Unlocking the ring: Occurrence and development of the uninterrupted intrabullar septum in Canidae

The maned wolf, Chrysocyon brachyurus, and some of the earliest canids are known for the peculiar ring-like shape of their intrabullar septum (an incomplete bony partition in the auditory bulla). In attempt to understand the origin of this character state, the auditory bullae in adult and juvenile dried skulls of 28 species of living Caninae were examined with a special emphasis on the occurrence of a ring-like (uninterrupted) septum. In addition to C. brachyurus, this morphology was invariably found in the bush dog, Speothos venaticus, what makes the presence of the uninterrupted septum the first osteological synapomorphy supporting the Chrysocyon–Speothos clade recently revealed by molecular systematics.The canine juvenile morphologies indicate that the intrabullar septum is ring-like at its early developmental stage. As ontogeny progresses, the septum resorbs differentially, depending on taxon, to become horseshoe- or crescent-like. By this transformation of the initially ring-like septum, Caninae seem to ontogenetically parallel a trend found in the phylogeny of Hesperocyoninae, an extinct canid group. The reverse recapitulation occurs, however, in the evolution of Caninae themselves, with their most-derived members having a least-resorbed septum. This implies that the ontogenetic criterion cannot be used for inference of the polarity of septal character states within the Caninae. The evolution of the canine intrabullar septum could have involved a series of heterochronies towards increasingly paedomorphic states in several lineages of the tribe Canini.New data were also obtained on the bone composition of the canid intrabullar septum. While the dorsal septum is entotympanic, the ventral one is in fact ectotympanic in adult Caninae. This compound structure results from penetration of the ventral entotympanic sinus into the ectotympanic.     

Effect of previous feeding conditions on survival in Daphnia pulicaria (Crustacea: Cladocera)

Survival of natural Daphnia populations without previous acclimatization in filtered water under laboratory conditions (starvation) was used as a parameter describing the actual competitive ability of populations in periods of food limitation. Two populations (1 and 2) of Daphnia pulicaria Forbes coming from two enclosures with a low (< 1 mgC l^–1) and a high (about 3 mgC l^–1) seston level were examined in their resistance to starvation. Juveniles of both populations survived better than adult females in the conditions without seston as well as in the control experiment, despite their much lower biomass. Animals from population 1 had lower mortality than those from population 2 in all variants of the experiment. The results confirm the hypothesis that the ability of daphnids to starve depends also on adaptation to previous feeding conditions.     

Aliesterase- (Ali-E) activity in Daphnia magna straus as a parameter for exposure to parathion

In vivo exposure of Daphnia magna Straus to parathion in concentrations from 0.05 to 5 µg/l resulted in a concentration-dependent decrease in aliesterase activity of the homogenate of the animals. Decrease in this activity after 48 hours of exposure was maximal for concentrations > 1.5 µg/l and amounted to about 90% of the activity of the blank.     

A hypothesis on the role of Aphanizomenon in translocating phosphorus

A mechanism for the upward translocation of phosphorus by flake-forming Aphanizomenon is hypothesized. Aphanizomenon germinates on oxic sediments and rises into the water column, then the flakes diurnally migrate. Observations from Spring Lake, Minnesota, U.S.A., indicate that Aphanizomenon translocates phosphorus to the summertime epilimnion. However, the observations cannot discriminate between which of two mechanisms related to Aphanizomenon flakes, upon germination or with subsequent diurnal migrations, is more important.     

Mechanisms of the filtering area adaptation in Daphnia

The changes of selected parameters of the filtering comb of the third thoracic limb were studied in a natural population of Daphnia pulicaria Forbes, as well as in experimental enclosures and in lab cultures, including individual life history. Two hypotheses were tested: 1. either these changes are related to the succession of clones coexisting within one population, or 2. the size of the filtering area changes gradually as an individual adaptation during the moulting. No evidence supporting the clonal hypothesis was found. On the contrary, the adaptability of the filtering comb is the same in a natural population as it is in a clone and in individuals.     

A note about Daphnia in Venezuelan waterbodies

After examining numerous water samples of 19 lakes and reservoirs in Venezuela, only two species of Daphnia were found: D. laevis (in colder, high altitude lakes) and D. gessneri. This observation supports the theory of the scarcity of Daphnia species in tropical inland waters.     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

The relationship between cladoceran body size and the growth of underyearling roach (Rutilus rutilus) (L.) in two shallow lowland lakes: a mechanism for density-dependent reductions in growth

Large planktonic Cladocera are typically the most important components of the diet of underyearling roach. Selection for large species and individuals by fish can result in a shift in the species composition of the cladoceran community as well as a reduction in the mean size of the individuals of large species and in the assemblage as a whole. Laboratory experiments demonstrated that underyearlings feeding on Daphnia hyalina smaller than 1 mm in length has a significantly lower intake of prey volume per unit time than when feeding on prey greater than 1.5 mm. A decrease in the nutritional quality of zooplankton prey, brought about by increasing predation pressure is suggested as the mechanism for density-dependent reductions in the growth of underyearling roach in eutrophic water bodies.     

Intersex and male development in Daphnia magna

Sex induction is environmentally stimulated in Daphnia and involves a cue-dependent response for sex determination. Somatic growth was shown to be similar in males and females during juvenile instars, but divergent due to a reduction in male somatic growth, at about the time that females produce ovaries. At this time, males appeared morphologically adult with respect to secondary sex characteristics. Intersex was rare and is unlikely to be important in natural populations. Intersex could be induced in both sexes, and observed more frequently after longer exposure to high temperature, or in the second generation following a temperature change. This indicates an impact on the ability of mothers to determine the sex of the offspring. It may be possible to use intersex characteristics for manipulative investigation of sex-determination mechanisms in Daphnia. Although sex-determination is initiated before birth, intersex occurrence suggests that development of male characters requires an additional process, probably involving hormone activity, during juvenile development and maturation.     

Grazer-induced changes in the desmid Staurastrum

In aquatic environments, predator kairomones have been shown to affect morphology of prey species. Past work on the interaction between zooplankton and phytoplankton was based mainly on the Daphnia–Scenedesmus model. Algae of the genus Staurastrum can produce mucilage, causing cell clumping and settling out of the water column. These clumps are too large to be eaten by daphniids. Thus, we hypothesised that this may be a grazer defence. We investigated whether Daphnia magna induces the formation of mucus globules in Staurastrum, how this occurs, and if the formation of clumps inhibits growth in juvenile Daphnia. Eight strains of Staurastrum were used to check whether mucus extrusion is induced by the presence of Daphniaor possibly by a chemical excreted by Daphnia magna. None of the strains reacted to the presence of Daphnia water alone, animals had to be present to induce clumping. Mechanical action (gentle stirring) caused the same strains to clump. The ecological relevance of clumping was then investigated. The different Staurastrum strains were used as food in a growth experiment with ecologically relevant densities of neonates of Daphnia hyalina. These small daphniids did not cause the same clumping observed for Daphnia magna when present in experiments at high densities. We observed that juvenile daphniids grew less well on strains with larger cell size.     

The costs of crest induction for Daphnia carinata

The effects of notonectid-induced crests on growth and reproduction, and resource allocation to crest construction, moult losses and eggs of Daphnia carinata were measured. An attempt was made to elucidate the mechanisms of physiological costs of crest induction for this species. The crested morph of d. carinata reached a significantly larger size than the uncrested form. Reproductive output was similar in early instars, but the crested morph produced more eggs in latter broods. Instar duration was longer for the crested morph and age at first reproduction was delayed. Survival was also lower in this form. Crest construction required significant resources (equivalent to 60 eggs over a life time) but evidence is presented that these resources were obtained primarily by re-allocation of available material rather than collection of extra resources. The crested morph allocated significantly more resources to moulting than its uncrested counter-parts. The uncrested morph produced large eggs in early instars and progressively smaller ones in later instars. The crested morph produced only small eggs. The hypothesis is presented that the crest-induction strategy of D. carinata involves at least two separate sets of responses, each with its own costs and trade-offs. The first response is production of the crest. The cost of crest production is an increased cost of moulting. D. carinata off sets this cost by increasing instar duration and thus age at reproduction. The second response is increased size. D. carinata achieves this by reducing the fraction of available resources allocated to reproduction. The cost is lower reproductive output.     

Genetic Identification of Hyalodaphnia Species and Interspecific Hybrids

Species of the genus Daphnia, in particular the subgenus Hyalodaphnia, represent a taxonomically problematic group due to their phenotypic plasticity, local races and the formation of interspecific hybrids and backcrosses. In this study, we present a genetic approach utilising nuclear DNA to unequivocally identify species and interspecific hybrids. Several nuclear loci (ITS1-ITS2, CA14 and GA13) were amplified by PCR and products were subjected to diagnostic restriction enzymes (restriction fragment length polymorphism; RFLP). The application of this approach to several populations across Europe revealed that the markers are highly consistent and reproducible. In addition, we illustrate with a number of examples how this approach contributed to unravel previously unrecognised taxa, increased the sensitivity of biodiversity studies or contributed to the analysis of resting egg banks. Advantages and disadvantages of this approach compared to existing techniques are discussed and several empirical studies and their results are summarised.     

Possible toxic effects on Daphnia resulting from the green alga Scenedesmus obliquus

We cultured individuals of two Daphnia species and their hybrid on two different algae, Scenedesmus obliquus and Chlamydomonas globosa, in different concentrations. Our results suggest that culture conditions of S. obliquus can be such that the algal cells become toxic to Daphnia     

The consequences of a drastic fish stock reduction in the large and shallow Lake Wolderwijd,The Netherlands. Can we understand what happened?

In 1990 an experiment started in the large and shallow lake Wolderwijd (2700 ha, mean depth 1.5 m) to improve the water quality. About 75% of the fish stock was removed (425 000 kg fish). The fish was mainly composed of bream and roach. In May 600000 young pikes (3–4 cm) were introduced.In May 1991 the water became very clear (Secchi depth 1.8 m) during a spring bloom of large Daphnia. Then the grazing by zooplankton was eight times higher than the primary production of algae and the total suspended matter concentration became very low. Compared to the situation before the fish reduction, the grazing had increased only slightly, while the primary production had decreased significantly in early spring. The fish stock reduction might have contributed to the reduction in primary production by a reduced internal nutrient load. The clear water period lasted six weeks. Daphnia disappeared in July due to food limitation, the algal biomass increased and the Secchi depth became 50 cm. Daphnia did not recover during summer, due to predation that was not caused by 0 + fish but by the mysid shrimp Neomysis integer. Neomysis could develop abundantly, because of the reduced biomass of the predator perch. The production of young fish had been low because of the cold spring weather. The cold weather was probably also responsible for the slow increase in density of macrophytes. After 1991, perch probably can control Neomysis. Due to lack of spawning places and shelter for 0 + pike, pike was probably not able to control the production of 0 + fish. In a lake of this scale, it will not be easy to get more than 50% coverage of macrophytes, which seems necessary to keep the algal biomass low by nutrient competition. Therefore, we expect also in the future a decrease in transparency in the summer. Locally, especially near Characeae, the water might stay clear.     

Low fish predation pressure in London reservoirs: II. Consequences to zooplankton community structure

The 1992 survey of zooplankton structure in fourteen London supply reservoirs showed the overall dominance of large-bodied zooplankton, mainly species of Daphnia. These reservoirs can be considered as ‘anti-fish’ by virtue of their steeply sloping concrete or brick sides. The average biomass of large Daphnia spp (retained on a 710 μm sieve) in the total zooplankton biomass was higher than 20% for twelve out of fourteen reservoirs. The cladoceran-copepod ratio was inversely correlated with both dominance of large-bodied Daphnia magna and cladoceran body-size structure. Parallelly, there were tendency of more efficient utilization of lowered algal crops in reservoirs dominated by large-bodied Daphnia spp. A graphical model is presented which relates daphnid species composition and zooplankton size structure to a presumed gradient of fish biomass in these reservoirs.     

The relationship between temperature,competition and the potential for colonization of a subtropical pond by Daphnia magna

The role that temperature plays in excluding large daphnid species from subtropical and tropical ponds, and competitive relationships between Daphnia laevis and Daphnia magna, are explored. D. magna, a large temperate species is probably not excluded from subtropical or tropical systems by elevated temperatures. However, D. magna was a poorer competitor, under a restricted set of laboratory conditions, than D. laevis, the only pond dwelling daphnid in subtropical Florida. Competition is proposed as one mechanism that may limit the number of daphnid species in subtropical and tropical ponds and lakes. Reduced environmental fluctuations in subtropical and tropical systems (compared with temperate systems) may allow zooplankton populations to reach an environmental carrying capacity where competition limits the number of similar, coexisting species in a habitat.     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Daphnia exhibit diurnal vertical migration in shallow rock-pools

We report diurnal changes in vertical distribution ofDaphnia longispina andD. magna in shallow (25–60 cm) rock-pools on Baltic islands, southern Finland. During the daytime shallow rock-poolDaphnia reside in water layers close to the sediment while at midnight they either are evenly distributed throughout the water column, or have a modal peak of distribution in the surface water. Contrary to this pattern, no changes were observed in vertical distribution ofD. longispina at noon and at midnight in a 150 cm deep pond. We suggest that the pattern ofDaphina vertical migration in shallow rock-pools is not satisfactorily explained by predation.