A review of cnidarians and ctenophores feeding on competitors in the plankton

Predation among pelagic cnidarians and ctenophores is reviewed. The diets of semaeostome scyphomedusae and hydromedusae commonly include other gelatinous zooplanktivores. However, few species of siphonophores and ctenophores are known to consume other gelatinous species. Most of these species can be said to exhibit intraguild predation, since they consume species that potentially compete with them for food. In addition, some hydromedusan and ctenophore species may consume other gelatinous zooplanktivores exclusively. Characteristics of cnidarians and ctenophores as predators and as prey of other gelatinous species are discussed.     

Interactions between pelagic metazoan and protozoan zooplankton,an experimental study

The population of the holotrich ciliate Vasicola lutea was studied for two seasons. An experimental reduction of competitors and predators within the metazoan part of the zooplankton resulted in an increase of the Vasicola population and an expansion of its range. The density of the population decreased and Vasicola retired to its original habitat after the recovery of the populations of predators and competitors. The results imply that competition and predation from metazoans may be important factors for the regulation of population size of protozoans in the zooplankton community. Competition and predation may, furthermore, be important selective forces in the evolution of mechanisms for partitioning resources and habitats.     

Predation by sprat and herring on pelagic fish eggs in a plaice spawning area in the Irish Sea

Predation on planktonic fish eggs was examined by stomach content analysis of sprat and herring sampled in a plaice spawning area to the east of the Isle of Man in March 1993. Plankton samples were taken to examine prey selection. The clupeids showed a selection for the later developmental stages of plaice eggs. Plaice eggs had a refuge in size from predation by sprat <80 mm total length which selected smaller non-plaice eggs. However, herring and sprat >80 mm total length showed a strong selection for plaice eggs, presumably due to their large size.     

A review of ecological interactions between native frogs and invasive cane toads in Australia

Translocated from their native range in the Americas in 1935, cane toads (Rhinella marina, Bufonidae) have now spread through much of tropical and subtropical Australia. The toad's invasion and impact have attracted detailed study. In this paper, I review information on ecological interactions between cane toads and Australian anurans. The phylogenetic relatedness and ecological similarity between frogs and toads creates opportunities for diverse interactions, ranging from predation to competition to parasite transfer, plus a host of indirect effects mediated via impacts of toads on other species, and by people's attempts to control toads. The most clear‐cut effect of toads on frogs is a positive one: reducing predator pressure by fatally poisoning anuran‐eating varanid lizards. However, toads also have a wide range of other effects on frogs, some positive (e.g. taking up parasites that would otherwise infect native frogs) and others negative (e.g. eating frogs, poisoning frogs, competing with tadpoles). Although information on such mechanisms predicts intense interactions between toads and frogs, field surveys show that cane toad invasion has negligible overall impacts on frog abundance. That counter‐intuitive result is because of a broad balancing of negative and positive impacts, coupled with stochastic (weather‐induced) fluctuations in anuran abundance that overwhelm any impacts of toads. Also, the impacts of toads on frogs differ among frog species and life‐history stages, and depend upon local environmental conditions. The impacts of native frogs on cane toads have attracted much less study, but may well be important: frogs may impose biotic resistance to cane toad colonization, especially via competition in the larval phase. Overall, the interactions between native frogs and invasive toads illustrate the diverse ways in which an invader's arrival can perturb the native fauna by both direct and indirect mechanisms, and by which the native species can curtail an invader's success. These studies also offer a cautionary tale about the difficulty of predicting the impact of an invasive species, even with a clear understanding of mechanisms of direct interaction.     

Polarization sensitivity as a contrast enhancer in pelagic predators: lessons from in situ polarization imaging of transparent zooplankton

Because light in the pelagic environment is partially polarized, it has been suggested that the polarization sensitivity found in certain pelagic species may serve to enhance the contrast of their transparent zooplankton prey. We examined its potential during cruises in the Gulf of Mexico and Atlantic Ocean and at a field station on the Great Barrier Reef. First, we collected various species of transparent zooplankton and micronekton and photographed them between crossed polarizers. Many groups, particularly the cephalopods, pelagic snails, salps and ctenophores, were found to have ciliary, muscular or connective tissues with striking birefringence. In situ polarization imagery of the same species showed that, while the degree of underwater polarization was fairly high (approx. 30% in horizontal lines of sight), tissue birefringence played little to no role in increasing visibility. This is most likely due to the low radiance of the horizontal background light when compared with the downwelling irradiance. In fact, the dominant radiance and polarization contrasts are due to unpolarized downwelling light that has been scattered from the animal viewed against the darker and polarized horizontal background light. We show that relatively simple algorithms can use this negative polarization contrast to increase visibility substantially.     

Effects of parasites on fish behaviour: a review and evolutionary perspective

Fish serve as hosts to a range of parasites that are taxonomically diverse and that exhibit a wide variety of life cycle strategies. Whereas many of these parasites are passed directly between ultimate hosts, others need to navigate through a series of intermediate hosts before reaching a host in (or on) which they can attain sexual maturity. The realisation that parasites need not have evolved to minimise their impact on hosts to be successful, and in many cases may even have a requirement for their hosts to be eaten by specific predators to ensure transmission, has renewed interest in the evolutionary basis of infection-associated host behaviour. Fishes have proved popular models for the experimental examination of such hypotheses, and parasitic infections have been demonstrated to have consequences for almost every aspect of fish behaviour. Despite a scarcity of knowledge regarding the mechanistic basis of such behaviour changes in most cases, and an even lower understanding of their ecological consequences, there can be little doubt that infection-associated behaviour changes have the potential to impact severely on the ecology of infected fishes. Changes in foraging efficiency, time budget, habitat selection, competitive ability, predator-prey relationships, swimming performance and sexual behaviour and mate choice have all been associated with – and in some cases been shown to be a result of – parasite infections, and are reviewed here in some detail. Since the behavioural consequences of infections are exposed to evolutionary selection pressures in the same way as are other phenotypic traits, few behavioural changes will be evolutionarily neutral and host behaviour changes that facilitate transmission should be expected. Despite this expectation, we have found little conclusive evidence for the Parasite Increased Trophic Transmission (PITT) hypothesis in fishes, though recent studies suggest it is likely to be an important mechanism. Additionally, since the fitness consequences of the many behavioural changes described have rarely been quantified, their evolutionary and ecological significance is effectively unknown.Potential hosts may also change their behaviour in the presence of infective parasite stages, if they adopt tactics to reduce exposure risk. Such `behavioural resistance', which may take the form of habitat avoidance, prey selectivity or avoidance of infected individuals, can be viewed as behavioural change associated with the threat of being parasitised, and so is included here. Actually harbouring infections may also stimulate fishes to perform certain types of simple or complex behaviours aimed at removing parasites, such as substrate scraping or the visitation of cleaning stations, although the efficacy of the latter as a parasite removal strategy is currently subject to a good deal of debate.The effects parasites have on shoaling behaviour of host fish have attracted a good deal of attention from researchers, and we have provided a case study to summarise the current state of knowledge. Parasites have been shown to affect most of the antipredator effects of shoaling (such as vigilance, co-ordinated evasion and predator confusion) and can also impair an individual's foraging ability. It therefore seems unsurprising that, in a number of species avoidance of parasitised individuals has evolved which may explain the occurrence of parasite-assorted shoals in the field. Parasitised fish are found more often in peripheral shoal positions and show a reduced tendency for shoaling in some fish species. Given the array of host behaviours that may be changed, the fitness consequences of shoal membership for parasitised hosts and their parasites are not always easy to predict, yet an understanding of these is important before we can make predictions regarding the ecological impact of infections on host fish populations.Clearly, there remain many gaps in our knowledge regarding the effects of parasites on the behaviour of host fish. We believe that a much greater understanding of the importance of infection-associated behaviour changes in fish could be gained from high quality research in comparatively few areas. We have completed our review by highlighting the key research topics that we believe should attract new research in this field.     

A golden age of gelata: past and future research on planktonic ctenophores and cnidarians

The study of the natural history of gelatinous zooplankton (‘gelata’) reached a high point at the end of the 19th century, when scientists first began to understand the phylogenetic and ecological links between cnidarians and ctenophores. Siphonophores, carefully figured in their entirety, and gauze-like lobate ctenophores too fragile to touch, were described by the dozens. In the ensuing years, focus on zooplankton shifted toward more ‘industrial’ goals such as quantitative sampling using plankton nets. While plankton scientists were busy summing tattered parts, they lost sight of the whole jellies themselves, and a crustaceocentric view of the ocean came to dominate. During this period, the most dramatic breakthroughs in cnidarian research came from laboratory studies of neurobiology, physiology, and development, particularly of certain model organisms. Now, at the turn of this century, we have the opportunity to bring gelata back into primacy. Submersibles and remotely operated vehicles allow us to study entire life histories of organisms that we did not even know existed. The tools of molecular biology allow us to answer questions about development, evolution, and phylogeny that had reached a stalemate. Even in the surface waters, where it might be thought that there is little left to learn, in situ observations have revealed unexpected interactions and hidden diversity. The critical roles that these organisms play in the health of the oceans, their position at the crux of many evolutionary debates, and the tools for biotechnology that they provide, have led to resurgent public appreciation and awareness. Although advanced tools do not necessitate good science, we have few excuses for failing to bring about another golden age of gelata. A plenary address of the 7th International Conference on Coelenterate Biology.     

Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment

1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe. 2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year‐to‐year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers. 3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites. 4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L^?1) when grazer biomass was high (>80–90 μg dry mass L^?1) or accounted for >30% of the grazer community. 5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass. 6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.     

A golden age of gelata: past and future research on planktonic ctenophores and cnidarians

Hydrobiologia - The study of the natural history of gelatinous zooplankton (‘gelata’) reached a high point at the end of the 19th century, when scientists first began to understand the...     

Small fish use the hypoxic pelagic zone as a refuge from predators

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Why plankton communities have no equilibrium: solutions to the paradox

In a classical paper, Hutchinson (1961) argued that the large number of species in most plankton communities is remarkable in view of the competitive exclusion principle, which suggests that in homogeneous, well-mixed environments species that compete for the same resources cannot coexist. Few ideas in aquatic ecology have evoked more research than this `paradox of the plankton'. This review is an effort to put the main solutions to the paradox that have been proposed over the years into perspective. Hutchinson himself already suggested that the explanation could be that plankton communities are not in equilibrium at all due to weather-driven fluctuations. Subsequent research confirmed that such externally imposed variability can allow many species to coexist. Another important point is that in practice the homogeneous well-mixed conditions assumed in the competitive exclusion principle hardly exist. Even the open ocean, for instance, has a spatial complexity resulting from meso-scale vortices and fronts that can facilitate coexistence of species. Perhaps most excitingly, theoretical work on species interactions has given a counter-intuitive new dimension to the understanding of diversity. Various competition and predation models suggest that even in homogeneous and constant environments plankton will never settle to equilibrium. Instead, interactions between multiple species may give rise to oscillations and chaos, with a continuous wax and wane of species within the community. Long-term laboratory experiments support this view. This chaotic behavior implies among other things that plankton dynamics are intrinsically unpredictable in the long run when viewed in detail. Nonetheless, on a higher aggregation level, indicators such as total algal biomass may show quite regular patterns.     

Interactions between the phloem-feeding speciesTomicus piniperda (Col.: Scolytidae) andAcanthocinus aedilis (Col.: Cerambycidae), and the predatorThanasimus formicarius (Col.: Cleridae) with special reference to brood production

Interactions betweenTomicus piniperda (L.) (Col.: Scolytidae),Acanthocinus aedilis (L.) (Col.: Cerambycidae) andThanasimus formicarius (L.) (Col.: Cleridae) were investigated in caged pine bolts. The treatments wereT. piniperda alone,A. aedilis alone,T. piniperda together withA. aedilis, T. piniperda together withT. formicarius and all three species together. T. piniperda offspring production per m² was reduced by 92% when reared withT. formicarius, by 78% when reared withA. aedilis, and by 94% when all three species were reared together, compared withT. piniperda reared alone.A. aedilis had a negative influence on the offspring production ofT. formicarius and vice versa. When both species were present in the same bolt (together withT. piniperda) offspring production was reduced by 74% forA. aedilis and by 42% forT. formicarius compared with their respective production values when each species was present alone with the bark beetle. The new generation ofT. formicarius emerged as larvae from June to August while most of theA. aedilis offspring emerged as adults from September to October, leaving only a few larvae in the bolts to hibernate.A. aedilis only reproduced in a small part of one of the bolts without bark beetles.     

Interactions between granivorous and omnivorous ants in a desert grassland: results from a long-term experiment

Abstract.  1. There is growing evidence that granivorous ants compete for seeds in desert ecosystems. But ants that diverge in diet may still interact strongly if existing colonies deter foundress queens. Granivores (seed eaters) and omnivores (ants that scavenge and feed on plant exudates) are common components of desert ecosystems.
2. The results of a 15-year granivore reduction experiment to explore the responses of two common omnivorous ants, Forelius cf pruinosus and Dorymyrmex insana , in a desert grassland were analysed. The food competition hypothesis predicts that granivore removal will have no net effect on omnivores. The spatial competition hypothesis predicts compensatory increases in omnivores on granivore removal plots.
3. A grain-based gut poison significantly reduced the densities of three of the four common granivorous ants. Densities of the large-colony (10⁵-worker) omnivore Forelius did not increase with granivore removal. In contrast, and consistent with the hypothesis of competition for space, densities of the small-colony omnivore (10²−10³ worker) Dorymyrmex increased on granivore removal plots; however, this effect was only observed in years of high Dorymyrmex abundance.     

Digenea parasites of jellyfish and ctenophores of the southern Atlantic

Parasites of planktonic cnidarians and ctenophores in the southern Atlantic Ocean are little known. The aim of this study was to describe three new metacercariae from jellyfish and ctenophores, and assess the importance of the gelatinous zooplankton as intermediate hosts in the life history of digeneans. During examination of zooplankton in Argentine Sea for digeneans that mature in fishes, two species of jellyfish (Phialidium sp. and Liriope tetraphylla Chamiso & Eysenhardt, 1821), and one ctenophore (Mnemiopsis mccradyi Mayer, 1900) were analyzed for parasites. The samples were obtained in Mar del Plata. Three metacercariae belonging to Faustulidae, Lepocreadiidae and Hemiuridae are described. The prevalence (percent of hosts infected) varied from 1.4–30% and the range of intensity (number of individuals of a parasite species in a single infected host) was from 1 to 30 for the different metacercariae. Given the important position of free-swimming cnidarians and ctenophores in the marine food web, and the great number of fishes that have been found with these organisms in their digestive tracts, their importance in the life histories of digeneans should not be underrated.     

A new species of Poly dor a (Polychaeta: Spionidae) from the Indo-West Pacific and first record of host hermit crab egg predation by a commensal polydorid worm

A new spionid polychaete, Polydora robi, is described from intertidal and shallow subtidal areas in the Philippine Islands and Bali, Indonesia. Polydora robi belongs to the Polydora ciliata/ websteri species group and is characterized by a rounded prostomium, triangular occipital tentacle, needlelike posterior notosetae, and a pygidium with digitiform composite cirri surrounding the anus. Adults burrow into empty gastropod shells inhabited by hermit crabs. The burrows of the worms typically extend from an external opening in the apex of the shells to an opening in the central body whorls along the columella. The species was found to ingest the fertilized eggs and developing embryos attached to the pleopods of host hermit crabs. The occurrence of egg predation and the symbiotic relationship between polydorids and hermit crabs is discussed. Known egg predators of hermit crabs are reviewed.     

Structuring of the cyanobacterial community by pelagic fish in subtropical reservoirs: experimental evidence from Australia

1. Subtropical reservoirs of Australia are commonly subject to summer blooms of cyanobacteria. The potential for food web manipulation to control cyanobacterial blooms was investigated in Lake Maroon, south east Queensland using enclosures in which the density of the Australian gudgeon Hypseleotris spp. was manipulated. 2. Zooplankton biomass and community structure were strongly affected by fish density. A size dependent predation effect of Hypseleotris on zooplankton was observed at ambient fish densities, and the community shifted towards a dominance of copepod juveniles and nauplii. Substantial increases in the populations of Ceriodaphnia and calanoid copepods were observed at low fish densities and in the absence of fish. 3. At ambient fish densities total phytoplankton and the proportion of cyanobacteria were maintained at levels similar to those prevailing at day 0. Total phytoplankton and the proportion of cyanobacteria decreased substantially at low fish densities and in the absence of fish. Chlorophytes became dominant in the ‘no fish’ treatment and the grazing‐resistant species Oocystis and Dictyosphaerium were significantly higher than at ambient fish densities. 4. The experiment demonstrated a strong positive relationship between Hypseleotris density and cyanobacteria, and the results suggest that subtropical reservoirs may be suited to food web manipulation as a means of controlling summer cyanobacterial blooms.     

The spatial distribution of zooplankton populations in the presence and absence of fish

The spatial distribution patterns of the main zooplankton groups in two small experimental ponds, one which contained fish and one which did not, were measured over a twelve month period from March 1983. Populations of all zooplankton groups were generally distributed non-randomly and with varying degrees of aggregation. The aggregation ofDaphnia longispina populations was inversely related to ambient wind speed, indicating a breakdown of behavioural clumping by wind-induced water currents. Evidence that populations ofDaphnia aggregate in response to fish predation was inconclusive. In contrast cyclopoid nauplii showed no evidence of behavioural clumping and the degree of aggregation was not related to wind speed. Nauplii aggregations were however greater in the absence than in the presence of fish.     

Long-term changes in the community of planktonic crustaceans in Lake Glubokoe in relation to predation and competition

Studies on the planktonic Crustacea in Lake Glubokoe (Moscow region) from 1897 to 1982 have been reviewed. After 1965, when most of the swamp water inflow was diverted from the lake, certain populations have penetrated into deeper layers, while the average overlap in the community has dropped. The summer dominant Mesocyclops leuckarti disappeared and Chydorus sphaericus appeared in mass quantities; this development was not associated with biomass changes of blue-greens. The Cladocera/Calanoida ratio and the relative abundance of certain species showed no considerable changes. The effect of Chaoborus, Cyclops and Leptodora in the 70's on the crustacean populations was insignificant in spite of the increasing abundance of these genera after 1965. Analysis of the dynamics of the clutch size and the density of lake populations in the 1970's and 1980's revealed food limitation and competition in herbivorous Cladocera. In laboratory experiments with lake water they always competed, though the intensity of their interaction varied depending on the species combinations.     

Major changes in pelagic rotifers during natural and forced recovery from acidification

Pelagic rotifers were studied in lakes with contrasting acidification histories situated in an acid-stressed region of southern Norway. Life histories and spatial distribution varied considerably between the investigated species, and influenced the recovery processes. Most headwater lakes have experienced strongly acidified environments during the last five decades, whereas lakes close to the Skagerrak coast have been stable within the same period. Rotifer diversity and abundance were reduced in the most acidic sites and increased towards the coast. Most surveyed species are known to possess sediment egg-banks, and after chemical recovery most rotifers dispersed into the plankton from these egg-banks and produced viable populations. Some species of the genera Polyarthra and Collotheca, and the species Kellicotta longispina and Keratella serrulata showed a striking ability to tolerate acidification, and were the dominant taxa in the acidmost environments. K. serrulata characterised, but did not numerically dominate, acid rotifer communities especially in the most coloured sites, and decreased following liming. The predominantly bacteriophageous genus Conochilus exploded in numbers shortly after liming, most probably because bacteria increased strongly during this transition phase. Planktivorous fish influenced indirectly rotifer abundance by consuming invertebrate predators and important rotifer competitors such as filter feeding cladocerans. Invertebrate predators, such as larvae of Chaoborus spp. and Heterocope saliens probably influenced rotifer distributional patterns in a complex top-down manner, both during chronic acidification and liming in environments with low fish predation. Important rotifer predators such as pelagic cyclopoid copepods, Bythotrephes longimanus and Leptodora kindti, were absent from the most acidic fishless lakes. Considerable populations of large-sized Daphnia longispina probably suppressed several rotifer species in sites with low fish predation, as did large populations of Bosmina longispina and Ceriodaphina quadrangula in lakes with intense fish predation.     

Relationship Patterns between Ichthyoplankton and Zooplankton: A Conceptual Model

Selected cases of plankton studies were analyzed to illustrate the main types of relationships between the zooplankton and the ichthyoplankton abundance observed in the pelagic realm. Such relationships may exhibit a positive, a negative, or a random pattern. In the conceptual model here proposed, short-term oscillations among these patterns were attributed to small-scale biological processes, such as competition, predation, and intraguild predation, acting in conjunction with water turbulence. A negative relationship between zooplankton and ichthyoplankton abundance may be caused both by predation on fish eggs and larvae, and by detrimental competition and intraguild predation interactions for fish larvae. In contrast, positive relationships emerge from the absence or low abundance of major predators on the ichthyoplankton, and from food availability for fish larvae and competing species. The random pattern may appear as a gradual transitional stage between the negative and positive patterns, or be promoted by strong water turbulence – which generates random movements of individuals. The size of zooplankters greatly influences these small-scale phenomena. Hence, their role in the trophic web, the success in competition interactions and vulnerability to water turbulence depends on their size. Intra- and interspecific competition may be reduced by variability in body size within or among fish larvae populations. Owing to a strong interaction among phenomena at different scales, these small-scale processes are also influenced by larger scale features, such as seasonal changes in zooplankton biomass, water currents, or spawning periods of fish. At the space level, some theoretical studies have emphasized the role of water currents as a vector for fish larvae to reach the nursery grounds (migration triangle hypothesis), or to allow them to remain within their own population’s distributional area (member/vagrant hypothesis). At the temporal level, the match/mismatch theory insists in a synchrony between reproductive strategies of fish and cyclical changes in abundance and size spectrum of potential prey items for their larvae. In any case, a coincidence between favorable abiotic and biotic features during the whole life-cycle of fish would assure a success in survival of larvae and their subsequent recruitment to adult population.