Larval photoresponses of the polyclad flatworm Maritigrella crozieri (Platyhelminthes,Polycladida) (Hyman)

Phototaxis by larvae of the flatworm Maritigrella crozieri was used to determine spectral sensitivity, the ontogeny of the phototactic pattern, and the lowest light intensity to induce a directional response (intensity threshold). Adult M. crozieri live in shallow water with the tunicate Ecteinascidia turbinata and have a planktonic larval phase lasting longer than 3 weeks. The primary spectral sensitivity maximum was at 500 nm, which is probably an adaptation to the spectrum available underwater at twilight. The phototactic threshold changed with age, as dark-adapted, 1-week-old larvae had a threshold (1.84×10¹⁷ photons m⁻² s⁻¹) an order of magnitude higher than that of 3-week-old larvae. Flatworm larvae are relatively insensitive to light as compared to other invertebrate larvae. Young larvae were positively phototactic at high light intensities and negative at low, a pattern typical of a predator avoidance shadow response. In contrast, older larvae were only positively phototactic, which would be useful for transport to shallow-water adult habitats.     

The embryonic development of the polyclad flatworm Imogine mcgrathi

In this paper we describe the embryonic development of the polyclad flatworm Imogine mcgrathi. Imogine is an indirect developer that hatches as a planctonic Goette’s larva after an embryonic period of approximately 7 days. Light and electron microscopic analyses of sections of staged embryos were combined with antibody stainings of wholemounted embryos to reconstruct the origin and movement of the primordia of the various organ systems, with particular emphasis on the nervous system. We introduce a system of morphologically defined stages aimed at facilitating future studies and cross-species comparisons among flatworm embryos. Imogine embryos undergo typical spiral cleavage. Micromere quartets 1–3 form an irregular double layer of mesenchymal cells that during gastrulation expands over micromere quartet 4. Micromere 4d divides into several large mesendodermal precursors whose position defines the ventral pole of the embryo. These cells, along with the animal micromeres that obtained a sub-surface position during cleavage, form a deep layer of cells that gives rise to all internal structures, including the nervous system, musculature, nephridia, and gut. Micromeres 4a–c are large yolky cells that are incorporated into the lumen of the gut, but do not themselves contribute to the gut epithelium. Shortly after gastrulation, cell differentiation sets in. Cells located at the surface adopt epithelial characteristics and form cilia that result in continuous movement of the post-gastrula stage embryo. Deep cells at the lateral margins of the embryo become organized into a protonephridial tube. A cluster of approximately 50 deep cells at the anterior pole forms the brain, in which we have identified sets of founder neurons of the brain commissure and the dorsal and ventral connectives. The early differentiating neurons, along with other cells forming stabilized microtubules (ciliated cells of the epidermis, gut and protonephridia; apical gland cells) could be analyzed in detail because of their labeling with an antibody against acetylated α-tubulin. Our findings indicate that, despite significant differences in the cleavage pattern and arrangement of blastomeres in the early embryo, morphogenesis and organ formation of a polyclad embryo follows a pattern that is very similar to the pattern observed by us and others in phylogenetically more evolved rhabdocoel flatworms. Received: 10 February 2000 / Accepted: 10 April 2000     

Embryonic and post-embryonic development of the Early Cambrian cnidarian Olivooides

Phosphatized specimens of Olivooides from the Early Cambrian of Shaanxi, China, represent a number of developmental stages. These include cleavage, gastrulation, organogenesis, cuticularization, pre-hatching, post-hatching and subsequent growth. This allows the reconstruction of a nearly full developmental sequence of this animal. Olivooides had large (600-870 μm in diameter), sphaerical eggs, indicating a high yolk content. Development was direct. Thus adult characters were forming already in the embryo, and there was no free larval stage. The embryonic development took place within a smooth protective membrane. Gastrulation probably was by polar ingression, and the blastopore appears to correspond to the aperture of the later stages. An embryonic cuticle formed which carried star-shaped structures, stellae, over the entire surface except for a radially folded non-stellate portion around the future aperture. At a later stage, the stellate cuticle was thrown into folds concentric with the aperture. This radially folded tissue then became more dominant. After hatching, the body assumed the shape of a strongly annulated cone, with the stellate cuticle forming the apical part and the folded cuticle forming a longitudinally striate cuticle around the aperture. Subsequent growth took place through the addition of striate tissue. A pentaradial symmetry of the body is suggested by lateral folds in the apical part. Olivooides is interpreted as a cnidarian, probably closely related to the scyphozoans. The conical test may have housed a polyp similar to the thecate polyps of modern coronate scyphozoans, but, unlike the latter, Olivooides had no visible attachment structures. There is no evidence for or against a free medusa stage. The prevalence of lecithotrophic direct developers in the Neoproterozoic and Cambrian, unless reflecting a preservational bias, casts some doubts on evolutionary models that assume larval planktotrophy to be primitive among metazoans.     

Ecological constraints, life history traits and the evolution of cooperative breeding

The ecological constraints hypothesis is widely accepted as an explanation for the evolution of delayed dispersal in cooperatively breeding birds. Intraspecific studies offer the strongest support. Observational studies have demonstrated a positive association between the severity of ecological constraints and the prevalence of cooperation, and experimental studies in which constraints on independent breeding were relaxed resulted in helpers moving to adopt the vacant breeding opportunities. However, this hypothesis has proved less successful in explaining why cooperative breeding has evolved in some species or lineages but not in others. Comparative studies have failed to identify ecological factors that differ consistently between cooperative and noncooperative species. The life history hypothesis, which emphasizes the role of life history traits in the evolution of cooperative breeding, offers a solution to this difficulty. A recent analysis showed that low adult mortality and low dispersal predisposed certain lineages to show cooperative behaviour, given the right ecological conditions. This represents an important advance, not least by offering an explanation for the patchy phylogenetic distribution of cooperative breeding. We discuss the complementary nature of these two hypotheses and suggest that rather than regarding life history traits as predisposing and ecological factors as facilitating cooperation, they are more likely to act in concert. While acknowledging that different cooperative systems may be a consequence of different selective pressures, we suggest that to identify the key differences between cooperative and noncooperative species, a broad constraints hypothesis that incorporates ecological and life history traits in a single measure of 'turnover of breeding opportunities' may provide the most promising avenue for future comparative studies. Copyright 2000 The Association for the Study of Animal Behaviour.     

Activity and habituation in the brain of the polyclad flatworm Freemania litoricola.

1. A variety of spontaneously active units was measured in the brain of the polyclad flatworm Freemania litoricola. Following application of MgCl2 there was both a decrease in number of active units and a decrease in frequency of firing of those cells which persisted in their activity. 2. Receptors which respond to vibration stimuli evoke potentials in the posterior part of the brain. Repetitive stimulation leads to habituation, the extent of which is dependent on both the number of times stimulated and the strength of the stimulus. Weaker stimuli habituate more rapidly than strong stimuli. Habituated responses can be dishabituated by tactile stimuli and also by stronger intensity stimuli of the same modality. The vibration-evoked potentials appear to occur in at least second-order cells, since vibration responses are abolished by the application of MgCl2. 3. Tactile responses can also be elicited from the posterior portion of the brain when the stimulus is applied to the periphery of the animal. These responses are insensitive to MgCl2. 4. Both vibration and tactile evoked responses are able to evoke further barrages of spike activity. 5. The presence of a dual sensitizing and inhibitory system during habituation is discussed.     

Life history of Xenodexia ctenolepis: implications for life history evolution in the family Poeciliidae

Xenodexia ctenolepis (Hubbs, 1950) is a uniquely asymmetrical species in the fish family Poeciliidae that is endemic to a remote region of Guatemala. In the present study, we describe its life history based on the dissection of 65 adult females from three different collections. We show that it is a livebearer, has superfetation, or the ability to carry multiple litters of young in different stages of development, and has matrotrophy, or placentation, which results in the dry mass of young at birth being three- to four-fold greater than the egg at fertilization. The size distribution of males is non-normal in a fashion that suggests a genetic polymorphism for age and size at maturity. Most phylogenies place Tomeurus gracilis as the sister taxon to the remaining members of the family Poeciliidae. Because Tomeurus is the sole egg-layer in the family, egg-laying is thought to represent the life history of the common ancestor. Because Xenodexia possesses three supposed derived traits (livebearing, superfetation and matrotrophy), this phylogenetic hypothesis suggests that Xenodexia has a highly derived life history with respect to other members of the family. By contrast, the most recent DNA-based phylogeny suggests Xenodexia is the sister taxon to the remainder of the family. If this proves to be true, it suggests that some or all of these life history traits may have been characteristic of the common ancestor to the family, then lost and re-evolved multiple times within the family.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 77–85.     

Thermal evolution of pre-adult life history traits in Drosophila melanogaster

Replicated lines of Drosophila melanogaster were allowed to evolve in population cage culture at 16.5° C or 25° C for five years. Their larval and pupal development times, larval growth rates, larval critical weights for pupariation and pre-adult survival rates were then measured at both temperatures. Pre-adult survival showed evidence of adaptation of the lines to their thermal selection regimes, with each set of lines showing superior survival when tested at the temperature at which they had been evolving. Pupal periods were similar for all lines when growing at 16.5° C but, at 25° C, the low temperature lines had the longer pupal periods. Irrespective of experimental temperature, low temperature lines grew faster and had shorter larval development periods than the high temperature lines. Larval critical weights for pupariation were higher in the low temperature lines at the low experimental temperature, and higher in the high temperature lines at the higher experimental temperature. The correlations between these traits induced by thermal evolution were in general different from or opposite to the genetic correlations found within a single temperature.     

Dental development and the evolution of life history in Hominidae

Development of the dentition is critically integrated into the life cycle in living mammals. Recent work on dental development has given rise to three separate lines of evidence on the evolution of human growth and aging; these three, based on several independent studies, are reviewed and integrated here. First, comparative study of living primate species demonstrates that measures of development (e.g., age of emergence of the first permanent molar) are highly correlated with the morphological attributes brain and body weight (as highly as r = 0.98, N = 21 species). These data predict that small-bodied, small-brained Australopithecus erupted M₁ at 3–3.5 years and possessed a life span comparable to that of a chimpanzee. Second, chronological age at death for three australopithecines who died at or near emergence of M₁ is now estimated as ～3.25 years based on incremental lines in teeth; this differs substantially from expectations based on human growth schedules (5.5–6 years). Third, developmental sequences (assessed by the coefficient of variation of human dental age) observed in gracile Australopithecus and great apes diverge from those of humans to a comparable degree; sequences become more like modern humans after the appearance of the genus Homo. These three lines of evidence agree that the unique rate and pattern of human life history did not exist at the australopithecine stage of human evolution. It is proposed that the life history of early Homo matched no living model precisely and that growth and aging evolved substantially in the Hominidae during the last 2 million years.     

Embryonic and post-embryonic development of the parietal and visceral peritoneum in white mice]

Light and electron microscopy were used in order to investigate histogenesis of the parietal and visceral peritoneum of white mice in embryonic and postembryonic periods of development. Four periods were distinguished, during which gradual differentiation of the primordium material into tissue structures (mesothelium and the connective tissue) of the peritoneum were observed. Asynchronous differentiation of the mesothelium as well as certain correlation in the degree of differentiation of mesothelial and mesenchymal cells took place at all stages of the embryonic and postembryonic development. More differentiated cells of prolonged shape were predominant in the mesenchyma even at early stages (11 days) in those portions where the lining of the secondary cavity of the body resembled mesothelim in its structure.     

Low humidity reduces ectoparasite pressure: implications for host life history evolution

A parasite's potential effect, or "pressure", can influence the life history strategy of its host. In environments with high parasite pressure, hosts invest more in anti-parasite defense, which may limit their investment in other life history components, such as survival. This tradeoff is difficult to study in natural populations because pressure is hard to quantify. Pressure is not necessarily correlated with the abundance of the parasite. A host population can be under high pressure, yet have few parasites, because members of the population have invested heavily in defense. Therefore, the extent to which parasite pressure varies among host populations, and the cause of such variation, remain largely undocumented. In this paper we show that birds in arid regions have fewer ectoparasitic lice than birds in humid regions. We show experimentally that low humidity reduces the number of lice on birds, even when host defense is held constant. Comparisons of ambient humidity to humidity beneath the plumage demonstrate that plumage does not provide a buffer for lice against low humidity. Our results confirm that an abiotic factor can cause substantial variation in parasite pressure among host populations. We suggest that humidity may influence host life history evolution through its impact on ectoparasites.     

Regulation of male traits by testosterone: implications for the evolution of vertebrate life histories

The negative co-variation of life-history traits such as fecundity and lifespan across species suggests the existence of ubiquitous trade-offs. Mechanistically, trade-offs result from the need to differentially allocate limited resources to traits like reproduction versus self-maintenance, with selection favoring the evolution of optimal allocation mechanism. Here I discuss the physiological (endocrine) mechanisms that underlie optimal allocation rules and how such rules evolve. The hormone testosterone may mediate life-history trade-offs due to its pleiotropic actions in male vertebrates. Conservation in the actions of testosterone in vertebrates has prompted the 'evolutionary constraint hypothesis,' which assumes that testosterone signaling mechanisms and male traits evolve as a unit. This hypothesis implies that the actions of testosterone are similar across sexes and species, and only the levels of circulating testosterone concentrations change during evolution. In contrast, the 'evolutionary potential hypothesis' proposes that testosterone signaling mechanisms and male traits evolve independently. In the latter scenario, the linkage between hormone and traits itself can be shaped by selection, leading to variation in trade-off functions. I will review recent case studies supporting the evolutionary potential hypothesis and suggest micro-evolutionary experiments to unravel the mechanistic basis of life-history evolution.     

The impact of diapause on the evolution of other life history traits in flesh flies

Summary Flesh flies (Sarcophagidae) collected in Costa Rica and Panama lack the pupal diapause that is characteristic of flesh flies from the temperate zone and tropical Africa. The absence of a diapause capacity in the neotropical species correlates with several other life history traits: in most species the post feeding wandering phase of the third larval instar is longer and duration is more variable, adult life is longer, clutch size is smaller, and more clutches are produced. Among species that have the capacity for diapause, risk is invested primarily in the diapausing stage and other life stages are brief. Though diapausing species are short-lived, they produce as many or more progeny than nondiapausing species by increasing clutch size. The slower and more variable developmental rate and increased adult longevity desynchronizes development and permits the nondiapausing species to spread an environmental risk over different stages of the life cycle, thus offering an alternative to diapause. Other traits such as body size, developmental velocity, thermal constant thresholds, thermal constants, age at first reproduction, and the interval between clutches do not appear related to the capacity for diapause.     

The Maturity Index,the evolution of nematode life history traits,adaptive radiation and cp-scaling

Nematodes are increasingly being used in environmental studies. One of the potential parameters to measure the impact of disturbances and to monitor changes in structure and functioning of the below-ground ecosystem is the nematode Maturity Index; an index based on the proportion of colonizers (r-strategists s.l.) and persisters (K-strategists s.l.) in samples. In this paper the original allocation of nematode taxa on the colonizer-persister scale, and the tolerance and sensitivity of colonizers and persisters are discussed from an evolutionary viewpoint. The phenomenon that neither relative egg size nor body length is an unequivocal character to scale nematodes suggests that the main selection for life history traits occurred independently in the major evolutionary branches. This revised version was published online in June 2006 with corrections to the Cover Date.     

Prey preferences of the polyclad flatworm Prostheceraeus roseus among Mediterranean species of the ascidian genus Pycnoclavella

In the present study we analyzed prey preferences of the polyclad flatworm Prostheceraeus roseus among three different species of colonial ascidians of the genus Pycnoclavella occurring sympatrically in the Northwestern Mediterranean (Spain). Palatability assays were conducted in laboratory conditions in order to test predator preferences in pairwise tests, and cycles of abundance of the predator and prey were monitored in the field. The results showed a clear preference of the predator for Pycnoclavella communis over Pycnoclavella nana and Pycnoclavella aurilucens. We suggest that chemical variation in defense compounds among species of this secondary metabolite-rich genus can drive the flatworm preferences. The ascidian had seasonal cycles in the area studied, with resting (aestivation) states in the summer months. The flatworm abundance showed no clear seasonal cycle, but it was less abundant in winter. The predator has been seen in the field feeding either on active zooids or on the reserve-laden basal mass of tunic during the aestivation phase of P. communis. Handling editor: I. Nagelkerken     

The post-embryonic development and life history of the millipede, Ommatoiulus moreletii (Diplopoda: Iulidae), introduced in south-eastern Australia

The life cycle of Ommatoiulus moreletii consists of an egg, pupoid and up to sixteen stadia. Sexes are differentiated at the sixth stadium. Males may mature at any moult from the eighth to the twelfth stadia, but most are mature by the tenth or eleventh. Maturation is more difficult to determine in females but seems similar to the males with respect to stadial age. O. moreletii is periodomorphic.
Field studies were made of O. moreletii in an open grassland and a dry sclerophyllous woodland in South Australia. Females matured their eggs in late summer-autumn. They mated and oviposited during autumn-winter. After one year, O. moreletii was in the seventh, eighth or ninth stadium. After two years, the tenth or eleventh stadia were reached and after three years the twelfth or thirteenth. The moulting of individuals older than about one year was confined to moults in (1) spring and (2) summer. Adult males were mature from summer to spring and intercalary from spring to summer.     

Characterizing the biological traits and life history of Acanthopagrus (Sparidae) hybrid complexes: implications for conservation and management

This study examined the spatio‐temporal reproductive patterns, population structure, maturity and growth of Acanthopagrus hybrid complexes, which comprise mainly black bream Acanthopagrus butcheri and the hybrids they form with yellowfin bream Acanthopagrus australis, in two Australian estuaries (Coila and Brou Lakes, NSW). There were no differences between pure A. butcheri and hybrids in terms of their population structure, growth and maturity, suggesting that these two breeds have similar life histories and may therefore be managed as single, naturally cohesive units. Sexual variation in size structure was only observed for the complex in Coila Lake. Although there was significant variation in age structure between estuaries, both complexes exhibited dominance of only a few year classes indicating episodic recruitment and high fishing mortality. Acanthopagrus eggs occurred in higher numbers within upstream creek habitats compared to lake habitats, thereby highlighting the need to protect these spawning habitats. Although maturity in the complexes was not influenced by gender or estuary, differences in growth were detected between the genders and estuaries.