Anisopary in compost earthworm reproductive strategies (Oligochaeta)

Soil Bio/. Biochem. Vol. 29, No. 314, pp. 731.-735, 1997

0 1997 Published by Elsevier Science Ltd. Al1 rights reserved

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ANISOPARY

IN COMPOST EARTHWORM REPRODUCTIVE STRATEGIES (OLIGOCHAETA) W. J. MEYER*

and H. BOUWMAN

Department of Zoology, Potchefstroom University for Christian Higher Education, Private Bag X6001, Potchefstroom 2520, South Africa (Accepted 22 June 1996) Snmmary-Individual

differences in the number of hatchlings and cocoons produccd by Eiseniu fetida

have been recorded. We propose to cal1 this phenomenon anisopary -

unequal reproduction (an -

not, iso - the same, parere -to producc).The reproduction of individual partners from pairs of E. fetida were studied. The earthworms were mated for 72 h and then separated. In 85% of al1 matings one partner produccd on average twicc as many hatchlings as the other - in the remaining 15% the partners produced on average equal numbers of hatchhngs. The biomass changes of the earthworm partners corresponded with the apparent reproductive functioning of the worms. We suggest that there are sperm donors - low producing (hypopary) and sperm receivers - high producing (hyperpary) earthworms. 0 1997 Published by Elsevier Science Ltd

INTRODUCTION

Many reproductive aspects of the vermicomposting earthworm Eisenia fetida have been studied. Venter and Reinecke (1987) Adel1 and Mensua (1989), Meyer and Bouwman (1995) and noted differences in the number of cocoons and hatchlings produced between the individuals of pairs. We propose to cal1 this phenomenon anisopary - unequal reproduction (an - not, iso - the same, parere - to prothe term for the higher duce). Hyperpary productive partner and hypopary - the lesser producer of a pair. In an experimental breeding programme this pbnomenon impeded the selection process (Meyer and Bouwman, 1995). The phenomenon of anisopary resembles basic bisexual reproduction (with karyogamy) cross fertilization and dioecism where one partner supply sperms and the other the ova. This may be called gonochorism (Blackwelder and Shepherd, 1981). In hermaphroditism the organism is capable of producing both sperms and ova - monoecious, and an equal opportunity to produce both, but not always simultaneously. A protandric mode of reproduction is also possible but has not yet been recorded. The double or single slime-tube around the clitellum during copulation may also influence the reproductive strategy of the earthworms (Stephenson, 1972). If hermaphroditism with gonochoristic functioning or hermaphroditism without simultaneous functioning, is the strategy of E. fetida, it wil1 also have an influence on experimental studies because of differ*Author for correspondence. Fax: (0148) 2992370 South Africa.

ences in individual fecundity. The performance of male (hypopary) functioning individuals VS female (hyperpaty) functioning individuals have to be taken into account as they may respond differently to environmental conditions. The aim of the present study was to detertnine the existente and extent of anisopary in the composting earthwortn species E. fetida. This species needs to practise amphimixis before viable cocoons can be produced (Venter and Reinecke, 1987; Cluzeau et aZ., 1992). This species was used as it is one of the most popular commercial earthworms and its biology is relatively wel1 known. MATERIALS AND METHODS

Cocoons were collected from a stock culture of in replidishes in tap water. The hatchlings were collected every day and stored as described in Bouwman and Reinecke (1987). This ensured that al1 the worms used in the experiments were in the same developmental stage at the start of the rearing process. When su8ìcient hatchlings (+200) were accumulated, rearing commenced. Al1 the hatchlings were individually reared in 150 ml plastic containers with a tight fitting lid with a smal1 hole. Rearing and maintenance were as described by Meyer and Bouwman (1995). Ten grams of culture medium (ground cattle manure, particle size 0-500 lm, 75% water content), was used as food at onset, and 5 g of culture medium was added every week to ensure sufficient food. The maturation was determined as described by Meyer and Bouwman (1995). The containers were E. jëtidu and incubated

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numbered and random number tables were used to combine individuals of pairs. Each worm was rinsed in tap water, dried and weighed in water. The original media of both worms (partners) were mixed and placed in one container together with the two worms to ensure that both were in familiar surroundings. The containers with worms, together with the empty containers of the partners, were kept for 72 h in an environmental control chamber at 25°C and RH of 75%. Three days was selected as the mating period according to the results of an unpublished study. The worms were separated, weighed and returned to their original containers together with half of the mixed medium. The worms were identified by their individual biomass which did not change significantly during the mating period. The control group consisting of 30 pairs which were not separated after three days, but kept paired for the experimental period. Cocoons were left in the medium to ensure optimum conditions for hatching. Hatchlings were recorded and removed every week. This was continued until no further hatchlings were found. The control group, however, never stopped cocoon production and recording was terminated, when their experimental counterparts ceased to produce hatchlings. The worms were weighed every second week to minimize the effect of handling on cocoon production. Some mortalities occurred leaving 35 pairs in the test group and 28 pairs in the control group at the

end of the study, which was enough for statistical purposes. At the end of the experiment the individuals of a pair were divided into a hyperpary and a hypopary group, according to the number of hatchlings produced by each worm of a pair. Within the pairs of the control group, individuals were scored as hyperparous or hypoparous, assuming that the heavier worm of each pair is hyperparous. This assumption was based on the biomass and fecundity results of the separated group of this study. The reproductive period was defined as the period from separation up to the day the last hatchling was collected.

RESULTS

Hatchling production

For the hyperparous worms the number of hatchlings produced per individual varied between 20 and 140 (Fig. 1). For the hypoparous worms the number of hatchlings per individual varied between 0 to 124. The hyperparous worms produced on average 53% more hatchlings per worm than the hypoparous worms. The differente between the mean number of hatchlings produced by the hyperparous (87.4) and hypoparous (46.4) worms (calculated for 35 pairs) was statistically significant (t-test, P < 0.001). Reproductive

period

The separated individuals (hyperpary + hypopary) started to produce cocoons sooner after mating than

Number of hatchllngs 160 i’Hyperpary-

14’ + Hypopary 120

20

25

30

35

Pairs Fig. 1. The total number of hatchlings produced by the individual hyperparous and hypoparous worms of each pair of the separated group.

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Anisopary in earthworm reproductive strategies

Mean number of hatchllngs

Tlme (days) Fig. 2. The mean number of hatchlings produced by the hyperparous (separated hyperpary) and hypoparous (separated hypopary) worms separated and combined (hyper + hypo, pairs), compared with the mean number of hatchlings produced by the control group (control pairs).

the control group and their production also peaked 30 days earlier (Fig. 2). The hatchling production of the separated worms decreased gradually after 30 days. The control group (mated for the duration of the study) started cocoon production more gradually, but their production peaked higher than the separated pairs (Fig. 2). Production by the control group decreased sharply after this peak and continued at a lower rate than the separated group.

30

44

Biomass changes

In the separated group the differente in mean biomass of the hyperparous (646.3 mg) and hypoparous warms (527.3 mg) at mating (day 0) was statistically significant (t-test, P < 0.05; Fig. 3). The mean biomass of both the control and separated groups increased during the mating period (Fig. 3). For both the separated and control groups the differences in mean biomass (between hypopar-

S8

72

86

100

Tlme (days) Fig. 3. The mean biomass changes of the hyperparous (separated hyperpary) and hypoparous (separated hypopary) worms during the mating and mproductive period and the mean biomass changes of the hyperparous (con. assumed hyper) and hypoparous worms (con. assumed hypo) of the control group.

W. J. Meyer and H. Bouwman

134

ous and hyperparous worms within each group) at day 3 were stil1 statistically significant (t-test, P < 0.05). For the separated group the hyperparous worms increased their biomass to 683.2 mg and the hypoparous worms to 569.7 mg during the mating period. For the separated group a biomass increase was evident til1 day 30 with consequent decreases, although the hypoparous worms lost less biomass than the hyperparous worms. After the production of hatchlings by a worm ceased, a rapid biomass increase was evident. This turning point was reached 44 days after mating by the hypoparous worms and 58 days after mating for the hyperparous worms and corresponded with hatchling production (Fig. 2). The differences in the mean biomass between the hyperparous worms and hypoparous worms, however, was not statistically significant any more 72 days after mating (f-test, P > 0.05). The mean biomass of the control group increased over the first three days and consequently decreased til1 day 44, which corresponded with the reproduction peak (Fig. 2). From 44 to 72 days an increase in biomass was evident, and from 72 to 86 days it decreased again.

DISCUSSION

For the separated group the differente between the number of hatchlings produced by the hyperparous worms, when compared with the hypoparous worms was 53%. This is an indication of anisopary. This trend is predictable in 85% (Browns prediction) of worms mated in pairs. In the remaining 15% the two partners produced more or less equal numbers of hatchlings, or the difference was less than 10 between partners (Fig. 1). There were also a number of pairs where one of the partners produced no hatchlings or less than 10 (Pairs: 1, 2, 3, 6, 13, 19 and 29). Individual male and female functioning may support the hypothesis of anisopary the best. A protandric mode of reproduction may also explain this differente, but then age would also play a role. Because al1 the earthworms were of the same age and random numbers were used to combine pairs, protandry could not have occurred. It may be that only a few sperms were transferred into the spermatheca of the hypoparous worms during copulation. With open grooves for sperm exchange, self fertilization cannot be ruled out. According to Stephenson (1972) sperms (spermatophores) could drift freely during copulation. Those worms that produced few or no hatchlings, had partners that produced well, indicating an uneven exchange of sperm during copulation. This may be related to the formation of a

double or single slime-tube during copulation (Stephenson, 1972). The reproduction pattern of al1 the groups followed approximately the same pattern as described by Reinecke and Viljoen (1990) (Fig. 2). The production of hatchlings of the separated and control groups differed, however. The production of the separated group peaked at 30 days which was 28 days earlier than the control group. This is probably due to mating activities continuing in the control group or other factors associated with keeping two worms together. It should also be kept in mind that a pair had twice the amount of space and food than a separated individual. The peak for the control group was higher than for the separated group (Fig. 2), this probably indicates that the restricted mating period of three days limited production of the separated group. The drop in production of the control group after the peak to a leve1 lower than that of the separated group also correlates with the results of Cluzeau et al. ( 1992). Because random numbers were used to combine the pairs, individual initial biomass at mating was not considered. The biomass of the hyperparous and hypoparous worms may be the most important factor which influenced the fecundity of the individual worms during and after mating. Sella (1990) found a positive correlation between body length, which is related to body mass, and fertilization efficiency in Ophryotrocha diadema (Polychaeta) and this may also be the case with E. jëtida. The biomass of both hyperparous and hypoparous worms increased after separation til1 day 30 and the biomass of the control group decreased (Fig. 3). This decrease in the control group may be the result of continued metabolic activity for sperm production and amphimixis (Skelton, 1993). After day 58 an increase in biomass of both hyperparous and hypoparous worms were evident when reproduction started to decrease. The biomass of the hypoparous worms increased faster than the biomass of the hyperparous worms. At 72 days after mating there was no significant differente in biomass (t-test, P 2 0.05). The biomass of the control group (assumed hyperpary and hypopary) also increased during the mating period (Fig. 3), but a sharp decrease followed, which correlated with their reproductive peak (Fig. 2). The lower biomass of the control group (30 days) when compared with that of the separated worms could be due to crowding (Reinecke and Viljoen, 1990) or other factors associated with keeping two worms together. The biomass changes and differences in reproduction support the anisopary theory. If this theory is proven valid it should be taken into account when working with this species and possibly others as well. Differences in sexual functioning could result

Anisopary in earthworm reproductive strategies in dissimilar responses under the same environmental and other conditions. The advantage of sexual functioning in earthworms could be explained by the increase in genetic diversity which could affect resistance to disease 4 (Ridley, 1993). Sexual functiomng may atso exptam the adaptability of E. fetida to a wide range of environmental conditions (Cluzeau et af., 1992; Reinecke et al., 1992). The advantage of a femaleto-male ratio greater than 1:l is wel1 known, and if that is the case with E. fetida, being a hermaphrodite, it may have the following implications: the anisopary strategy may be beneficial under normal and favourable conditions. Under unfavourable conditions or with low population densities, the normal hermaphroditic strategy might be applied to increase fecundity. Aspects that need further clarification are the effects of age, biomass, stress (food, temperature), stocking rate and reversibility of sexual functioning, as wel1 as the presence of this phenomenon in other species. Acknowledgements-We would like to thank our colleagues in the Departments of Zoology and Statistics for their help and support. We would like to thank the Dean of the Faculty of Natura1 Sciences, the Foundation for Research Development and the Anglo American Chairman’s Bducational Fund for financial support.

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