Contrasting mortality in young freshwater leeches and triclads

A guild of leeches and triclads coexist and are the most numerous invertebrate predators on the stony shores of productive British lakes. Populations of all species are food-limited. Mortality of recruited young is considerably higher in leech than in triclad populations, and this paper investigates reasons for this. In particular, the feeding success of young leeches and triclads in relation to prey species, prey size, prey condition (alive or crushed), spatial heterogeneity (with or without the presence of stones or gravel), and the presence or absence of other young or adults predators (leeches or triclads) of the same or different species are investigated in the laboratory. Feeding success by young leeches and triclads on crushed prey without the presence of stones was high, but declined dramatically in leeches but not triclads when stones were present. Young leeches and triclads were inept at capturing live prey, of a small or large size, with the exception of soft-bodied prey such as oligochaetes. Feeding success by young predators on live prey was not increased by the presence of other young predators of the same or different species. With only a few exceptions, the presence of adult leeches, and to a much lesser extent adult triclads, increased the feeding success, growth and survival of young leeches and triclads. It is concluded that the high mortality of young leeches, compared to triclads, in field populations is due to their inability to locate damaged food in an environment with spatial heterogeneity due to a poorly developed chemosensory system. High and low levels of juvenile morality are accompanied by high and low reproductive rates in leech and triclad populations, respectively. It is unusual for a food limited population to have a high level of recruitment, but it is speculated that the characteristically high reproductive output in parasitic leeches, from which predaceous leeches are derived or have affinities, has been retained to counterbalance high juvenile mortality rates.     

Population stability of the sea snail at the southern edge of its range

The sea snail Liparis liparis population in the Bristol Channel on the Atlantic coast of Britain is close to the south-western extremity of its range and yet it is one of the more abundant species present. Using a 17-year time series of monthly samples collected at Hinkley Point power station in Bridgwater Bay, Somerset, U.K., the coefficient of variation of annual abundance of sea snail was estimated as 70·48%. Analysis of variance of the six replicate time series showed that 94% of the total variance was attributable to between-year variation in local abundance and only 6% to sampling error. A comparison of the coefficients of variation in the annual number of captures of all the common fish found that the sea snail had one of the more stable populations in the estuary and bottom-living fish tended to show the lowest variability in population number. Arguments based on either the assumed maladaptation of populations at the periphery of their geographical range or the rapid response of short-lived animals to change would have predicted sea snail to have one of the more variable populations. As might be expected for a fish adapted to cooler, more northern and eastern waters, sea snail winter abundance was negatively correlated with seawater temperature. Using the unusual oscillation in water temperature observed during the winter of 1996/199 7 , it is argued that the correlation of sea snail abundance with temperature reflects changes in distribution rather than mortality, with the fish avoiding inshore waters in warm winters. It is hypothesized that demographic plasticity and biotic interactions such as parasite and pathogen attack can result in fish populations showing neither latitudinal nor longitudinal trends of increasing population variability from the centre towards the periphery of the range.     

Competitive interactions between the lake-dwelling leeches Glossiphonia complanata and Helobdella stagnalis: an experimental investigation of the significance of a food refuge

The glossiphoniid leeches Helobdella stagnalis and Glossiphonia complanata co-exist on the stony shores of British lakes. Because both species have a similar life-style and are food limited, the question arises as to how coexistence persists. Laboratory experiments investigated if either leech has a competitive advantage on three foods, viz. oligochaete worms (Tubifex sp.), snails (Lymnaea peregra), and a worm-plus-snail mixture. A variety of densities with a range of ratios of the two leeches, and monospecific controls were used. The level of feeding ensured increasing food shortage with rising leech density. Experiments, conducted at 14° C, lasted 3 months. Regression values for growth against density in the controls indicated the occurrence of intraspecific competition in both leech species. Light mortality occurred at the highest densities. G. complanata gained significantly less weight in mixed leech cultures than in monospecific cultures at some of the highest densities when fed worms, but more weight when fed on snails. The opposite was true for H. stagnalis. When both prey types were provided, no differences at any density were observed for either leech. Differences in bodysize and feeding strategy between the leeches may be implicated in determining their feeding success on the prey types. Because the anti-predatory responses (e.g. shaking) exhibited by L. peregra are less when encountering an inactive than an active leech, the sit-and-wait strategy of G. complanata may aid snail capture. In contrast, the seek-out strategy of H. stagnalis, coupled with its relatively smaller size, may result in fewer successful captures. The active hunting strategy of H. stagnalis may be advantageous for the capture of oligochaetes which are relatively less mobile than snails. Offering both prey types simultaneously would have had an ameliorating influence on interactions between the two leeches. Findings provide support for the hypothesis that long-term co-existence between the two glossiphoniids in British lakes may depend, in part at least, on the occurrence of a snail food refuge for G. complanata.     

The consequence of a food refuge collapse on a guild of lake-dwelling triclads and leeches

The diets ofPolycelis nigra, P. tenuis, Dugesia polychroa, Dendrocoelum lacteum, Glossiphonia complanata, Helobdella stagnalis andErpobdella octoculata in an English lake were examined, using a serological technique, and compared between 1981–82 and 1989–90. Leech, triclad and prey abundances were also recorded. Between the two studies, snail numbers crashed whereas the abundances ofAsellus andGammarus increased.Dugesia andGlossiphonia numbers decreased substantially, whilstPolycelis tenuis andHelobdella abundances increased. In the second study, the snail component in the diet was greatly reduced resulting in a broader food niche, particularly forDugesia andGlossiphonia, and greater food overlap between the predators with the exception ofDendrocoelum andErpobdella which do not eat molluscs. It is postulated that the reduced size of the snail refuge, and consequent increase in severity of interspecific competition with other predators, particularlyPolycelis andHelobdella, led to the observed decrease in abundances ofDugesia andGlossiphonia. The decline in the last two genera, perhaps coupled with increased crustacean abundance, could have contributed to the numerical increase of the competitively superiorPolycelis tenuis andHelobdella.     

Does body size difference in the leeches Glossiphonia complanata (L.) and Helobdella stagnalis (L.) contribute to co-existence?

The effect of predator and prey body size on the feeding success of the British lake-dwelling leeches Glossiphonia complanata and Helobdella stagnalis was examined in the laboratory, and any involvement of size difference between the leeches in allowing coexistence in the field assessed. G. complanata breeds in advance of H. stagnalis and maintains a body size advantage throughout their annual life-cycle. In experiments, conducted at 14 °C and a photoperiod of 16 hrs L: 8 hrs D, three size classes of leeches of each species were each exposed to each of three size classes of each of five prey species, viz. Tubifex sp., Chironomus sp., Asellus aquaticus, Lymnaea peregra and Potamopyrgus jenkinsi. For each prey species, three different types of experiments were performed: one leech exposed to four prey individuals; four leeches of the same species with sixteen prey; and two leeches of each species with sixteen prey. In the first experiment, all sizes of G. complanata were capable of feeding on all sizes of the prey types offered; the same was true for H. stagnalis with exceptions of feeding on large A. aquaticus and large L. peregra. For both species, but especially for G. complanata, there was a trend within each size class of leech for decreasing proportions of fed leeches with increasing prey size, and within each size class of prey for an increasing proportion of fed leeches with increasing leech size; however there were several exceptions to these trends. Both leeches fed extensively on Tubifex sp. but there were significant differences in the proportions feeding on other prey types; G. complanata fed more on A. aquaticus and the two snail species, and less on Chironomus, than H. stagnalis. The effect of increasing the number of leech individuals from one to four individuals, of the same or mixed species, had little effect on the proportion of leeches which had fed. It is concluded that large G. complanata will have access to large individuals of certain prey taxa denied H. stagnalis, which may lessen the intensity of interspecific competition.     

Food partitioning by lake-dwelling triclads and glossiphoniid leeches: field and laboratory experiments

The triclads Polycelis tenuis and Dugesia polychroa and the glossiphoniid leeches Glossiphonia complanata and Helobdella stagnalis are abundant on the stony shores of productive British lakes. All species are food limited and there is considerable overlap in the diets of these triclads and leeches. This paper investigates interactions between the two groups using field and laboratory experiments to try to identify the mechanism of their co-existence. Triclad and leech numbers were manipulated inside experimental enclosures, mathced by controls, erected on the stony shore of an eutrophic English lake. Increasing the numbers of P. tenuis and D. polychroa prior to the reproductive season in spring resulted in a significant decrease in the numbers and body size of G. complanata and H. stagnalis compared with control populations in the summer months, and vice versa. However, increases and decreases were temporary with a readjustment of numbers and body size to control levels in the autumn after reproduction had ceased. It is suggested that increasing the numbers of either group elevated the severity of both intra- and interspecific competition for food. The condition of prey may, in part, determine the strength of competition, and this was examined in laboratory experiments in which different densities and ratios of P. tenuis and H. stagnalis were offered either live of recently crushed Asellus aquaticus. In monospecific controls, growth rates of P. tenuis were greater when fed on crushed than live Asellus, but there was no significant difference in the growth of H. stagnalis fed either live or crushed prey. In mixed cultures of predators, P. tenuis and H. stagnalis were the superior competitors when fed on crushed and live Asellus, respectively. However, when competitive pressure was low, at low densities of predators, the presence of H. stagnalis in mixed cultures fed on live prey was beneficial to the growth of P. tenuis. These results are explained in terms of the greater ability of triclads to detect damaged prey, leaking body fluids, due to their sophisticated chemosensory system, and the ability of leeches to capture live prey due to the presence of suckers. It is concluded that co-existence of the two groups in British lakes is assisted by the partitioning of food on a live or damaged basis.