Detrital Food Sources Research Articles

Macroalgal input into the coastal food web along a gradient of seasonal sea ice cover along the Western Antarctic Peninsula

Coastal food webs that are supported by multiple primary producer sources are considered to be more stable against perturbations. Here, we investigated how declining macroalgal abundance and diversity might influence coastal food web structure along an annual sea ice cover gradient along the Western Antarctic Peninsula (WAP). The most common benthic invertebrate consumers, macroalgae, and surface particulate organic matter were collected at 15 stations along the WAP. Stable carbon and nitrogen isotope values of primary producers changed negligibly in relation to the sea ice cover gradient, while isotope values of most invertebrate feeding groups increased with higher sea ice cover, although at low explanatory power. Food web length became shorter and consumer trophic niche width smaller in regions with higher sea ice cover. Changes in food web structure were mostly associated with shifts in trophic position of lower trophic levels. Food web structure in higher ice-covered regions resembled that of more generalist feeders with a loss of specialist species, concurrent with an increased reliance on a more reworked detrital food source. These results suggest that a number of benthic invertebrates are able to adjust to differences in basal energy sources. Conversely, these food webs dominated by generalist feeders are likely less efficient in energy transfer, which can create less-stable systems with lower adaptive capacity to disturbance. The predicted sea ice loss along the WAP may ultimately lead to a longer food web with higher macroalgal abundance, more specialist species, and wider consumer trophic niches in the currently more ice-covered regions.

Solubilization of nutritional lipids from three coastal and estuarine primary producers using sodium taurocholate as a model surfactant to mimic typical consumer gut-fluids

Coastal and estuarine food webs receive large inputs of both autochthonous and allochthonous detritus, which serves as an important dietary input for a variety of detritivorous and omnivorous organisms. Surfactants, which aid digestion by solubilizing hydrophobic lipids, are found in the gut-fluids of many fish and invertebrate consumers. To develop a biomimetic assay to explore how source of primary production and age may affect the bioavailability of nutritional lipids from detrital food sources, we assessed the capacity of a model surfactant, sodium taurocholate (STC), to solubilize fatty acids from three primary producers, a haptophyte (Isochrysis galbana), a rock-weed (Ascophyllum nodosum), and a marsh grass (Spartina alterniflora). Comparing lipids solubilized by a constant concentration of STC to traditional Folch-extracted yields, we assessed relative bioavailability of nutritional lipids from fresh and decayed material of each primary producer. We focused on the solubilization of storage triacylglycerols (TAG) and the fatty acid constituents of all lipids (FA). With a temperature of 5 °C, 28.7 mM STC solubilized 44 ± 12% of total FA from S. alterniflora and about half that fraction for I. galbana and A. nodosum after 15 h in the dark unstirred. In the form of TAG, STC solubilized 53 ± 19% of pure triolein as a reference and similar proportions from S. alterniflora (56 ± 19%) and I. galbana (48 ± 2%) but only 9 ± 3% from A. nodosum. Both approaches (TAG and FA) thus show significant differences in bioavailability among these three sources. Two months of decay had inconsistent effects on overall lipid yields but substantial effects on composition. Degradation led to decreased total phospholipids and long-chain polyunsaturated FA and increased break-down products (free FA and diacylglycerols), saturated and monounsaturated FA, and bacterial FA. Lower FA bioavailability and reduced degradation rates in A. nodosum may be due to its higher polyphenol and alginate concentrations. Although we mimicked only one component of the digestion process of lipids, this study represents a first step in developing a biomimetic assay to assess the relative bioavailability of nutritional lipids.

Implications for the mesopelagic microbial gardening hypothesis as determined by experimental fragmentation of Antarctic krill fecal pellets.

Detritivores need to upgrade their food to increase its nutritional value. One method is to fragment detritus promoting the colonization of nutrient‐rich microbes, which consumers then ingest along with the detritus; so‐called microbial gardening. Observations and numerical models of the detritus‐dominated ocean mesopelagic zone have suggested microbial gardening by zooplankton is a fundamental process in the ocean carbon cycle leading to increased respiration of carbon‐rich detritus. However, no experimental evidence exists to demonstrate that microbial respiration rates are higher on recently fragmented sinking detrital particles.Using aquaria‐reared Antarctic krill fecal pellets, we showed fragmentation increased microbial particulate organic carbon (POC) turnover by 1.9×, but only on brown fecal pellets, formed from the consumption of other pellets. Microbial POC turnover on un‐ and fragmented green fecal pellets, formed from consuming fresh phytoplankton, was equal. Thus, POC content, fragmentation, and potentially nutritional value together drive POC turnover rates.Mesopelagic microbial gardening could be a risky strategy, as the dominant detrital food source is settling particles; even though fragmentation decreases particle size and sinking rate, it is unlikely that an organism would remain with the particle long enough to nutritionally benefit from attached microbes. We propose “communal gardening” occurs whereby additional mesopelagic organisms nearby or below the site of fragmentation consume the particle and the colonized microbes.To determine how fragmentation impacts the remineralization of sinking carbon‐rich detritus and to parameterize microbial gardening in mesopelagic carbon models, three key metrics from further controlled experiments and observations are needed; how particle composition (here, pellet color/krill diet) impacts the response of microbes to the fragmentation of particles; the nutritional benefit to zooplankton from ingesting microbes after fragmentation along with identification of which essential nutrients are being targeted; how both these factors vary between physical (shear) and biological particle fragmentation.

Complementary information from fatty acid and nutrient stoichiometry data improve stream food web analyses

Our study characterized the food web of a shaded stream to investigate consumer food choices and food source quality. We measured fatty acids (FA) and elemental carbon (C), nitrogen (N), and phosphorus (P) of autochthonous and allochthonous resources and macroinvertebrates (Hydropsyche sp., Ephemerella sp., Rhyacophila sp., and crayfish-Cambaridae). We examined trophic links using FAs and identified food source quality based on essential fatty acids (EFAs) and elemental nutrients. Autochthonous food sources had greater EFA content (20:5ω3 in periphyton, 20:4ω6 in bryophyte Hygrohypnum) than terrestrial matter. FAs confirmed the grazing nature of Ephemerella. Periphyton may release this invertebrate from N and P limitation, and FA biochemical constraints. Limitation by elemental nutrients, but not FAs, may exist for Hydropsyche if feeding solely on transported matter. Crayfish FA signature suggests consumption of the bryophyte Hygrohypnum as well as terrestrial matter. Our data demonstrate that autochthonous sources are crucial for many invertebrates in shaded streams, despite limited light availability. However, detrital food sources can also be important and their contribution to stream food webs should not be overlooked. Our study highlights the importance of measuring nutrient and biochemical constraints in order to understand factors driving secondary production in streams.

Bt Proteins Exacerbate Negative Growth Effects in Juvenile Rusty (F. rusticus) Crayfish Fed Corn Diet

The adoption of genetically modified (GM) crops has occurred rapidly in the United States. The transfer of GM corn byproducts from agricultural fields to nearby streams after harvest is significant and occurs well into the post-harvest year. These corn leaves, stems, and cobs then become a detrital food source for organisms, such as shredders in the stream ecosystem. Considering that the nontarget effects of Bt corn have been observed in some terrestrial organisms, we assessed whether Bt toxins affect an important aquatic organism, juvenile F. rusticus crayfish. Juvenile crayfish were fed six distinct diet treatments: two varieties of Bt corn, two non-Bt controls of herbicide tolerant corn, and two controls: fish gelatin and river detritus. Juveniles were fed these diets while housed in flow-through artificial streams that received natural stream water from a local source. Specific growth rate and survivorship of the crayfish were measured throughout the study. Juveniles fed corn diets grew significantly less and had reduced survival compared with juveniles fed fish gelatin or river detritus diets. Furthermore, juveniles fed one Bt variety of corn (VT Triple Pro®) exhibited significantly less growth than those fed one of the herbicide tolerant varieties (Roundup Ready 2®). Our study shows that corn inputs to streams may be detrimental to the growth and survivorship of juvenile crayfish and that certain Bt varieties may exacerbate these negative effects. These effects on crayfish will have repercussions for the entire ecosystem, because crayfish are conduits of energy between many trophic levels.

Worm-lobopodian assemblages from the Early Cambrian Chengjiang biota: Insight into the “pre-arthropodan ecology”?

Well-preserved assemblages dominated by scalidophoran worms (Cricocosmia, Mafangscolex) and lobopodians (Paucipodia, Microdictyon, Onychodictyon and undetermined xenusiid) are described from the Early Cambrian Chengjiang biota of China. This fossil material provides the opportunity to reassess key aspects of the functional morphology and lifestyles of the two groups. Both lobopodians exemplified here by Paucipodia and worms relied on a hydrostatic skeleton for support and locomotion and have direct functional analogues in extant onychophorans and priapulids, respectively. Retractor-like muscles attached to terminal claws probably assisted leg elevation in the lobopodians. Whereas lobopodians were essentially walkers and climbers, Cricocosmia and Mafangscolex were most likely subhorizontal burrowers. Their external ornament (phosphatic circular microplates or spiny sclerites) is interpreted as a possible adaptation for tunnelling close to the water-sediment interface. These assemblages seem to represent in-situ faunal associations and suggest that lobopodians and worms might have shared the same habitat and possibly exploited the same detrital food source. However, no evidence indicates that they interacted in predator-prey relationships. Finally, we hypothesize that during a time interval preceding Cambrian Stage 2, scalidophoran worms and lobopodians were the prevalent ecdysozoan component of early animal communities, and introduce the notion of a possible “pre-arthropodan ecology”.

ICHNOLOGICAL EVIDENCE FOR ENDOBENTHIC RESPONSE TO THE K–PG EVENT, NEW JERSEY, U.S.A.

Degree of bioturbation, Thalassinoides isp. morphology, and diameters were compared across the Cretaceous–Paleogene (K–Pg) boundary interval at three localities along the New Jersey coastal plain. Within this regionally extensive ichnoassemblage, mean burrow diameters decrease abruptly by 26–29% (n = 1767) at the base of the Main Fossiliferous Layer (MFL) or laterally equivalent horizons. The base of the MFL has been previously interpreted as the K–Pg boundary based on the last occurrence of Cretaceous marine reptiles, birds, and ammonites, as well as iridium anomalies and associated shocked quartz. Along with the mean, the maximum and minimum burrow diameters exhibit a negative shift, which indicates that the changes are the result of a directional reduction in diameter, rather than an artifact of decreased variance. As a proxy for the size of the tracemaker, a change in burrow diameter indicates a decrease in thalassinid crustacean body size. We interpret this shift as dwarfing within the endobenthic community as detrital food sources became scarce following the mass extinction. Despite the difference in size, there is no change in framework geometry. Ichnofabric indices generally increase up-section at each site across the K–Pg chronostratigraphic boundary, indicating a regional reduction in sedimentation rate, which is supported by a gradual increase in glauconite maturity. Overall, the ichnological evidence at these localities suggests that a prolonged period of negative feedback followed a short-term positive endobenthic response across the K–Pg boundary.

Sex- and habitat-specific movement of an omnivorous semi-terrestrial crab controls habitat connectivity and subsidies: a multi-parameter approach.

Distinct habitats are often linked through fluxes of matter and migration of organisms. In particular, intertidal ecotones are prone to being influenced from both the marine and the terrestrial realms, but whether or not small-scale migration for feeding, sheltering or reproducing is detectable may depend on the parameter studied. Within the ecotone of an upper saltmarsh in the United States, we investigated the sex-specific movement of the semi-terrestrial crab Armases cinereum using an approach of determining multiple measures of across-ecotone migration. To this end, we determined food preference, digestive abilities (enzyme activities), bacterial hindgut communities (genetic fingerprint), and the trophic position of Armases and potential food sources (stable isotopes) of males versus females of different sub-habitats, namely high saltmarsh and coastal forest. Daily observations showed that Armases moved frequently between high-intertidal (saltmarsh) and terrestrial (forest) habitats. Males were encountered more often in the forest habitat, whilst gravid females tended to be more abundant in the marsh habitat but moved more frequently. Food preference was driven by both sex and habitat. The needlerush Juncus was preferred over three other high-marsh detrital food sources, and the periwinkle Littoraria was the preferred prey of male (but not female) crabs from the forest habitats; both male and female crabs from marsh habitat preferred the fiddler crab Uca over three other prey items. In the field, the major food sources were clearly vegetal, but males have a higher trophic position than females. In contrast to food preference, isotope data excluded Uca and Littoraria as major food sources, except for males from the forest, and suggested that Armases consumes a mix of C4 and C3 plants along with animal prey. Digestive enzyme activities differed significantly between sexes and habitats and were higher in females and in marsh crabs. The bacterial hindgut community differed significantly between sexes, but habitat effects were greater than sex effects. By combining multiple measures of feeding ecology, we demonstrate that Armases exhibits sex-specific habitat choice and food preference. By using both coastal forest and saltmarsh habitats, but feeding predominantly in the latter, they possibly act as a key biotic vector of spatial subsidies across habitat borders. The degree of contributing to fluxes of matter, nutrients and energy, however, depends on their sex, indicating that changes in population structure would likely have profound effects on ecosystem connectivity and functioning.

Contributions of detrital subsidies to aboveground spiders during secondary succession, revealed by radiocarbon and stable isotope signatures

Prey subsidies originating from detritus add nutrients and energy to arboreal communities. Measurement of this subsidy is required in the understanding of how food web dynamics respond to changes in surrounding environments. Shrub spiders are one of the key predators involved in food web coupling. We evaluate the effects of potential changes in prey availabilities during secondary succession on the contribution of subsidy from detrital food webs to shrub spiders and how different spider feeding guilds used the subsidy of prey from detrital food webs. We measured the relative importance of the subsidy for the spider feeding guilds, using the ratios of stable isotopes of C (δ(13)C), and N (δ(15)N) and C isotope discrimination (Δ(14)C). Diet age was calculated from Δ(14)C values, because old diet ages of spiders indicate that the spiders consume prey from detrital food sources. Dominant aerial prey (Diptera) had a distinctively old diet age compared with arboreal prey, which indicates that aerial prey were subsidized from detrital food webs. Sit-and-wait spiders tended to have an older diet age than active hunting spiders, which indicates that sit-and-wait spiders depended more on subsidies. Diet age varied only slightly for spiders in stands of different ages, indicating that rates at which spiders use grazing and detrital prey are probably determined more by foraging strategies and not by stand age. A dominance of sit-and-wait predators will lead to higher detrital subsidy inputs in shrub habitats. This study highlights the effect of shrub spider community structure (feeding guild composition) on the volume of the subsidy received from the detrital food web.

Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis

The North Bay of Biscay continental shelf is a major French demersal fishery, but little was known on the trophic food web of its benthic communities. In order to determine the benthic trophic web, the objectives of this study are to describe the macro- and megafaunal benthic community structure (species richness, abundance and biomass) and to establish the trophic pathways (food sources and trophic levels) by applying carbon and nitrogen stable isotopic analysis to the main benthic and demersal species (invertebrates and fish). Two distinct benthic communities have been identified: a muddy sand community within the central part of the bay, and an outer Bay of Biscay Ditrupa sand community of higher species richness, abundance and biomass than the muddy sand community. Deposit-feeders, suspension feeders and predators, distributed in three main trophic levels, dominate both communities. Large differences in stable carbon ratio values within the primary consumers provide evidence of two different food sources: i) a pelagic food source made up of recent sedimenting particulate organic matter on which zooplankton and suprabenthos feed and ii) a benthic detrital food source supplying deposit feeders and partly benthic suspension feeders. Differences in isotopic signatures were also observed within the upper trophic levels that allowed estimation of the contribution of each food source component to the diet of the upper consumers. Finally, the use of stable isotopic composition together with the species' feeding strategy allow identification of the main differences between the trophic functioning of the two benthic communities and highlight the importance of the role of detrital pathways in the carbon cycling within the continental shelf benthic trophic web.

Enhanced exoenzyme activities in sediments in the presence of deposit‐feeding Chironomus riparius larvae

Summary1. The combined effects of deposit‐feeding, bioturbation and bioirrigation by benthic macrofauna on the enzymatic hydrolysis of organic matter were studied in microcosms. Chironomus riparius larvae (Insecta, Diptera) served as model macrofauna and stinging nettle leaves (Urtica dioica) were used as a detrital food source.2. In the upper 10 mm of the sediment (the habitat of C. riparius larvae), the activities of several exoenzymes, the contents of several fractions of particulate organic matter (POM), and the concentrations of dissolved oxidants (O2, NO) were measured on a small scale. Fluorescent particles (luminophores) were used to quantify the vertical redistribution of particles within the same layer.3. In control sediment, the addition of detrital food enhanced exoenzyme activities in the 0–2 mm layer only. In the presence of C. riparius larvae, exoenzyme activities increased to 10 mm depth. Further, the content of POM in the 0–2 mm layer was lower in the presence than in the absence of larvae, suggesting ingestion and subduction of the added detritus. After prolonged incubation without further food addition, exoenzyme activities returned close to background values in both treatments, whereas the vertical distribution of POM remained unchanged.4. The overall penetration depth of O2 and NO into the sediment was greater in the presence than the absence of C. riparius, the differences being more pronounced after prolonged incubation. Locally high O2 and NO concentrations due to bioirrigation by C. riparius were measured deep in the sediment. Net downward transport of particles was observed only in the presence of C. riparius larvae and only at the beginning of the incubation.5. I conclude that deposit‐feeding and bioturbation by macrofauna can quickly remove freshly deposited POM from the sediment surface and transfer it to less oxygenated sites (i.e. animal guts and deep sediment layers). Bioirrigation also increases the availability of oxidants deep in the sediment. The oscillation of oxidant supply to POM particles by ingestion–egestion, burial and re‐burial, and the intermittent bioirrigation of subsurface sediment, is probably the cause of the increased rate of organic matter hydrolysis, the rate‐limiting step in mineralization.

Effects of detrital food sources on growth of a physid snail

Physid snails (Pulmonata: Physidae) are widespread in freshwater habitats throughout the Holartic and are generally considered to feed on periphyton. However, several studies have shown that they also consume living plant material and detritus. In streams where multiple food sources are available, snails may preferentially feed on resources based on palatability or accessibility. It is likely that snail feeding influences food availability and that, when preferable items (e.g. periphyton) are lacking, less desirable food sources can be consumed (e.g. detritus). In Florida, USA, six species of physid snails occur in low-order tributaries of the St Johns River, of which Haitia pomilia (Conrad) has been found to be abundant (M.A. Chadwick, unpublished data). The food sources available in different streams where H. pomilia have been collected vary dramatically due to the presence of a closed or open canopy. Streams with closed canopies are lined with red maple (Acer rubrum ), sweet gum (Liquidambar styraciflua ) and water oak (Quercus nigra ) trees. Streams with an open canopy (due to urbanization and stream-channel modifications) have macrophytes, particularly the invasive Hydrilla verticillata. This plant was introduced from Asia to North America in the 1960s and its role as a potential food source is not well known. The goal of this study was to assess how differences in putative detrital food sources affect H. pomilia growth. Due to their ubiquitous distribution, understanding how snails respond to changes in the availability of different food sources may yield important insights into the processing of organic matter. In June 2004, H. pomilia were collected from small tributaries of the St Johns River in the vicinity of Jacksonville, Florida and transported to the Experimental Mesocosm Facility at the University of Alabama. Snails were kept in temperature-controlled recirculation tanks (800 l, 5 m length, 0.4 m width and depth, 188C, 15 cm/s flow velocity) and provided food sources including live H. verticillata and detritus composed of air-dried red maple, sweet gum and water oak leaves (collected immediately after leaf abscission) and H. verticillata. In October 2004, feeding/growth experiments were initiated. Forty plastic Petri dishes were prepared with one of four treatments composed of detritus from H. verticillata, red maple, sweet gum or water oak (i.e. ten replicates per treatment). In order to homogenize the detritus, air-dried material of each source was conditioned in the mesocosm stream for 2 weeks, re-dried and then ground with a Wiley mill (no. 40 sieve). In each dish, 25 ml of distilled water and 75 mg of fine particulate organic matter (FPOM) were added. Snails were randomly selected from the mesocosm (total shell length 3–6 mm), photographed (RT SE digital camera, Diagnostic Instruments, Inc.), and then randomly assigned to a food source treatment. The treatments were then placed in a randomly blocked arrangement in an 188C incubator, which was used to eliminate light and the potential for periphyton growth. Digital pictures of each snail were then retaken 18 days later. Shell lengths were measured directly from each picture using image analysis software (SPOT, Diagnostic Instruments, Inc.). Snail instantaneous growth rates (g ) were calculated as g 1⁄4 ln(Wfinal/Winitial)/Dt, where W is snail mass (mg) and t is time (d). After 18 days, snails were removed from the dishes and all remaining material was dried and weighed. The remaining material was a mix of dissolved organic carbon, unconsumed FPOM and snail faeces, but this measurement was used as a proxy for food consumption. We developed a predictive model to estimate snail ash-free dry mass from shell length (Table 1). One-way ANOVA was used to assess differences among treatment for both snail growth rates and food consumption. Pair-wise comparisons among treatments were accomplished using 95% confidence intervals. Survival rates for snails among treatments ranged from 70 to 100%. Snail instantaneous growth varied significantly among treatments (F3,32 1⁄4 5.86, P 1⁄4 0.003) with theH. verticillata treatment having much higher growth rates than the other three treatments (Fig. 1). Food consumption also varied significantly among the treatments (F3,32 1⁄4 8.31, P, 0.001) with the least amount of organic matter being found in the H. verticillata treatment (Fig. 2).

Relationship between benthic foraminifera and food resources in South San Francisco Bay, California, USA

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 297:131-145 (2005) - doi:10.3354/meps297131 Relationship between benthic foraminifera and food resources in South San Francisco Bay, California, USA Amy E. Lesen* Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley, California 94720-3140, USA Present address: Department of Math and Science, Pratt Institute, 200 Willoughby Avenue, Brooklyn, New York 11205, USA *Email: alesen@pratt.edu ABSTRACT: Foraminifera are amongst the most abundant known protists found in marine habitats, and are ecologically important constituents of the benthic meiofauna. The seasonality of San Francisco Bay, USA, makes it an ideal system in which to study the relationships between primary production, organic matter fluxes, and food resources for the benthos. Samples were collected at one site at a depth of 8.8 m in South San Francisco Bay on monthly cruises for 2 yr from November 1999 through November 2001. Each month, water column parameters (chlorophyll concentration) and sediment parameters (chlorophyll, total organic carbon, nitrogen, amino acids, bacterial abundance) were measured, and benthic foraminiferal population size and biovolume was counted and calculated. Water column chlorophyll peaked in the spring of 2000 and 2001, and the fall of 2000, with sediment parameters peaking 1 to 3 mo later. The benthic foraminiferal standing crop peaked during the spring of both study years, and showed a large peak in the fall of 2001 dominated by the small-sized foraminifer Fursenkoina pontoni. The data strongly suggest that benthic foraminifera increase in number following phytoplankton blooms when many kinds of sediment organic matter also increase. Foraminiferal biovolume does not necessarily follow this pattern, especially when peaks in standing crop mainly consist of small-sized individuals. Foraminiferal biovolume and standing crop generally increase when the sediment C:N ratio increases, suggesting that benthic foraminiferal populations in South San Francisco Bay may be exploiting a detrital food source in addition to recently deposited fresh phytoplankton. Thus, foraminifera are probably quick to exploit sediment organic matter, and may be important remineralizers of nutrients in this system. KEY WORDS: Benthic ecology · Foraminifera · Marine protists · Population dynamics · Food webs · Estuarine ecology · Meiofauna · Organic matter Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 297. Online publication date: August 01, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Seasonality in Dietary Shifts in Size-Structured Freshwater Fish Assemblages in Three Reservoirs of Sri Lanka

We studied seasonal changes in feeding habits of size-structured fish assemblages in three freshwater reservoirs in Sri Lanka. We obtained fish samples for diet analysis from three reservoirs during the months of rising water level (RIWL) and receding water level (REWL). During RIWL, peripheral areas with terrestrial/semi-terrestrial macrophytes get inundated, which resulted in increased food availability for macrophyte feeding fish. During REWL, detrital food sources increase due to decaying terrestrial plant material, which has inundated. Based on the dietary habits of individual species in the three reservoirs during RIWL and REWL, it is evident that detritivorous and phytoplanktivorous fish species, which belong to low trophic levels do not show significant variation in dietary habits between the two seasons (e.g. Oreochromis mossambicus, Oreochromis niloticus and Amblypharyngodon melettinus). Stenophagous species such as benthic invertebrate predators (Puntius chola and Puntius dorsalis), macrophytophagous species (Etroplus suratensis and Tilapia rendalli) and zooplanktivorous Hemirhamphus limbatus do not exhibit significant variation in dietary habits between seasons. Also their feeding habits do not vary with body size. Euryphagous species such as Puntius filamentosus on the other hand, exhibit variations in dietary habits between seasons as well as with body size. Stenophagous species, which feed on detritus and phytoplankton and euryphagous species, which exhibit temporal dietary plasticity, are therefore abundant in fish assemblages of these reservoirs.

Effect of dietary 1,3,6,8-tetrachlorodibenzo-p-dioxin on the Japanese freshwater fish Oryzias latipes (Medaka) and aquatic snail Indoplanorbis exustus (Indohiramakigai)

During the Japanese spring, many tons of herbicide are used to control weeds in rice paddies. Some of these herbicides are contaminated with 1,3,6,8-tetrachlorodibenzo-p-dioxin (1368-TCDD) and have given rise to residue levels of up to 40 ppb in some Japanese aquatic species. Japanese Medaka ( Oryzias latipes) and Indohiramakigai ( Indoplanorbis exustus) snails were chosen to represent aquatic species that might be exposed to elevated levels of agriculture derived 1368-TCDD. The Medaka were challenged with food contaminated with 40 ppb - 400 ppm 1368-TCDD, and the Indohiramakigai allowed to feed on the resulting detritus. Grab samples of both fish and snails were analysed for 1368-TCDD by GC-HRMS-SIM. Results show that the Medaka absorbed only 0.05 – 0.1 % of the dioxin content of their food and that the absorbed 1368-TCDD was almost totally depurated within 56 days. The Medaka appeared to suffer no harmful effects, even at the highest doses. Indohiramakigai snails were very much more efficient at absorbing 1368-TCDD from their detrital food source. Their populations were reduced dramatically, even at the lowest dioxin dose.

Herbivory and Detritivory on Freshwater Macrophytes by Invertebrates: A Review

Aquatic macrophytes have been viewed as important to invertebrates as substrates for periphyton and epiphytic food, refuges from predation, heterogeneous substrates for co-existence, and upon senescence and death as detrital food sources. Direct herbivory on living macrophytes has been considered unimportant and has been understudied until recently. A review of the literature shows that, when looked for, invertebrate herbivory of macrophytes is more common than generally thought. Most herbivory on macrophytes is usually by specialized oligophagous herbivores which are placed phylogenetically in families and orders of primarily terrestrial insect groups (secondary invaders) that include a number of pest herbivores in terrestrial systems (e.g., chrysomelid and curculionid beetles, aquatic lepidopterans, and specialized dipterans). Herbivores from primarily aquatic invertebrate groups (primary aquatic invertebrates) are often generalists and also detritivores. These patterns and the use of macrophyte detritu...

THE EVOLUTION OF BIOLUMINESCENCE IN BACTERIA

Individuals of aerobic, saprophytic bacterial species, encountering a changing environment in which they were regularly exposed to periods of severe hypoxia, would have gained a major selective advantage in fatty acid metabolism if a mutation in a flavoprotein oxygenase permitted them to oxidize accumulated C8‐C18 aliphatic aldehydes to fatty acids at oxygen concentrations below which the cytochrome oxidase mediated electron transport pathway became inhibited. Such mutants, able to continue to metabolize exogenous lipid fatty acids under hypoxia, might have outproduced their wild type ancestors. Colonies of those mutants which utilized reduced flavin mononucleotide (FMNH2) as the flavin cofactor would have been luminous, owing to the fortuitous coincidence that the aldehyde oxygenation resulted in an enzyme‐flavin excited electronic state which emitted blue light with a high fluorescence yield. If this accidental “proto‐bioluminescence” were initially of sufficient brightness to have elicited phototactic responses in nearby motile organisms, increasing thereby detrital food sources or the potential for dispersal and colonization for the bacteria, a new and completely different selective advantage, that of bioluminescent signalling, would have arisen from the original metabolic function. This is the biochemical analog of Darwin's principle of functional change in structural continuity. It is proposed that bacterial luciferase, the FMNH2‐oxygenase in luminous bacteria, was such a mutation. The present ubiquitous distributions of luminous bacterial species in marine waters, on the surfaces of marine animals and as symbionts in the specialized light organs and digestive tracts of many fish species have resulted from subsequent environmental selection and optimization for this original proto‐bioluminescent reaction. By extension it is suggested whereas “protobioluminescence” arose in many species independently whenever metabolic oxygenation of a substrate resulted in an adventitiously efficient chemiluminescence, the function of bioluminescence arose only upon favorable interaction between the “proto‐bioluminescence” and its ecosystem.

Reproduction and genetic variation in the deposit-feeding sea star Ctenodiscus crispatus

The gonad index in the deposit-feeding asteroid Ctenodiscus crispatus (Retzius) in the Gulf of Maine (USA) is seasonally less variable than in any other sea star, ranging from 2.99 to 4.98% of dry body weight in females and from 2.28 to 3.42% in males, and varies in concert with, rather than reciprocal to, the pyloric caecum index. Biochemical composition and, hence, caloric content, of the gonads also show little seasonal change, suggesting that reproduction is aseasonal and continuous in this population. Oocyte development is asynchronous, all females having a full size range of oocytes (from less than 30 μm to greater than 400 μm diameter) throughout the year. Seasonally determined size-frequency distributions of juveniles, oocyte cytology and size-frequency distributions, responsiveness of adult females to 1-methyladenine, and oxygen uptake rates indicate that variations in reproductive intensity are superimposed on continuous reproduction, and seem related to changes in phytoplankton production rather than to temperature. The rich neutral lipid content (ca. 50% of total lipid) and large egg size (>400 μm) in the ovaries suggest that development is direct. The population is extremely variable genetically, polymorphism among 13 enzyme-coding genes being 77% and average heterozygosity being 0.174. The reproductive pattern and genetic variation in the eurybathic C. crispatus are similar to those in deep-sea echinoderms. This may be related to the constancy of the population's detrital food source, to small-scale heterogeneity of its physical environment, and to low individual vagility.