Ecological and evolutionary consequences of linked life-history stages in the sea

Naturalists and scientists have been captivated by the diversity of marine larval forms since they were discovered following the advent of the microscope. Because they often bear little resemblance to adults, larvae were identified initially as new life forms, classified into different groups based on the similarity of their body plans and given new names that are still with us today. The radically different body plans and lifestyles of marine larvae and adults have led most investigators historically to study the two phases of complex life cycles in isolation. More recently, important ecological insights have sprung from taking a holistic view of marine life cycles. Meanwhile, the evolutionary (phenotypic and genetic) links among life-history phases remain less appreciated. In this review, our objective is to evaluate the evolutionary links within marine life cycles, and explore their ecological and evolutionary consequences. We provide a brief overview of marine life histories, discuss the phenotypic and genetic links between the two phases of the life cycle and pose challenges to advance our understanding of the evolutionary constraints acting on marine life histories.     

Animal behaviour shapes the ecological effects of ocean acidification and warming: moving from individual to community‐level responses

Biological communities are shaped by complex interactions between organisms and their environment as well as interactions with other species. Humans are rapidly changing the marine environment through increasing greenhouse gas emissions, resulting in ocean warming and acidification. The first response by animals to environmental change is predominantly through modification of their behaviour, which in turn affects species interactions and ecological processes. Yet, many climate change studies ignore animal behaviour. Furthermore, our current knowledge of how global change alters animal behaviour is mostly restricted to single species, life phases and stressors, leading to an incomplete view of how coinciding climate stressors can affect the ecological interactions that structure biological communities. Here, we first review studies on the effects of warming and acidification on the behaviour of marine animals. We demonstrate how pervasive the effects of global change are on a wide range of critical behaviours that determine the persistence of species and their success in ecological communities. We then evaluate several approaches to studying the ecological effects of warming and acidification, and identify knowledge gaps that need to be filled, to better understand how global change will affect marine populations and communities through altered animal behaviours. Our review provides a synthesis of the far‐reaching consequences that behavioural changes could have for marine ecosystems in a rapidly changing environment. Without considering the pervasive effects of climate change on animal behaviour we will limit our ability to forecast the impacts of ocean change and provide insights that can aid management strategies.     

Biodiversity among haptophyte algae

The division Haptophyta is represented only by about 300 extant species showing wide diversity in morphology, biochemistry and ecology. They have a world-wide distribution and are numerically important in phytoplankton populations in nearly all marine environments. Evidence from the geological record shows that they have been the major constituent of calcareous deposits since the Late Triassic and, as they have evolved quickly through time, their coccoliths have always shown wide morphological diversity. In today's oceans they occasionally produce extensive blooms, visible by satellite imagery, which have ecological impact. As a consequence of these blooms the haptophyte algae are now receiving greater attention, as their role in the global sulphur and carbon cycles may influence the world's climate, and their potential as nuisance bloom algae have implications for commercial fishing and the marine ecosystem. As it is likely that these organisms have always produced such blooms, these effects may have been in operation for the last 200 million years.     

What has happened to the “aquatic phycomycetes” (sensu Sparrow)? Part II: Shared properties of zoosporic true fungi and fungus-like microorganisms

Many species of zoosporic heterotrophic parasites, saprotrophs and mutualists in the Phyla Perkinsozoa (dinoflagellates), Oomycota, Hyphochytriomycota, Labyrinthulomycota and Phyomyxea share morphological characteristics with zoosporic true fungi especially with some of the Chytridiomycota and with fungus-like organisms in the Phyla Mesomycetozoea, Chytridiomycota and Aphelidae. These characteristics include chemotactic motile zoospores, zoosporangia which produce zoospores, thick walled resistant cysts, rhizoid-like structures, hyphal-like structures and cell walls surrounding the cells in several phases of their life cycle. These assemblages also inhabit both marine and freshwater ecosystems in which aquatic fungi and fungus-like organisms are found, have similar life cycles, grow on similar substrates, use similar infection strategies and infect some of the same host plants and animals. Many of these species were once included in the aquatic phycomycetes, an ecological assemblage of microorganisms but not a valid taxonomic group. Some of the shared characteristics are discussed in this review.     

A comparison of ecological responses among aclonal (unitary), clonal and coalescing macroalgae

Based on growth patterns, regeneration capabilities and genetic make up, benthic macroalgae include three groups of species. Similar to land plants, they include clonal and aclonal species, and, similar to colonial aquatic animals, seaweeds also include coalescing species, that have the capacity to fuse forming composite (chimeric) entities. Since the awareness of the differences between these three kinds of seaweeds is rather recent, most ecological studies have not discriminated among them. However, ecological models based on one kind of seaweeds will not necessarily apply to all kinds of seaweeds. This study reviews ecological responses of algae at the individual and community levels, and describes similarities and differences among both the three algal groups and with parallel groups in land plants and chimeric marine animals. The ecological responses reviewed are plant sizes and shapes; patterns of resource acquisition; algal life phases, reproduction and dispersal; genetic variability, intraspecific and interspecific competition and herbivory. Analysis of these responses supports the idea in distinguishing among the above three algal group, reveals the need for numerous additional ecological studies and advices on incorporating concepts from the biology of chimeric aquatic animals and from clonal theory of land plants into the study of benthic macroalgae.     

REGULATION OF GEOGRAPHIC VARIABILITY IN HAPLOID:DIPLOD RATIOS OF BIPHASIC SEAWEED LIFE CYCLES1

The relative abundance of haploid and diploid individuals (H:D) in isomorphic marine algal biphasic cycles varies spatially, but only if vital rates of haploid and diploid phases vary differently with environmental conditions (i.e. conditional differentiation between phases). Vital rates of isomorphic phases in particular environments may be determined by subtle morphological or physiological differences. Herein, we test numerically how geographic variability in H:D is regulated by conditional differentiation between isomorphic life phases and the type of life strategy of populations (i.e. life cycles dominated by reproduction, survival or growth). Simulation conditions were selected using available data on H:D spatial variability in seaweeds. Conditional differentiation between ploidy phases had a small effect on the H:D variability for species with life strategies that invest either in fertility or in growth. Conversely, species with life strategies that invest mainly in survival, exhibited high variability in H:D through a conditional differentiation in stasis (the probability of staying in the same size class), breakage (the probability of changing to a smaller size class) or growth (the probability of changing to a bigger size class). These results were consistent with observed geographic variability in H:D of natural marine algae populations.     

Dispersal and gene flow in free-living marine nematodes

Dispersal and gene flow determine connectivity among populations, and can be studied through population genetics and phylogeography. We here review the results of such a framework for free-living marine nematodes. Although field experiments have illustrated substantial dispersal in nematodes at ecological time scales, analysis of the genetic diversity illustrated the importance of priority effects, founder effects and genetic bottlenecks for population structuring between patches <1 km apart. In contrast, only little genetic structuring was observed within an estuary (<50 km), indicating that these small scale fluctuations in genetic differentiation are stabilized over deeper time scales through extensive gene flow. Interestingly, nematode species with contrasting life histories (extreme colonizers vs persisters) or with different habitat preferences (algae vs sediment) show similar, low genetic structuring. Finally, historical events have shaped the genetic pattern of marine nematodes and show that gene flow is restricted at large geographical scales. We also discuss the presence of substantial cryptic diversity in marine nematodes, and end with highlighting future important steps to further unravel nematode evolution and diversity.     

赤潮过程中“藻-菌”关系研究进展

微生物对促进海洋物质循环,维持水生环境的生态平衡具有重要作用。在赤潮事件中,基于微生物(尤其是细菌)的多样性和重要性,它们与藻类之间的相互关系成为了研究的热点。过去20年里,人们从不同角度对"藻-菌"间的关系进行了探索,包括物理学过程、生物学过程、环境过程以及化学过程。就化学过程而言,它作为一种较早出现的技术,在以往的研究中带给人们许多认识藻菌关系的方法。随着学科的渗透,化学法有了拓展与延伸,为人们认识藻菌关系带来了新的契机。从化学生态学领域来梳理"藻-菌"关系中涉及的现象和行为,包括菌对藻的有益面、菌对藻的有害面、以及藻类应答细菌行为的化学途径;并从信号语言(群体感应、化感作用)的角度来阐释两者之间的互生或克生关系。通过文献综述的方式来解读藻菌关系的互作过程和机理,为认识赤潮的发生和防控方法提供借鉴。     

Where Turbellaria? Concerning knowledge and ignorance of marine turbellarian ecology

Present knowledge of marine free-living turbellarians is reviewed in the light of a standard ecological text. Little is known of their ecological adaptations or fitness but they are clearly successful animals. Diversity within communities can be considerable, possibly relating to the small size of individuals and temporal and spatial resource gradients. Little is known about genetic diversity, environmental tolerances and demography. Local distribution patterns are better known but rarely proven with appropriate statistics. New data from work in Northern Ireland on behaviour and habitat choice of Procerodes littoralis and on species richness, population aggregation and microdistribution of meiofaunal turbellarians is presented.Nothing appears known about intra-specific competition but more about inter-specific relationships including responses to biogenic structures. Dietary information is sparse and mostly observational with little known about feeding efficiency. Studies at Sylt have revealed mainly univoltine or polyvoltine annual life cycles partly related to habitat preference and possibly giving some evidence of r and K selection. Factors controlling local abundance are largely uninvestigated and the reasons for the apparent rarity of many species not understood.Lists and counts of turbellarians within particular habitats provide some knowledge of community structure but very few numerical analyses, e.g. of diversity and similarity, exist. Contribution to energy flux and to structuring communities has not been determined in spite of the turbellarians' assumed major role as meiofaunal predators. Nevertheless marine turbellarians appear particularly fitted to studies such as habitat heterogeneity, community stability and biogeography and are surely suitable subjects for field and laboratory experiments. There is no lack of direction for studies to proceed. Impulsion is required.     

Distribution patterns of marine bird digenean larvae in periwinkles along the southern coast of the Barents Sea.

An important component of the parasite fauna of seabirds in arctic regions are the flukes (Digena). Different species of digeneans have life cycles which may consist of 1 intermediate host and no free-living larval stages, 2 intermediate hosts and 1 free-living stage, or 2 intermediate hosts and 2 free-living larval stages. This study examined the distribution of such parasites in the intertidal zones of the southern coast of the Barents Sea (northwestern Russia and northern Norway) by investigating 2 species of periwinkles (Littorina saxatilis and L. obtusata) which are intermediate hosts of many species of digeneans. A total of 26,020 snails from 134 sampling stations were collected. The study area was divided into 5 regions, and the number of species, frequency of occurrence and prevalence of different digenean species and groups of species (depending on life cycle complexity) were compared among these regions, statistically controlling for environmental exposure. We found 14 species of digeneans, of which 13 have marine birds as final hosts. The number of species per sampling station increased westwards, and was higher on the Norwegian coast than on the Russian coast. The frequency of occurrence of digeneans with more than 1 intermediate host increased westwards, making up a larger proportion of the digeneans among infected snails. This was significant in L. saxatilis. The prevalence of different species showed the same pattern, and significantly more snails of both species were infected with digeneans with complicated life cycles in the western regions. In L. saxatilis, environmental exposure had a statistically significant effect on the distribution of the most common digenean species. This was less obvious in L. obtusata. The causes of changing species composition between regions are probably (1) the harsh climate in the eastern part of the study area reducing the probability of successful transmission of digeneans with complicated life cycles, and (2) the distribution of different final hosts.     

DEMOGRAPHIC MECHANISMS DETERMINING THE DYNAMICS OF THE RELATIVE ABUNDANCE OF PHASES IN BIPHASIC LIFE CYCLES1

Studies investigating the demographic traits that drive the patterns of phase dominance (the ploidy ratio) in isomorphic biphasic life cycles have not found an integrative solution. Either fertility or survival has been suggested independently as the main driver. Here, we provide a global theoretical framework on how demographic mechanisms determine the ploidy ratio, unifying previous numerical and observational attempts at this question. The analytical solutions of both the ploidy ratio and its elasticities to model parameters of a stage/size‐structured model patterned after the life cycle of a marine alga were derived and analyzed. A complex interaction among vital rates determines the patterns of phase dominance of biphasic life cycles. Three co‐occurring processes—growth, fertility, and looping—may dominate the dynamics of the population, determining both its growth rate and ploidy ratio. Our analyses show that in species where fertility is low, the ploidy ratio is highly elastic to looping transitions (survival, breakage, and clonal growth). Consequently, the subtle morphological, ecophysiological, and biochemistry phase differences that have been reported in isomorphic life cycles as not explaining the observed ploidy ratios, may, in fact, explain them if they translate into slight phase differences in looping transitions. In species where fertility is low, the looping dissimilarities between phases cannot be too high favoring simultaneously one phase, as the population structure would be completely dominated by that phase. In the case of ecological similarity between phases (equal looping and growth rates between phases), a ploidy ratio different from one can only be set by strong phase differences in fertility.     

珊瑚礁生态系统中珊瑚藻的生态作用研究进展

珊瑚藻是海洋红藻中的大型钙化藻类,全球分布623种,中国现有记录共77种。随着生态科学研究的广泛展开,人们越来越认识到,珊瑚藻在海洋生态系统中,尤其在维持珊瑚礁生态系统的生物多样性及生态功能中发挥着重要作用。目前,科研人员对有关珊瑚藻的初级生产力、钙化作用以及在诱导底栖无脊椎动物幼虫的附着与变态等方面已有多方面的研究和探索。然而,有关珊瑚藻生态功能的深层次机理问题有待进一步深入研究。文章着重围绕目前珊瑚藻研究中的一些热点问题,从近年来珊瑚藻在珊瑚礁生态系统中的生态功能方面的研究概况进行综述,以期加深人们对珊瑚藻的认识,并促进对珊瑚藻生态功能的进一步深入研究。     

Ecological effects of larval trematode infestation on littoral marine invertebrate populations

Evidence is presented that, contrary to common scientific “belief”, larva digeneans have profound effects on various components at various levels of the littoral marine ecosystem. Their ecological capacity includes: —Reduction of the breeding potential of host populations by “parasitic castration”; —Structural modification of host populations by generation of erratic growth patterns, size-class differential mortality and “negative growth”; —Induction of host-population mortality and control by increased susceptibility to environmental stress; —Induction of changes in community structure by removal of hosts from their normal trophic levels; —Interference with major energy-flow pathways by precocious removal of hosts from their normal food-web position; —Interference with host-biomass, production and turnover-rate estimations by by-passing of hostassimilated energy; —Interference with predator-prey systems by affecting either component(s) of such systems.The notorious neglect of these factors by marine ecologists and, even more, their total unawareness of the effects these factors produce, raise serious doubts about the validity of marine ecological data and concepts. For the parasitologist, on the other hand, the study of ecological aspects of marine parasite (digenean) biology may open new avenues of research. With a true synthesis of both scientific disciplines, we may eventually arrive at a point where “more complete knowledge of life cycles will permit more intelligent and more effective regulatory methods, the reduction of morbidity, the advancement of health, and the conservation of natural resources.”The latter statement has not been cited from a recent issue of a scientific journal; it has been written down as long as 46 (!) years ago by one of the most outstanding investigators of marine digenean life cycles—Horace W. Stunkard (1940, p 15), but has lost nothing of its actuality. It is hoped that Stunkard's far-sighted words might encourage parasitologists to devote some of their scientific power and skill to the study of marine ecoparasitological problems—for the sake of a better understanding of ecological processes and to the benefit of the endangered marine life.     

For adults only? Supply-side ecology and the history of larval biology

When ecologists study organisms with multiphasic life cycles, they must often confront the problem of which phase to scrutinize. In principle, the dynamics and interactions of all stages could play a major role in the regulation of adult populations and species assemblages. In practice, however, the roles of larger and more sedentary phases - being easier to count and manipulate than motile propagules - have been emphasized. Nonetheless, several recent studies on the small, dispersing larval phase of marine invertebrate life cycles reach the conclusion that the spatial distribution and supply of propagules can control the distribution and abundance of populations of benthic adults. To some, the present emphasis on planktonic propagules amounts to a resurrection of ideas developed during a long and rich history of larval biology. To others, studies of demographic and ecological connections between larval and adult populations represent a substantial revision of ecological paradigms.     

ALTERNATION OF GENERATIONS IN ALGAE: ITS COMPLEXITY, MAINTENANCE AND EVOLUTION

Life histories of photosynthetic eukaryotes traditionally-termed algae exhibit a considerably greater degree of complexity than those of ‘higher cryptogams.’ Some algae have a so-called ‘obligate’alternation between spore-producing and gamete-producing phases, but the majority seem capable of following other pathways depending upon environmental conditions. In only four algal classes do life histories show a change in morphological and/or nuclear phases. The following basic life histories are recognized in the Chlorophyceae, Phaeophyceae and Rhodophyceae:(a) monophasic, a diploid or haploid phase, (b) two or more phases, most commonly an alternation of an isomorphic or heteromorphic haploid gametangial phase and a diploid sporangial phase, and (c) three phases (unique to florideophyte Rhodophyceae), with a diploid spore-producing phase (carposporophyte) developing on the gametangial phase, a diploid phase (tetrasporophyte if meiosis is sporic) and a haploid gametangial phase. Evidence from recent research indicates that in many algae there is an uncoupling of the morphological and nuclear phases. The dominance of one phase and suppression of another has been suggested to be due to the common occurrence in algae of apogamy, apomeiosis and parthenogenesis. Free-living morphs in heteromorphic life histories may be morphologically so dissimilar that formerly they were attributed to different genera. Evolution of the carposporangial phase in red algae is speculated to be a means of achieving zygotic amplification to compensate for the infrequency of syngamy. Such amplification allows the production of a large number of dispersible products from a single fertilization. The direct development of a free-living tetrasporangial phase is considered another mechanism for achieving amplification. In freshwater red algae the growth of an upright phase from a perennial microscopic one is considered an adaptation for maintaining their upstream position. Life history pathways in algae are controlled by subtle environmental influences (e.g. photoperiodism, temperature, light quality, nutrients). Experimental evidence is lacking to support the contention that spatial and/or temporal partitioning of the environment is a mechanism favouring the maintenance of heteromorphy. Herbivory is known to be an important selective force suppressing some morphs and accentuating the seasonal dominance of others. Differential resistance of morphs to herbivory in environments where grazing intensity is predictable may lead to the selective maintenance of heteromorphy. Algal life history patterns are unexplored in terms of evolutionary processes. Various models for the evolution of biphasic or polyphasic life histories stress the importance of the capacity for both asexual dispersal of successful genotypes and for the generation of new genotypes via meiosis and syngamy. All evidence points to the fact that many life history processes operative in algae differ significantly from those described for other cryptogams.     

Dinoflagellates,diatoms, and their viruses

Since the first discovery of the very high virus abundance in marine environments, a number of researchers were fascinated with the world of "marine viruses", which had previously been mostly overlooked in studies on marine ecosystems. In the present paper, the possible role of viruses infecting marine eukaryotic microalgae is enlightened, especially summarizing the most up-to-the-minute information of marine viruses infecting bloom-forming dinoflagellates and diatoms. To author's knowledge, approximately 40 viruses infecting marine eukaryotic algae have been isolated and characterized to different extents. Among them, a double-stranded DNA (dsDNA) virus "HcV" and a single-stranded RNA (ssRNA) virus "HcRNAV" are the only dinoflagellate-infecting (lytic) viruses that were made into culture; their hosts are a bivalve-killing dinoflagellate Heterocapsa circularisquama. In this article, ecological relationship between H. circularisquama and its viruses is focused. On the other hand, several diatom-infecting viruses were recently isolated and partially characterized; among them, one is infectious to a pen-shaped bloom-forming diatom species Rhizosolenia setigera; some viruses are infectious to genus Chaetoceros which is one of the most abundant and diverse diatom group. Although the ecological relationships between diatoms and their viruses have not been sufficiently elucidated, viral infection is considered to be one of the significant factors affecting dynamics of diatoms in nature. Besides, both the dinoflagellate-infecting viruses and diatom-infecting viruses are so unique from the viewpoint of virus taxonomy; they are remarkably different from any other viruses ever reported. Studies on these viruses lead to an idea that ocean may be a treasury of novel viruses equipped with fascinating functions and ecological roles.     

Effects of species biology on the historical demography of sharks and their implications for likely consequences of contemporary climate change

Climate variation is an important factor shaping the demographic histories of many marine species, though impacts likely differ depending on species life history, habitat preferences and ecology. Investigating how species responded to historic climate fluctuations may provide critical insights into a species’ response to current climate change. Despite their ecological diversity, shark species share many similar life history characteristics and may be especially vulnerable to anthropogenic and climate impacts. We compared patterns of genetic variability, mismatch distributions and demographic reconstructions from coalescence approaches among temperate and tropical shark species with differing ecological characteristics, to investigate the effect of the past glaciation cycles on population abundance. Genetic diversity at two mitochondrial DNA regions (ND2 and control region) was assayed in four North Pacific species, Pacific spiny dogfish, Pacific sleeper sharks, salmon shark, and bluntnose sixgill shark. In addition, control region sequences acquired from GenBank for five shark species [tope shark (California/Australia), white shark (California), blacktip shark (eastern and western Gulf of Mexico), lemon shark (Bahamas), and whale shark] were analyzed. General patterns in genetic diversity, mismatch analyses and Bayesian skyline plots supported our hypothesis that species biology affected the impact of climate variation on demographic history. Consequently, our results suggest that effects of contemporary climate change on sharks may be to some degree predictable from species biology, distribution, habitat and the impact of past climate events.     

Complex life cycles and the responses of insects to climate change

Many organisms have complex life cycles with distinct life stages that experience different environmental conditions. How does the complexity of life cycles affect the ecological and evolutionary responses of organisms to climate change? We address this question by exploring several recent case studies and synthetic analyses of insects. First, different life stages may inhabit different microhabitats, and may differ in their thermal sensitivities and other traits that are important for responses to climate. For example, the life stages of Manduca experience different patterns of thermal and hydric variability, and differ in tolerance to high temperatures. Second, life stages may differ in their mechanisms for adaptation to local climatic conditions. For example, in Colias, larvae in different geographic populations and species adapt to local climate via differences in optimal and maximal temperatures for feeding and growth, whereas adults adapt via differences in melanin of the wings and in other morphological traits. Third, we extend a recent analysis of the temperature-dependence of insect population growth to demonstrate how changes in temperature can differently impact juvenile survival and adult reproduction. In both temperate and tropical regions, high rates of adult reproduction in a given environment may not be realized if occasional, high temperatures prevent survival to maturity. This suggests that considering the differing responses of multiple life stages is essential to understand the ecological and evolutionary consequences of climate change.     

Transitions between marine and freshwater environments provide new clues about the origins of multicellular plants and algae

Marine–freshwater and freshwater–marine transitions have been key events in the evolution of life, and most major groups of organisms have independently undergone such events at least once in their history. Here, we first compile an inventory of bidirectional freshwater and marine transitions in multicellular photosynthetic eukaryotes. While green and red algae have mastered multiple transitions in both directions, brown algae have colonized freshwater on a maximum of six known occasions, and angiosperms have made the transition to marine environments only two or three times. Next, we review the early evolutionary events leading to the colonization of current habitats. It is commonly assumed that the conquest of land proceeded in a sequence from marine to freshwater habitats. However, recent evidence suggests that early photosynthetic eukaryotes may have arisen in subaerial or freshwater environments and only later colonized marine environments as hypersaline oceans were diluted to the contemporary level. Although this hypothesis remains speculative, it is important to keep these alternative scenarios in mind when interpreting the current habitat distribution of plants and algae. Finally, we discuss the roles of structural and functional adaptations of the cell wall, reactive oxygen species scavengers, osmoregulation, and reproduction. These are central for acclimatization to freshwater or to marine environments. We observe that successful transitions appear to have occurred more frequently in morphologically simple forms and conclude that, in addition to physiological studies of euryhaline species, comparative studies of closely related species fully adapted to one or the other environment are necessary to better understand the adaptive processes.