The evolutionary history of the Chydoridae (Crustacea: Cladocera)

Although much is known about the evolutionary history of the pelagic 'cladocerans', there is little information on benthic families such as the Chydoridae. In this study, we examine the phylogenetic history of 37 chydorid species using sequence variation in two mitochondrial genes, COI and 16S rDNA, and one nuclear gene, 18S rDNA. The four recognized subfamilies of chydorids (Eurycercinae, Saycinae, Aloninae and Chydorinae) were well supported, being separated by large sequence divergences of 14.3–16.4%. By contrast, the existing taxonomic system appears to be less clear at a generic level, since many genera (e.g. Alona , Chydorus , Pleuroxus ) consist of an amalgam of distantly related species. However, among those genera which are monophyletic, levels of divergence are very high, suggesting that they originated somewhere in the mid-Palaeozoic. The factors involved in promoting diversification in this group are discussed.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79, 629–643.     

Eurycercus glacialis in Ireland (Cladocera,Chydoridae)

The taxon Eurycercus glacialis of Western Europe has been discovered living in the turloughs (tuar loch = dry lake) of Western Ireland. Only four populations were found during three field excursions, but because of the multitude of these waterbodies in the region, there must be many populations present. Turloughs are mainly periodic waterbodies occupying depressions in the drift overlying limestone. Most of them dry every year, and the bottoms are then used for grazing cattle. This periodicity prevents the establishment of fish populations, which would rapidly eliminate the cladoceran by size-selective predation. Only one of the many populations known from Western Europe survives in the presence of fishes. A species of the glacialis group in Newfoundland (not yet described), which is the southernmost known location of E. glacialis sens. lat. in the world, occurs commonly with salmonid or gasterosteid fishes, but this is a completely different species. Scanning electron micrographs of E. glacialis and E. lamellatus in Western Europe will aid in understanding their morphological differences.     

On age morphological changes of males of Chydoridae (Cladocera)

Summary Young and adult males of 11 species of Chydoridae are studied, their figures being published here (fig. 1–15). The necessity is stressed to distinguish young forms of males and gynandromorphic individuals.Pleuroxus balatonicus is considered to be described from the population ofPleuroxus unicatus having under Balaton Lake conditions retarded transformation of young males into adult form, and accordingly having unusually numerous young males. \qO\qs\qn\qo\qv\qn\qy\ye \qr\ye\qz\qu\ql\Qj\qt\qa\qt\qy 11 (. 1–15). . , Pleuroxus uncinatus , Pleuroxus balatonicus.     

Life histories of some species of Chydoridae (Cladocera: Crustacea)

SUMMARY 1. Life history parameters of two species of Chydoridae ( Leydigia leydigi and Disparalona rostrata ) were studied in the field and in the laboratory.
2. For both species the number of juvenile instars was variable and tentatively related to initial size of neonate. Egg volume increased with increasing parent length and with decreasing temperature; possible advantages gained by the offspring are discussed in relation to invertebrate predation.
3. The life history strategy employed by the littoral Chydoridae is compared to that of large and small planktonic cladocerans. Small planktonic cladocerans and the Chydoridae (except the Eurycercinae and the Saycinae) produce large young relative to their size at maturity and mature early. However, unlike the small planktonic cladocerans, growth in the Chydoridae is curtailed after the onset of reproduction and in this they resemble the large planktonic cladocerans. This strategy may be related to vertebrate predation and the presence of vegetation in the habitat.     

Subfossil chydorid (Cladocera, Chydoridae) ephippia as paleoenvironmental proxies: evidence from boreal and subarctic lakes in Finland

Chydorids (Cladocera, Chydoridae) have two reproductive strategies: asexual reproduction that prevails during favorable environmental conditions and sexual reproduction that is induced by environmental stimuli associated with seasonal or aperiodic environmental stresses. These modes of reproduction can be recognized in the subfossil sedimentary records as parthenogenetic shells of females (asexual reproduction) and by ephippia (sexual reproduction). We studied the interrelations between subfossil chydorid ephippia and environmental variables by analyzing surface sediment samples obtained from 76 Finnish lakes across a latitudinal gradient (60–70°N). The results showed that the total chydorid ephippia (TCE) increases along the climate gradient from ~2 to 3% in the south to ~25% in the north and suggested a significant dependence (r ~ −0.8, P < 0.001) with several climate factors, especially that of mean July air temperature. We used this relationship to create a model for reconstructing past mean July air temperatures. A linear regression of the log₁₀ transformed TCE as a single independent variable explained 76% (SE ± 0.76°C) of the variance of the observed mean July air temperatures. Accordingly, we propose that this novel tool may be highly suitable for reconstructing paleotemperatures in cold-temperate environments.     

Laboratory studies on the life cycle of Leydigia acanthocercoides Fisher (1854) (Cladocera: Chydoridae)

The biology of Leydigia acanthocercoides has been studied under laboratory conditions with reference to longevity, instar duration, growth, fecundity and embryonic development at a temperature range of 28–30 °C. It has three preadult and thirteen adult instars. Under the given laboratory conditions this species produces 20 eggs during a life span of 23 days. The number of eggs produced is uniformly constant in all adult instars. The growth rate seems to be exponential in the early phase of the life cycle as in other Cladocera. The general pattern of embryonic development of L. acanthocercoides is similar to those of other tropical cladocerans though differences in the duration of total developmental period have been recorded.Part of Ph.D. thesis.     

Alonella and Dunhevedia (Chydoridae,Cladocera): Morphology of trunk limbs

Summary The legs of some species of genera Alonella and Dunhevedia are described, including the type species of these genera. The structure of the endites and gnathobases is depicted and described in detail. The structure of legs of the studied species is compared with that of species of some other genera, including Eurycercus and Saycia.     

A New Species of the Chydorus sphaericus Group (Cladocera,Chydoridae) from Western Montana

Chydorus biovatus differs from C. sphaericus sens. str. in that the ephippial female carries two resting eggs, for which the brood pouch is expanded dorsally. resulting in a sharp angle in lateral contour near the junction of the head and shell and in the postpore distance being much elongated. The mature male has the pre-anal angle of the postabdomen rounded and the rostrum broadly rounded, both features being much different than in C. sphaericus sens. str. In North America there is a progressive replacement of C. brevilabris-group species in the South to C. sphaericus-group species in the Far North. Production of two resting eggs by Chydorus piger is recorded.     

A study of summer mortality factors for natural populations of Chydoridae (Cladocera)

In order to test the hypothesis that predation controls summer population growth of the Chydoridae in Lake Itasca, Minnesota two types of in situ predator-exclusion experiments were conducted. The first series of experiments excluded all predators from chydorid populations by enclosing the chydorids within clear plexiglass chambers suspended in their natural habitat. The second series of experiments excluded fish, but not invertebrates, from chydorids by placing polyethylene exclosures directly on the submerged vegetation. The concomitant abundance of natural chydorid populations was monitored by pattern sampling adjacent vegetation. Populations of Chydoridae that were protected from all predation increased exponentially, while natural populations remaining exposed to predation decreased dramatically. Populations protected from predation by fish but not invertebrates were not significantly different from unprotected control populations. Thus predation by fish and invertebrates combined was sufficient to limit chydorid density. The hypothesis that the predation was due primarily to fish was not supported. It is suggested that because chydorids are smaller and less accessible than preferred prey items, fish are able to significantly utilize them only during periods of high chydorid abundance.     

The reticulated species of Chydorus (Cladocera,Chydoridae): two new species with suggestions of convergence

Of the two small, compact, dusky, reticulated species of Chydorus from Sri Lanka, one arbitrarily has been selected as the taxon to which Daday's name reticulatus henceforward will be attached, the other herein being described as new. A third species from eastern North America, also described herein as new, resembles the two Asian species in certain gross features but otherwise is very distinct. Particularly noteworthy among these differences are the structure of the labrum and of the male postabdomen and copulatory hook. The question is raised whether the gross morphological features that seem to unite the taxa or the features of the labrum and of the male that separate them are the more conservative. No unequivocal answer is provided. Because of their obvious close similarity, the two Asian species are established as a species group, the one from North America being divergent and therefore not included. The possibility of convergence is considered.The North American taxon in north Florida occurs in waterbodies with a pH less than 5 (down to 4.2), conductivity less than 40 µS, and usually with no more than a trace of dissolved color. They are naturally acidic, non-bog lakes. The number of species of chydorids and of total littoral Cladocera in them are considerably greater than found by Fryer in waterbodies of comparable acidity in England. The occurrence also of a diversity of macrophytes and fishes in these lakes indicates that pH per se is not the factor forcing the decreasing diversity associated with changes resulting from acid precipitation.     

A contribution to the taxonomy ofC. sphaericus sens. lat. (Cladocera,Chydoridae)

Two distinct morphotypes ofChydorus sphaericus are recognised in Ireland. A reference collection of scanning electron micrographs of male and femaleC. sphaericus sens. str. has been compiled. This collection is used to compare the varying morphological features of Danish and Irish material to determine their taxonomic significance. However, no taxonomic decisions can be made about the Irish morphotypes until mature males and ephippial females become available. Comparison of population structures and stages of reproduction are accomplished by size-frequency distribution. This study illustrates the ever increasing value of scanning electron microscopy (SEM) in chydorid taxonomy, where characters such as shell-reticulation features may easily be misinterpreted using only light microscopy.     

The population dynamics of Chydoridae and Macrothricidae (Cladocera: Crustacea) from the River Thames, U.K.

SUMMARY.

• 1 A study was made of the population dynamics of benthic cladocerans in the River Thames, U.K. These included three species of Chydoridae (Alona affinis Leydig, Disparalona rostrata (Koch 1841) and Leydigia leydigi Schodler) and one species of Macrothricidae Iliocryptus sordidus (Lievan).
• 2 The pattern of chydorid abundance was similar to that found in many lakes: a midsummer peak followed by a sharp decrease in numbers and subsequent low abundance. This was caused by a temporary decrease in natality, possibly due to food limitation, and a more permanent increase in mortality, thought to be due to predation.
• 3 Iliocryptus sordidus differed considerably from the chydorids, the major population peak occurring in the autumn. The lifestyle and adult size of Iliocryptus sordidus (Lievan) must render adults less vulnerable to invertebrate and fish fry predation.
• 4 A literature survey showed that the assemblage of species found in the unvegetated littoral of lakes and rivers included a taxocene of cladocerans.

     

Reproduction in Laboratory Stocks of Pleuroxus (Chydoridae,Cladocera) Under the Influence of Photoperiod and Light Intensity

The effects of photoperiod and light intensity on the reproduction of laboratory populations of three species in Pleuroxus from different geographic origins were determined statistically. There were highly significant differences in the numbers of gamogenetic individuals produced between the populations at different photoperiods but not at different light intensities. More males than ephippial females appeared at long-day photoperiods. Pleuroxus denticulatus stocks showed greater variation of gamogenetic response to photoperiod and geographic origin suggesting that local populations have evolved different reproductive patterns to meet the major environmental stresses of the region.     

Discovery of groundwater-inhabiting Chydoridae (Crustacea: Cladocera), with the description of two new species

The groundwater along the Couze (= river) Pavin in Auvergne, France, yielded four chydorid Cladocera, among which two are known but rare species, while two are new species. One of the new species, Alona phreatica, has reduced eye and ocellus-sizes, but no other adaptations to a groundwater-mode of life are apparent. The likelihood that more phreatic species exist in other parts of Europe and elsewhere is discussed.     

The unique headpore of Monospilus dispar (Cladocera,Chydoridae) is not lonesome: new record of minute lateral pores

The headshield of Monospilus dispar (Cladocera, Chydoridae, Aloninae) was characterized by the presence of a unique headpore. Scanning Electron Microscopy shows the occurrence of minute pores close to the main headpore. The intimate structure of the main headpore is formed by concentric lamellae. This new information leads to new considerations about phylogeny.     

Responses of litter invertebrate communities to litter manipulation in a Japanese conifer plantation

We examined how the litter invertebrate communities were affected by the temporal changes in the mass and structural complexity of the litter resources by adding and removing litter on the forest floor of a temperate conifer plantation (Cryptomeria japonica) in Japan. We showed that litter mass and depth in the litter-addition (L+) plots changed rapidly into a steady-state condition similar to those in the control plots, mainly due to accelerated decomposition processes during the rainy season. Higher area-based densities of litter invertebrates in the L+ plots, similar mass-based densities between the L+ and control plots, and significant positive correlations between litter mass and the number of individuals implied that the abundance of litter invertebrates would be governed by litter mass rather than by the litter depth. Many litter invertebrates including detritivores were collected even in the litter-removal (L−) area. The relative abundances of invertebrate predators collecting pitfall traps were higher in the L− plots and lower in the L+ plots compared to those in the control plots, whereas those collecting Tullgren funnels were higher in the L+ plots than in the control plots. In the L+ plots, the range of variation in the community compositions among the samples decreased significantly over time in response to a drastic decrease in litter mass, in contrast to the control plots, which showed a relatively constant community composition during the study period. Our litter manipulation experiment reveals some of the mechanisms responsible for maintaining an equilibrium state of forest-floor litter mass and for the responses of litter invertebrate communities to temporal changes in the litter.