Sexual dimorphism in calanoid copepods: morphology and function

Mate location and recognition are essentially asymmetrical processes in the reproductive biology of calanoid copepods with the active partner (the male) locating and catching the largely passive partner (the female). This behavioural asymmetry has led to the evolution of sexual dimorphism in copepods, playing many pivotal roles during the various successive phases of copulatory and post-copulatory behaviour. Sexually dimorphic appendages and structures are engaged in (1) mate recognition by the male; (2) capture of the female by the male; (3) transfer and attachment of a spermatophore to the female by the male; (4) removal of discharged spermatophore(s) by the female; and (5) fertilization and release of the eggs by the female. In many male calanoids, the antennulary chemosensory system is enhanced at the final moult and this enhancement appears to be strongly linked to their mate-locating role, i.e. detection of sex pheromones released by the female. It can be extreme in calanoids inhabiting oceanic waters, taking the form of a doubling in the number of aesthetascs on almost every segment, and is less expressed in forms residing in turbulent, neritic waters. Mate recognition is a process where chemoreception and mechanoreception presumably work in conjunction. The less elaborate male chemosensory system in the Centropagoidea is counterbalanced by females playing a more active role in generating hydromechanical cues. This is reflected in females in the shape of the posterior prosomal margin, the complexity of urosomal morphology and the size of the caudal setae. Visual mate recognition may be important in the Pontellidae, which typically show sexual dimorphism in eye design. The most distinctive sexual dimorphism is the atrophy of the mouthparts of non-feeding males, illustrating how copepod detection systems can be shifted to a new modality at the final moult. In the next phase, the male captures the female using the geniculate antennule and/or other appendages. Three types of antennulary geniculations are recognized, and their detailed morphology suggests that they have originated independently. Grasping efficiency can be enhanced by the development of supplemental hinges. The scanty data on capture mechanisms in males lacking geniculate antennules are reviewed. It is suggested that the loss of the antennulary geniculation in many non-centropagoidean calanoids has evolved in response to increasing predator pressure imposed on pairs in amplexus. Spermatophore transfer and placement are generally accomplished by the modified leg 5 of the male. In some males, leg 5 consists of both a chelate grasping leg and a spermatophore-transferring leg, whereas in others, only the latter is developed. Tufts of fine setules/spinules and/or sclerotized elements on the terminal portion of the leg are involved in the transfer and attachment of the spermatophore. The configuration of gonopores, copulatory pores and their connecting ducts in the female genital double-somite is diversified in the early calanoid offshoots such as Arietellidae and Metridinidae, whereas in more derived groups, it is constant and invariable, with paired gonopores and copulatory pores located beneath a single genital operculum. The absence of seminal receptacles in most Centropagoidea limits the female's ability to store sufficient sperm for multiple egg batches, suggesting that repeated mating is necessary for sustained egg production. Discharged spermatophores are usually removed by the female leg 5 and/or specialized elements on other legs. In Tortanus (Atortus) Ohtsuka, which has rudimentary fifth legs in the female and complex coupling devices in the male, a spermatophore supposedly remains on the female urosome, since eggs appear to be released from a ventral opening of the spermatophore. The type of sexual dimorphism is closely related to habitat and biology. Some hyperbenthic families never show multiplication of aesthetascs on the male antennule, whereas families of the open pelagic realm such as the Aetideidae always have non-feeding males exhibiting secondary multiplication of antennulary aesthetascs. The various aspects and diversity of calanoid sexual dimorphism are herein considered in an evolutionary context.     

Daytime grazing and assimilation rates of planktonic copepods Acanthodiaptomus denticornis and Cyclops vicinus vicinus. Comparison of spatial and resource utilisation by rotifers and cladoceran communities in a eutrophic lake

The clearance rates of ¹⁴ carbon-labelled Chlamydomonas sp. by the copepodite and adult stages of the 2 dominant species of Copepoda were studied in eutrophic Lake Aydat. They varied from 30 to 3,007 µl ind^–1 h^–1 for Acanthodiaptomus denticornis (mean = 600 µl ind^–1 h^–1) and from 20 to 1,133 µl ind^–1 h^–1 for Cyclops vicinus (mean = 340 µl ind^–1 h^–1). Their maximum assimilation efficiencies were 58% and 50%, respectively. These populations collectively could consume the available food in 4 days during September (maximum daily grazing rate = 24%). Mean individual clearance rates could be ranked Acanthodiaptomus denticornis > Ceriodaphnia quadrangula > Chydorus sphaericus > Daphnia longispina > Cyclops vicinus vicinus > Bosmina longirostris > K. cochlearis > K. quadrata and Kellicottia longispina. Like cladocerans and rotifers, the copepods living in this eutrophic lake can feed at low oxygen concentrations.     

Sexual selection in fungi

The significance of sexual selection, the component of natural selection associated with variation in mating success, is well established for the evolution of animals and plants, but not for the evolution of fungi. Even though fungi do not have separate sexes, most filamentous fungi mate in a hermaphroditic fashion, with distinct sex roles, that is, investment in large gametes (female role) and fertilization by other small gametes (male role). Fungi compete to fertilize, analogous to ‘male‐male’ competition, whereas they can be selective when being fertilized, analogous to female choice. Mating types, which determine genetic compatibility among fungal gametes, are important for sexual selection in two respects. First, genes at the mating‐type loci regulate different aspects of mating and thus can be subject to sexual selection. Second, for sexual selection, not only the two sexes (or sex roles) but also the mating types can form the classes, the members of which compete for access to members of the other class. This is significant if mating‐type gene products are costly, thus signalling genetic quality according to Zahavi's handicap principle. We propose that sexual selection explains various fungal characteristics such as the observed high redundancy of pheromones at the B mating‐type locus of Agaricomycotina, the occurrence of multiple types of spores in Ascomycotina or the strong pheromone signalling in yeasts. Furthermore, we argue that fungi are good model systems to experimentally study fundamental aspects of sexual selection, due to their fast generation times and high diversity of life cycles and mating systems.     

Sexual Selection and Mate Choice

After a long period of dormancy, Darwin's theory of sexual selection in general, and mate choice in particular, now represents one of the most active fields in evolutionary research. After a brief overview of the history of ideas and a short introduction into the main mechanisms of sexual selection, I discuss some recent theoretical developments and empirical findings in the study of mate choice and review the various current models of mate choice, which can be grossly divided into adaptive models and nonadaptive models. I also examine whether available primate evidence supports various hypotheses concerning mate choice. Although primatologists were long aware that nonhuman primates have preferences for certain mating partners, until recently the functions and evolutionary consequences of their preferences remained obscure. Now there is growing evidence that mate choice decisions provide primates with important direct or indirect benefits. For example, several observations are consistent with the hypothesis that by direct or indirect mate choice female primates lower the risk of infanticide or enhance the chance of producing viable offspring. Nevertheless, there are also significant holes in our knowledge. How the male mandrill, one of Darwin's famous examples, got his brightly colored face, is still unknown.     

Random mating with respect to mating status in the simultaneously hermaphroditic land snail Arianta arbustorum

In promiscuous species with sperm storage, males are expected to show a preference for mating with virgin and young females to reduce the risk of sperm competition. In the simultaneous hermaphrodite land snail Arianta arbustorum, sperm production precedes egg production by 2–4 weeks, resulting in a short period of protandric hermaphroditism before shell growth is completed. In natural populations, copulating pairs involving individuals which have not yet completed shell growth (virgins) have been observed. We ran a series of mate-choice experiments to examine whether virgin and nonvirgin (experienced) individuals of A. arbustorum discriminate between virgin and nonvirgin mating partners. We also assessed the number of sperm delivered to partners with different mating status. Neither virgin nor nonvirgin snails showed any preference for mating with a virgin partner. In all test situations mating was random and the number of sperm delivered was not adjusted to the mating status of the partner. Mating success was mainly determined by the activity of the individual. The random mating pattern does not imply random fertilization of eggs because the presence of a sperm-digesting organ and the morphology of the sperm storage organ allow a selective storage and use of sperm in A. arbustorum.     

Cuticular hydrocarbons are heritable in the cricket Teleogryllus oceanicus

The ability of individuals to respond differentially to conspecifics depending on their genetic relatedness is a widespread phenomenon across the animal kingdom. Despite this, little is known about the selection processes that act on the phenotypic variation of traits used during recognition. Here we use a quantitative genetic approach to examine the patterns of genetic variation in cuticular hydrocarbon (CHC) profiles, a pheromonal system used extensively in insect communication. Using gas chromatography, we found family specificity in the CHC profiles of male crickets, Teleogryllus oceanicus. Across CHC peaks, our mean coefficient of additive genetic variation was 10.8%. Multivariate principal component analysis showed that most axes of variation were weighted by CHC peaks with significant additive genetic variation. Our results provide evidence that variation in CHC profiles can reflect genetic relatedness, supporting the widely held belief that this phenotypic trait is used as a mechanism for chemosensory kin recognition.     

The rotifers of Lake Peipus

In the northern part of Lake Peipus, 140 taxa of rotifers were identified, with species of Anuraeopsis, Conochilus, Keratella, Polyarthra and Synchaeta dominating. Two main periods of sexual reproduction occur, in the spring and autumn. Different life cycle patterns are represented. Rotifer number and biomass have two maxima between spring and early autumn. The contribution of rotifers to total zooplankton production varies from 13.6% (Oct.) to 89.8% (May). The average production of grazing rotifers is 485.1 kJ m^–2, while that of predatory rotifers (Asplanchna) is 10.0 kJ m^–2.     

Sexual cannibalism by Octopus cyanea on a Pacific coral reef

Sexual cannibalism is rare in the animal kingdom. Although cannibalism is not uncommon in cephalopods, here we report the first documented case of sexual cannibalism. A large female Octopus cyanea was observed continuously for 2.5 days in Palau, Micronesia, when she was out of her den. On the second day, a small male followed and mated her 13 times during 3.5 h while she continued to forage over 70 m distance. After the 12th mating, she aggressively chased a different small octopus that barely escaped by jetting, inking and swimming upwards. Shortly thereafter, the original small male mated her a 13th time, but subsequently she attacked and suffocated him and spent 2 days cannibalizing him in her den. This sort of intraspecific aggression helps to explain several reports of octopuses mating out in the open, a behaviour that may serve to allow the smaller mate to escape cannibalism.     

Sexual cannibalism by Octopus cyanea on a Pacific coral reef

Sexual cannibalism is rare in the animal kingdom. Although cannibalism is not uncommon in cephalopods, here we report the first documented case of sexual cannibalism. A large female Octopus cyanea was observed continuously for 2.5 days in Palau, Micronesia, when she was out of her den. On the second day, a small male followed and mated her 13 times during 3.5 h while she continued to forage over 70 m distance. After the 12th mating, she aggressively chased a different small octopus that barely escaped by jetting, inking and swimming upwards. Shortly thereafter, the original small male mated her a 13th time, but subsequently she attacked and suffocated him and spent 2 days cannibalizing him in her den. This sort of intraspecific aggression helps to explain several reports of octopuses mating out in the open, a behaviour that may serve to allow the smaller mate to escape cannibalism.     

Epizoic and parasitic rotifers

Many rotifer species live in close association with plants or other animals. Most of these associations are of a commensal or synoecious nature, some rotifer species having lost the ability to live independently. Few rotifers are true parasites, actually harming their hosts.The Seisonidae, Monogononta and Bdelloidea include epizoic and parasitic species. The most widely known are probably the parasites of colonial and filamentous algae (e.g. Volvox, Vaucheria). However, rotifers are also found on a wide range of invertebrates: colonial, sessile Protozoa; Porifera; Rotifera; Annelida; Bryozoa; Echinodermata; Mollusca, especially on the shells and egg masses of aquatic gastropods; Crustacea, including the lower forms (e.g. Daphnia, Asellus, Gammarus) and in the gill chambers of Astacus and Chasmagnathus; the aquatic larvae of insects. There appear to be few records of epizoic or parasitic rotifers among vertebrates, apart from Encentrum kozminskii on carp, Limnias ceratophylli on the Amazonian crocodile, Melanosuchus niger, and an unidentified Bdelloid apparently living as a pathogenic rotifer in Man.     

Mating behavior and mate recognition pheromone blocking of male receptors in Brachionus plicatilis Müller (Rotifera)

Copulatory behavior of three S and three L type B. plicatilis strains from different geographic areas was analyzed. A 29 KD surface glycoprotein on females, characterized as a Mate Recognition Pheromone (MRP), binds to receptors in the corona of males and blocks mate recognition. Blocking was observed in all S and L strains even though the MRP was isolated from a single L-type strain. Binding was quantified using image analysis and a 20-fold difference was observed among strains. A direct relationship between the male discrimination of females and the intensity of MRP binding to male receptors was found. This relationship might be useful as a tool to examine variation in the mate recognition systems of other rotifer species.     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

Models of female choice in acoustic communication

In this article we present generalized theoretical models ofmate attraction in acoustic communication.The models use simpledecision rules for females and show theoretical differencesbetween the rules in relative attraction of females to soundsources (calling males). The important variables of the modelsare the relative powers of the sources, the distance betweenthe sources and the number of competing signalers.Relative attractionamong sources is a function of their relative power output withmore powerful signals attracting more females. Using differentassumptions about the spacing of sources with respect to femalesensitivity, data from phonotaxis studies can be explained wellby either random pick or active choice models. Bioacousticiansmust reexamine their use of, and conclusions based on, the two-choiceplayback paradigm in studying female mate choice.     

Mating asymmetry and the direction of evolution in the Hawaiian cricket genus Laupala

Based on studies from native Hawaiian Drosophila, a model was proposed to explain sexual isolation and mating asymmetry, from which one could potentially infer the 'direction of evolution'. We examined sexual isolation between allopatric cricket species of the genus Laupala, another endemic Hawaiian insect with an elaborate mating system, to begin to explore the nature of sexual isolation and mating asymmetry in closely related Hawaiian organisms. We studied sexual isolation and mating asymmetry in two contrasts. First, an inter-island comparison, including L. makaio from the older island of Maui and L. paranigra from the younger island of Hawaii, and second, an intra-island (Hawaii) comparison, including L. nigra from the older volcano of Mauna Kea and L. paranigra with a primary distribution on the younger volcanoes of Mauna Loa and Kilauea. We used a 'no-choice' experimental design, pairing individual males and females in homospecific or heterospecific combinations. Several behavioural aspects of courtship (proportion of male singing, latency to male singing, production of spermatophores and courtship initiation speed) were quantified as well as the success or failure of matings. We demonstrate asymmetry in sexual isolation between reciprocal combinations of L. makaio and L. paranigra. This result is examined in light of the differences in courtship behaviour manifest in the experiments with these two species. We did not find evidence of asymmetry in sexual isolation between L. nigra and L. paranigra, although differences in courtship initiation speed were evident between reciprocal combinations of these two species. In addition to the geological argument that species on older islands and older volcanoes give rise to species on younger islands and younger volcanoes, we discuss phylogenetic evidence consistent with these biogeographic hypotheses of relationships among the focal taxa. The patterns of asymmetrical sexual isolation and mating asymmetry are consistent with those found in the native Hawaiian Drosophila.     

Vibrational Communication in Two Sympatric Species of Mantophasmatodea (Heelwalkers)

The communication via percussion of the abdomen on the substrate for species recognition and mate location of males and females of two sympatric species of the recently described insect order Mantophasmatodea (Heelwalkers) was investigated. Each sex produced a single and distinctive call. The female call consisted of repeated single pulses, whereas the more complex male call comprised repeated pulse trains. The calls of males and females of the two species were of similar general structure, but differed in most temporal characters such as pulse and pulse train repetition time. In behavioral playback experiments females reacted to the call of conspecific males by calling and decreasing locomotion. When stimulated with the call of the heterospecific, sympatric male, females showed no reaction. Males exhibited abdominal rubbing, high tapping rates, increased activity (both movement and active searching) as well as characteristic searching behavior at branch nodes, when presented with the conspecific female call. Being stimulated with the playback of the heterospecific female call and the conspecific male call respectively, males responded with less intense locomotor and searching behavior. The drumming behavior in the control situation (no playback) suggests that males sometimes call in the absence of other individuals.     

The significance of mating processes for the genetics and for the formation of resting eggs in monogonont rotifers

Crossbreeding experiments with three geographically distinct strains (E, S, and L) of the rotifer Brachionus plicatilis have been carried out in an attempt to elucidate the apparent male sterility of strain L, which is unable to produce resting eggs.The 9 crossing possibilities of the three strains have been investigated in 27 experiments. The results refute the concept of male sterility. L-males copulate successfully with mictic as well as with amictic females of strains E and S. Fertilized amictic E and S females produce defective resting eggs, which have only one thin shell and, which disintegrate after deposition. L-females cannot, as a rule, be fertilized. Moreover, crosses between strain E and S are succesful only in one direction; the reciprocal crosses failed.To explain the present results a hypothesis is suggested that the thickness of the body wall of newborn females differs in the three strains, and between mictic and amictic individuals. A sequence of gradually increasing body wall thickness of all types of females involved, together with a comparably increasing penetration ability of the males of strain E, S, and L explains the success or failure of all crosses, including the unilateral cross E × S.     

Relation to habitat in rotifers

Rotifera should be especially suited for an analysis of habitat relations because this group contains such a high number of species, inhabiting diverse environments. Furthermore, rotifers are to a large extent cosmopolitan, implying that ecological barriers, rather than geographical, are decisive of their distribution. In this review a short characterization of the rotifer fauna in different habitats is given, whereby macroenvironments and microenvironments are reported separately. The macroenvironments are classified as follows: harmonious lakes and ponds, arctic and antarctic waters, hot springs, hypertrophic-saprobic environments, mires, strongly acidic waters, saline waters, temporary water bodies, subterranean waters, running waters, oceans, terrestrial environments. The following microenvironments are distinguished: macrophytes (housing periphytic rotifers), open water (with planktic forms), minerogenous sediments (with psammon and hyporheos), organogenous sediments, other organisms (i.e. parasites and epizoans).Many rotifers are more or less euryecious, while relatively few are strongly restricted in their choice of habitat. In extreme environments a low number of species is found, but often a high number of individuals within these species. These rotifers are usually primary consumers, and for natural reasons extreme environments are characterized by a low number of trophic levels.In environments with a high species number the separate species differ very much in their morphology, making it difficult to find common traits which may be interpreted as adaptations to the respective habitats. The most apparent adaptations ought to be found among the planktic rotifers, and these adaptations seem to constitute largely a protection against predators. Rotifers in extreme environments are usually not very apart in a morphological or taxonomical respect, with their most close relatives living in normal habitats and sometimes euryecious (an apparent exception from this rule is formed by the class Seisonidea). Adaptations to deviating chemical and physical environments may develop relatively rapidly (seen from a geological perspective), while the more fundamental changes (occurring during a longer period of time) seem to be a response to biotic factors (e.g., the development of different types of trophi for facilitating food collection).     

Female discrimination thresholds frequently exceed local male display variation: implications for mate choice dynamics and sexual selection

Among the factors that can influence female mate choice decisions is the degree to which females differentiate among similar displays: as differences decrease, females are expected to eventually stop discriminating. This discrimination threshold, in conjunction with the magnitude of male trait variation females regularly encounter while making mate choice decisions, may have important consequences for sexual selection. If local display variation is above the discrimination threshold, female preferences should translate into higher mating success for the more attractive male. But if display variation is frequently below the threshold, the resulting increased pattern of random mating may obscure the existence of female mate choice. I investigated the interplay between female discrimination and male display variation in green treefrogs (Hyla cinerea) and found that call trait differences between nearest neighbour males were frequently smaller than what females are expected to discriminate. This finding has two important consequences for our understanding of sexual selection in the wild: first, low display variation should weaken the strength of selection on male display traits, but the direction of selection should mirror the one predicted from females choice trials. Second, caution is needed when interpreting data on realized mating success in the wild: a pattern of random mating with respect to male display traits does not always mean that female preferences are weak or that conditions are too challenging for females to express their preferences. Rather, insufficient display variation can generate the same pattern.