may be phenotypic in nature. Russell-Hunter (1978) has suggested that the adaptive variation displayed by many freshwater animals is phenotypic in nature, and that selection has produced genotypes which can provide phenotypic flexibility. Populations of S. striatinum display adaptive variation in life-history characteristics that may account for the widespread distribution of this species in North America.

Acknowledgements The authors are grateful to the many people who made

valuable contributions to this study: Steven Seilkop for his assistance with statistical and computer analyses; Michael Galloway for sorting benthic samples; and the many students who aided in field collections. This study was funded in part by a grant-in-aid from Sigma Xi to Daniel J. Hornbach.

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