Chromosome bands in freshwater triclads

Four species of Triclads belonging to three families, Dugesia polychroa and Dugesia mediterranea (Dugesiidae), Planaria torva (Planariidae) and Dendrocoelum lacteum (Dendrocoelidae) were studied for chromosome banding: C-banding for all the species, ASG banding for Dugesia polychroa and G-banding for Dugesia polychroa and Dugesia mediterranea. C-banding results for Dugesia lugubris-polychroa group suggest a high level of heterochromatin evolution in the group. Small bands strictly limited to the centromeric region were seen in Dugusia lugubris and this pattern is similar to the patterns of some species belonging to other families, notably Planaria torva and Dendrocoelum lacteum. This could be considered a primitive character. In contrast numerous heterochromatin bands which are useful to characterize the different karyotypes were seen in Dugesia polychroa and Dugesia mediterranea. Our data suggest that heterochromatin is important in freshwater triclad speciation.     

The food niches of the invasive Dugesia tigrina (Girard) and indigenous Polycelis tennis Ijima and P. nigra (Müller) (Turbellaria; Tricladida) in a Welsh lake

The invasion of Llyn Coron in North Wales by the American immigrant Dugesia tigrina resulted in the almost entire displacement of the native Polycelis tenuis and P. nigra populations. Because competition for food is the most important factor controlling and regulating populations of British lake-dwelling triclads, the diets of the invasive and native triclads in the lake were examined in an attempt to explain the successful invasion.A serological technique, the precipitin test, identified the gut contents of field-collected triclads. Niche breadth, electivity and niche overlap indices were used in analysis of the data. A broad food niche was recorded for all the triclad species. Oligochaetes predominated in the diet of D. tigrina followed by Asellus and chironomids, in almost equal proportions, snails and caddisflies, with mayflies scarcely eaten. Both Polycelis species fed heavily on oligochaetes followed by Asellus. P. tenuis ate slightly higher proportions of chironomids and caddisflies than snails and mayflies, whilst P. nigra consumed similar proportions of these four prey taxa. Gammarus, scarce in Llyn Coron, and cladocerans were not eaten by the Polycelis species and by only a few D. tigrina.To-date there has been no apparent shift in the diet of the Polycelis species as a consequence of the invasion by D. tigrina. In the absence of any evidence for the partitioning of food on the basis of prey size or condition, the considerable overlap in the diets of D. tigrina and the Polycelis species suggests the potential for severe inter-specific competition for food. The superior exploitation by D. tigrina of the available food resource requires explanation.     

Observations on the comparative ecology of lake-dwelling triclads in southern Sweden,Finland and northern Britain

Summary There are more lake-dwelling species of triclad in northerm Britain (10) than in southern Sweden (7) and the distribution of some differ in the two countries over approximately comparable types of lake. Dendrocoelum lacteum (Müller) occurs in a wide range of lake types in s. Sweden but is restricted to more productive lakes in n. Britain. Polycelis nigra (Müller) (absent from Sweden) has a similar, wide occurrence in Britain. Planaria torva (Müller) is more common in s. Sweden in lakes of all types but particularly in the less productive lakes. The Polycelis species P. tenuis (Ijima) and P. hepta E. H. and Y. Melander, also the Dugesia lugubris species complex show much the same distribution in both areas. Polycelis felina (Dalyell) does not occur in Sweden. The small nuber of Finnish lakes sampled confirm the data for s. Sweden. There is significant correlation between the total abundance of triclads and both the hardness and dissolved matter contents of the Swedish lake waters. Quantitatively the s. Swedish triclad populations compare most closely with those of Mid-Scotland in the n. Britain area.

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Zusammenfassung Es gibt im Norden Britanniens eine grössere Anzahl von Tricladen (10) die Süsswasserseen bewohnen, als in Süd-Schweden (7) und einige Sorten verbreiten sich in ungefähr gleichen Arten von Seen in verschiedenem Maasse in den beiden Ländern. Dendrocoelum lacteum (Müller) erscheint in einer grossen Reihe verschiedenartiger Seen in Süd-Schweden, beschränkt sich aber im Norden Britanniens auf die Seen mit grösserem Reichtum an Lebewesen. Polycelis nigra (Müller) — in Schweden nicht vorhanden-, hat eine ähnlich weite Verbreitung in Britannien. In Süd-Schweden ist in Seen aller Art ein reicherer Bestand an PLanaria torva (Müller), besonders in den Seen mit geringerem Bestand an Lebewesen. Die Polycelis Art P. tenuis (Ijima) und P. hepta E. H. und Y. Melander und auch der Dugesia lugubris Arten-Komplex zeigen dieselbe Verbreitung in beiden Gegenden. Polycelis felina (Dalyell) kommt in Schweden nicht vor. Die kleine Zahl finnischer Seen, denen Proben entnommen wurden, bestätigen die Befunde von Süd-Schweden. Wesentlicher Zusammenhang besteht zwischen der gesamten Reichhaltigheit von Tricladen und dem Gehalt an Kalk und aufgelöster Substanz bei den schwedischen Süsswasserseen. Mengenmässig ist die Verbreitung von Tricladen in Süd-Schweden der im Norden Britanniens am echesten zu verleichen, soweit es sich um Mittel-Schottland handelt.

     

Effects of Ration and Temperature on Reproduction of a Semelparous and an Iteroparous Triclad

Summary

This paper compares aspects of the reproduction of a semelparous triclad, Dendrocoelum lacteum, with those of an iteroparous triclad, Dugesia polychroa. Cocoon output was less sensitive to ration and possibly temperature for the former as compared with the latter species. Cocoons of D. lacteum were on average bigger and contained more hatchlings than those of D. polychroa. The sizes of hatchlings did not differ between species, but for D. lacteum only, the hatchlings emerging from late cocoons were significantly larger, and less numerous, than those emerging from earlier cocoons. Possible reasons for these differences are considered.     

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.     

The reaction time of leech and triclad species to crushed prey and the significance of this for their coexistence in British lakes

1. Three species of leeches, Erpobdella octoculata, Glossiphonia complanata and Helobdella stagnalis, and four species of triclads, Polycelis nigra, P. tennis, Dugesia polychroa and Dendrocoelum lacteum, commonly coexist on stony shores in productive British lakes. All species are food limited and there is much overlap in their diet. For both leech and triclad communities, coexistence of species is through the occurrence of food refuges. Leeches are more successful than triclads at capturing live prey, whereas both groups feed on damaged prey, comprising incapacitated, live or dead animals that are leaking body fluids. If triclads are better than leeches at exploiting damaged prey, this could be a mechanism for their coexistence. 2. Laboratory experiments investigated the comparative speeds at which leeches and triclads responded to crushed prey. Young and adult predators were offered a crushed specimen of the oligochaete Tubifex tubifex, the snail Lymnaea peregra, the crustacean Asellus aquaticus or the chironomid Chironomus sp., and their reaction times recorded. These four prey groups constitute the main diet of the predators in the field. Only D. polychroa and D. lacteum showed a significantly different reaction time between young and adults to crushed prey, and the reason for this is unclear. All predators, except H. stagnalis and D. polychroa, showed a difference in reaction time to the four types of prey, presumably a consequence of differences in both the ‘quality’ and ‘concentration’ of the different prey fluids, and there were some differences between predators in their speed of reaction to the same prey type. The following sequence, from fastest to slowest, in general reaction time to prey was obtained: E. octoculata, D. polychroa, P. tenuis, D. lacteum, P. nigra, H. stagnalis and G. complanata. 3. The location of the damaged food by the predators can be explained partly in terms of their foraging behaviour, with E. octoculata, D, polychroa and P. tenuis exhibiting a more seek-out strategy than other species which have a more sit-and-wait behaviour, and partly on the level of sophistication of their chemosensory system used to detect leaked prey fluids. This system is highly developed in triclad species but poorly developed in leeches. 4. In a second type of experiment in which prey, L. peregra, A. aquaticus or Chironomus sp., were offered at different time intervals after crushing to H, stagnalis and P. tenuis, few predators fed on food crushed for 24 h or longer, although a few leeches fed on Chironomus crushed for up to 72 h. 5. It is concluded that coexistence of leech and triclad species on stony shores in lakes is assisted by partitioning of food on a damaged or live basis.     

Triclads (Turbellaria: Tricladida) as predators of lake-dwelling stonefly and mayfly nymphs

It has been demonstrated by using a serological technique that Polycelis nigra (Müller) and P. tennis (Tjima) feed on mayfly and stonefly nymphs in unproductive lakes. Such feeding is less in productive lakes where these insects form a smaller proportion of the invertebrate fauna of the stony littoral zone. These nymphs form a significantly larger part of the diet of P. nigra than of P. tenuis; both triclad species feed more on stoneflies than mayflies.     

Factors affecting the occurrence of Dendrocoelum lacteum (Turbellaria) in Swedish lakes

By discriminant analysis on data from 115 lakes in South Sweden it was shown that the distribution and abundance of the triclad Dendrocoelum lacteum was predominantly governed by the density of the isopod Asellus aquaticus, its main food. The occurrence of the latter was mainly correlated with the nutrient richness of the lakes, most probably via presence of appropriate detritus, chiefly allochtonous material. As opposed to British oligotrophic lakes, most Swedish nutrient-poor lakes are surrounded by deciduous vegetation, which indirectly promotes the existence of A. aquaticus and consequently D. lacteum. Presence of mat-forming plants generally seemed to depress triclad density, whereas smaller stones promoted their occurrence, at least in nutrient-rich lakes.     

A comparative study of the food niches of lake-dwelling triclads and leeches

The food niches of four species of triclads and three species of non-parasitic leeches living in the littoral zone of British lakes of different trophic status were investigated, over one year, using a serological technique. Antisera against ten potential prey groups were employed. The basic data were adjusted to compensate for differences in predator size and seasonal changes in field temperatures. Data on two of the leech species have been presented elsewhere, but comparisons of the diet of all seven predator species are made in this paper. Much overlap in diet between the various predator species occurs, but, with one exception, each of the genera has a major food resource. The Polycelis spp. feed extensively on oligochaetes, Dugesia polychroa on molluscs, Dendrocoelum lacteum on Asellus, Erpobdella octoculata on chironomids and Glossiphonia complanata on molluscs. Helobdella stagnalis is a generalist feeder. Problems of coexistence of the leech species, and of the leech and triclad species are discussed.     

Regeneration of photoreceptor organs in freshwater planarians at different levels of accumulation of natural methylmercury compounds

The effects of natural methylmercury compounds on regeneration of photoreceptor organs were studied in three freshwater planarians: Polycelis tenuis, Dugesia lugubris, and D. tigrina. Accumulation of methyl mercury in the planarian body suppressed regeneration of P. tenuis with numerous photoreceptor organs to a greater extent than in two other planarians that have only two eyes. High methyl mercury concentrations inhibited the restoration of photoreceptor organs in asexual and sexual D. tigrina races     

Regeneration of freshwater planarians <Emphasis Type="Italic">Dugesia tigrina</Emphasis> and <Emphasis Type="Italic">Polycelis tenuis</Emphasis> under the influence of methyl mercury compounds of natural origin

We studied the effects of methyl mercury compounds of natural origin on regeneration of the planarians Dugesia tigrina and Polycelis tenuis. Accumulation of methyl mercury in the planarian body leads to a delayed formation of photoreceptor organs in planarians of both species. After a significant traumatic load, the regeneration is suppressed and the death of some control and most experimental animals was observed. The intensity of joining additional cuts depends on the localization of body fragment with a cut and localization of a cut itself.Translated from Ontogenez, Vol. 36, No. 1, 2005, pp. 35–40.Original Russian Text Copyright © 2005 by Medvedev, Komov.     

Reproductive ecology of Dendrocoelum lacteum (Turbellaria) in a rapid stream in southern Sweden and comparisons with a lake population

Rapid streams in southern Sweden are often inhabited by the triclad Dendrocoelum lacteum. Outside Fennoscandia, this species is mainly restricted to lentic habitats. The normal food refuge for D. lacteum, the isopod Asellus aquaticus, is seldom found in the stream habitats, and probably the prey here is the amphipod Gammarus pulex. With respect to spatial and trophic niche components, it seems that D. lacteum has taken the place of Crenobia alpina in southern Sweden.Two parameters of reproduction, namely production of cocoons and of hatchlings, both peaked approximately one month later in a stream than in an adjacent lake. This difference was attributed to a low temperature regime in the stream. Number of hatchlings per cocoon and cocoon sterility were higher in the stream than in the lake. Similar cocoon and hatchling output per adult was found in the two habitats, indicating a similar reproductive effort for the two triclad populations. I suggest that D. lacteum, by virtue of its choice of microhabitat, viz., under stones, is not so vulnerable to the more severe stream environment.     

Regeneration processes in various species of planarians

Blastema growth and functional maturation of the pharynx during regeneration in various planarian species were compared. The intensity of blastema growth was highest in Polycelis tenuis; the lowest, in Schmidtea mediterranea. In the sexual and asexual races of Girardia tigrina blastema growth differed inconsiderably. The function of the pharynx during the regeneration of caudal fragments lacking pharynx was manifested in G. tigrina in the usual amount of time, while in the regeneration of head fragments lacking pharynx, this function occured earlier. In other planarian species of the other two typed, the times of pharynx regeneration had no regular character and took longer compared to the same process in G. tigrina.     

Fine structural observations on the protonephridium of the terrestrial triclad Geoplana pasipha

Summary The protonephridial system of the terrestrial triclad Geoplana pasipha was studied by electron microscopy. In general construction it conforms to that of the freshwater planarian Dugesia tigrina (McKanna, 1968, 1968 a).Ultrastructural details permit the identification of proximal, collecting, osmoregulatory and nephridiopore regions. Intralumenal material is frequently observed within the tubules, whose cells are always joined by septate junctions. Cilia are numerous and possibly contribute to the flow of water and solutes along the system. Proximal tubules have a peculiar, particulate membrane coat, while a continuous coat, densely stainable with ruthenium red lines the more distal, unciliated tubules.Supported in part by Fundação de Amparo à Pesquisa do Estado de São Paulo and Banco Nacional de Desenvolvimento Econômico     

On the karyology of Dugesia gonocephala s.l. (Turbellaria,Tricladida) from Montpellier,France

Two species belonging to the Dugesia gonocephala group are found in the area of Montpellier, France. The karyology of these two species, D. gonocephala s. str. and S. subtentaculata, and of fissiparous Dugesia races has been studied.Two populations belonging to D. gonocephala s. str. are diploids with a chromosome number of 16, whereas the specimens of a third population are sexual aneuploids; the majority of cells possess 24 chromosomes, but some cells contain 23 or 25. The specimens attributable to D. subtentaculata are triploids, the most notable karyological feature being the presence of a single unmatched acrocentric chromosome. The fissiparous Dugesia strains are all aneuploids, the most common chromosome number being 27 with up to three small B-chromosomes.     

The distribution of Naididae (Oligochaeta) in the littoral zone of selected lakes in North Wales and Shropshire

SUMMARY. The distribution of twenty species of Naididae found in seventeen lakes in North Wales and Shropshire is described. Fewer species were observed in the mountain lakes in Snowdonia in comparison to the more productive lakes of Anglesey and Shropshire.
Substrate was an important factor influencing the distribution of naids within lakes, A small number of sexually mature individuals was found in more productive lakes in the summer and autumn months. Sexually mature individuals of Stylaria lacustris were cultured in the laboratory; the population doubled its numbers, by asexual reproduction, in 12 days at 15°C. In Llyn Coron a natural population of S. lacustris doubled its numbers in 40 days. The gut contents of S. lacustris consisted of periphyton.     

Fine characterization of turbellarian chromosomes I. Giemsa and quinacrine banding in Dugesia polychroa (O. Schmidt)

Giemsa and quinacrine banding was routinely produced in metaphase spreads of the freshwater triclad Dugesia polychroa. The techniques reported here may help eliminate the problems in chromosome banding which have prevented the application of differential chromosome banding in karyological studies of this taxon. More detailed karyological and phylogenetic comparisons with other species now seems possible.     

The effects of group size on per capita ingestion in flatworms

1. We evaluated the effect of group size on the per capita ingestion rates of three species of flatworm, two of which actively group with conspecifics (Dugesia tigrina, D, dorotocephala) and employ mucus to capture prey, and a third species (Mesostoma ehrenbergii) that does not actively group but does use mucus to capture prey. 2. As flatworm group size increased, daily per capita ingestion first increased and then decreased for D. tigrina and D. dorotocephala. In the case of D. tigrina this pattern was observed even at low predator densities. Ingestion rates of M. ehrenbergii were largely unaffected by group size. 3. Results suggest that the observed changes in per capita ingestion rates with changes in group size previously reported for D. tigrina are related to their tendency for active grouping and are not directly a consequence of prey capture technique or experimental design. 4. We argue that freshwater triclads in general, and D. tigrina in particular, represent an ideal model system for the development and testing of group foraging theory.     

A Quantitative Study of the Comparative Distribution of Non-parasitic Leeches and Triclads in the Stony Littoral of British Lakes

For each of the three species of leeches and four species of triclads inhabiting the stony littoral of 100 British lakes of diverse trophic status, numbers, biomass and mean weight were correlated against a wide range of physical, chemical and other variables. Very few significant correlations were found between the various biological variables and the non-chemical variables. Significant, positive correlations were obtained for all leech and triclad variables against all of the chemicals, with the exception of Erpobdella octoculata numbers, Helobdella stagnalis mean weight and all of the Polycelis nigra variables which did not correlate with any. Relationships between biological variables and calcium content are given special consideration. Over the range of lakes investigated, total triclad numbers exceeded leech numbers but the reverse was usually true for total biomass. Correlations between the various leech and triclad variables are examined.     

cDNA cloning and partial sequencing of homeobox genes in Dugesia japonica

In the freshwater planarian Dugesia japonica, four types of cDNAs of homeobox-containing genes have been isolated by screening a cDNA library using a homeobox guessmer. Partial sequencing analysis of two types of cDNAs revealed that one was a homolog of Dth2 which is a homeobox gene in Dugesia tigrina and another was similar to Distal-less gene in Drosophila. This suggests that planarians have many homeobox genes.