The biomass and nutritive potential of Vallisneria americana Michx in navigation pool 9 of the upper Mississippi river

Vallisneria americana Michx (wild celery) was studied to determine the biomass and nutritive potential of all morphological structures. A 2.6-ha stand of uniform V. americana was sampled during the summer and autumn of 1980, and the spring and summer of 1981 in the southern portion of Navigation Pool 9 of the Upper Mississippi River.The maximum production rate of 3.2 g m^?2 day^?1 was coincident with rapid rosette production and flowering, and occurred mid- to late-July 1980. The maximum biomass of 217.3 g dry wt. m^?2 was on 1 September 1980, when fruit development was also at a maximum. Leaves composed 60–70% of the summer biomass; winter buds constituted all of the winter biomass.Winter buds and fruits had the greatest nutritive potentials. Both organs contained relatively high dry matter concentrations and were low in ash (less than 10%) and fiber content. The potentially-digestible ash-free non-cell-wall fraction (NCF) was composed of an average of 75.7 and 82.2% of the dry weight of fruits and winter buds, respectively. In contrast, the nutritive potential of leaves, rootstocks, peduncles and stolons was reduced because of high moisture (less than 8% dry matter), ash and fiber concentrations. Staminate inflorescences and pistillate flowers were high in crude protein (averaged 21.8% and 16.1% of the dry-weight, respectively) and ash-free non-cell-wall fractions, but they accounted for only 2.7% of the plant biomass. The maximum calorific content of V. americana was approximately 3200 kJ m^?2 at peak biomass on 1 September 1980.     

Seasonal changes in flesh weight and biochemical composition of mussels (Mytilus edulis L.) in the Conwy Estuary,North Wales

Sublittoral Mytilus edulis L. from natural mussel beds of the Conwy Estuary fishery exhibited an annual cycle of dry flesh weight, biochemical composition, and caloric value during 1971–1974.Mean percentage values of the main components of dry flesh, with seasonal extremes in parentheses, were: protein 58.7 (46.4–73.1), carbohydrates 22.5 (4.1–36.7), lipids 7.0 (3.9–9.6), and ash 11.8 (8.8–17.2). The mean caloric values of dry flesh were 5.57 kcal/g ash-free and 4.92 kcal/g ashinclusive.Individual dry flesh weight was highest in summer and autumn, when protein and carbohydrate were maximal, and decreased through the winter to a post-spawning minimum in spring. Weight loss in winter resulted from rapid utilization of carbohydrate (glycogen) reserves and a depletion of both protein and lipid content. It is considered likely that mussels were then stressed by food shortage at the time when metabolic demands, due to gametogenesis, were highest.During the six main fishing months, October to March, the average weight of dry flesh fell by 30–50 % and caloric content by 35–45 %. The likelihood of similar losses in cultivated mussel stocks is indicated and the need for comparable information in fishery management is emphasized. The extent to which predators compensate for these seasonal changes in prey quality also requires examination.     

Nutrient availability affected shallow-lake ecosystem response along the Late-Glacial/Holocene transition

In temperate headwater streams, intra-annual variation in energy input is critical in influencing aquatic food webs. We used stable isotope analysis to explore how seasonal variation in energy sources likely influenced invertebrate composition together with isotopic niche patterns of a cyprinid minnow, the chubbyhead barb Enteromius anoplus within a southern temperate headwater stream. Basal food sources had lower carbon and nitrogen stable isotope values in winter and spring compared to summer and autumn. Seasonal variations in the aquatic invertebrate communities were depicted by differences in the composition of gatherer and predator groups together with occurrence of scrapers in summer. Temporal changes in the isotopic values of these invertebrate groups appeared to coincide with that of basal food resources. The chubbyhead barb exhibited seasonal variation that was characterised by larger isotopic niches in winter compared to summer. Tissue-specific comparisons revealed larger isotope niche size based on fin tissue than muscle tissue during the cold period but comparable isotope niche sizes in the warm period. Contrasting inter-tissue isotopic niche patterns, the cold period suggested the likelihood of tissue physiological differences. Comparable isotopic niche patterns in summer and autumn suggested the possibility of synchronised physiological processes for both tissues. This suggested the potential for fin tissue as non-lethal alternative when inferring food web dynamics for this species.

     

Nahrung und Freßverhalten bei Sedimentfressern dargestellt am Beispiel von Sipunculiden und Holothurien

Using a Phillipson microbomb calorimeter, the energy values of the “infauna-substrate-feeders”Sipunculus nudus andPhascolosoma vulgare and the nutritive value of their food substrate were determined. Calorific measurements were made of the total food substrate, of meiofauna organisms living in it and of fecal pellets from other invertebrate animals which are an important part in the food supply. Analyses of the gut content of the sipunculids and their feces were compared with the surrounding sediment. The total sediment had an ash content of about 97%, and an energy content of approximately 0.14 cal mg^?1 dry weight; this is equivalent to 165 kcal m^?2. For the meiofauna in this substrate a biomass of 800 mg dry weight was calculated. Using calorific determinations of important meiofauna groups (nematodes 5274 kcal kg^?1, ostracods 5884 and 6000 kcal kg^?1), one square meter of the sediment surface — the sipunculid food source — yielded a caloric content of 3.78 kcal for the meiofauna, which means a contribution of 2.3% to the total food substrate. The largest part of the calorific contents is by far provided by fecal pellets with 0.6 cal mg^?1 dry weight or 150 kcal m^?2; this accounts for 92% of the total calorific content of the food substrate. Peritrophic membranes and attached microorganisms may be responsible for this spectacularly high value. Coprophagy plays an important role. The rest of the total value was contributed by particulate and dissolved detritus as well as protozoans with a calculated sum of 10 kcal m^?2 or 6%. Food uptake is selective with regard to small sediment grain sizes. The amount of meiofauna in the gut is 80 times lower than in the surrounding environment, the energy content in the anterior gut 10 times higher. The energy loss inS. nudus from the anterior gut to the middle gut is more than 70%, to the posterior gut a further loss of 19% was observed. Calorific measurements in the feces were no longer possible. The utilization of food appears to be almost 100%. A correlation between ash content and the quantity of the sand grain-size fraction 37–125 μm and the calorific content is shown. The meiofauna does not represent a significant part of the total food structure, but it is taken up and utilized as food by the sediment feeding macrofauna.     

Variation in the calorific value and total energy content of the lesser sandeel (Ammodytes marinus) and other fish preyed on by seabirds

Wet and dry calorific values (energy densities) and total energy content of lesser sandeel Ammodytes marinus , sprat Sprattus sprattus , Atlantic herring Clupea harengus , whiting Merlangius merlangus , saithe Pollachius virens and cod Gadus morhua , were measured. Calorific values varied both within and between species. Larger fish tended to have higher calorific values than small ones, particularly when considered in terms of wet weight, but there was considerable temporal variation. The calorific values and body weights of sandeels larger than 10 cm showed marked seasonal trends and in consequence the total energy content of a sandeel of given length in summer is approximately double the spring value. The calorific values of herring and sprat also varied from month to month but seasonal cycles were less obvious. Whiting varied least. Both calorific values and total energy content of individual sprat were very variable in summer (the spawning season of this species). Because there is so much intraspecific variation, care must be taken when assessing the relative merits (in terms of energy) of different species as prey. The practical difficulties of obtaining reliable data on wet calorific values are discussed and the use of dry calorific values and dry weight/length relationships is recommended.     

Seasonal changes in organic composition and caloric content of an Arctic marine amphipod, Onisimus (= Boeckosimus) affinis H.J. Hansen

Protein, lipid, carbohydrate, chitin, ash, and caloric energy contents of the Arctic marine benthic amphipod Onisimus (= Boeckosimus) affinis H.J. Hansen were determined at intervals over two and one half years. The lipid content exhibited the most pronounced seasonal change, from a minimum of about 14–17% of dry wt to a maximum of 25–27% of dry wt. An increase in lipid content in the spring coincided with the reproductive period of the species. Lipid and caloric content decreased during the summer to levels that were maintained through the winter, indicating that the animals do not store significant nutritional reserves for use during the winter. The dry weight proportions of other components were: protein 27.7–38.3%; carbohydrate 1.8–4.0%; chitin 7.8–8.2%; ash 21.3–27.2%. Caloric content varied between 3.29 and 4.18 cal/mg dry wt.     

Trade-offs between growth and defence in two phylogenetically close invasive species

The success of an invasive plant species could be explained by trade-off between growth and defence. The aim of this paper was to explore the responses of two non-native aquatic macrophytes Elodea canadensis and Elodea nuttallii to herbivores in their introduced range. We assessed the palatability of the two phylogenetically close aquatic plant species in field and their responses to gammarid consumption in spring, summer and autumn in a microcosm experiment. We measured the variation of functional traits for each season. The traits selected were those judged most closely related to the allocation of resources to growth or to resistance against herbivores. We clearly established that the strategies of the two species were different and that their consumption rate differed in summer. In summer, E. canadensis allocated more of its resources to structural defence (leaf toughness). The increase in leaf thickness reduced the palatability of E. canadensis, whereas E. nuttallii stimulated its growth. Moreover, a decrease in dry matter content in E. nuttallii was found during the growing season in field. In autumn, both plant species accumulated nitrogen and phosphorus in their tissues. We also demonstrated that neither species induced efficient chemical defences against the herbivores. The different strategies of these two Elodea species could be explained by their different resident times in the introduced area and by an adaptation of the naturalised E. canadensis to herbivores.     

Seasonal variation in epifaunal communities associated with giant kelp (Macrocystis pyrifera) at an upwelling‐dominated site

Kelp forests are highly productive and species‐rich benthic ecosystems in temperate regions that provide biogenic habitat for numerous associated species. Diverse epifaunal communities inhabit kelp sporophytes and are subject to variations in the physical environment and to changes experienced by the kelp habitat itself. We assessed seasonal variations in epifaunal invertebrate communities inhabiting giant kelps, Macrocystis pyrifera, and their effects on this seaweed. Six seasonal samplings were conducted over a year at an upwelling‐dominated site in northern‐central Chile where physical conditions are known to fluctuate temporally. More than 30 taxa were identified, among which peracarid crustaceans stood out in both diversity and abundance. Species richness and abundance differed among sporophyte sections (holdfast and fronds) and throughout the year. The frond community was dominated by two grazers (the amphipod Peramphithoe femorata and the isopod Amphoroidea typa), while suspension feeders, grazers, and omnivores (the amphipod Aora typica, the isopod Limnoria quadripunctata, and polychaetes) dominated the holdfasts. Abundances of the dominant species fluctuated throughout the year but patterns of variation differed among species. The most abundant grazer (P. femorata) had highest densities in summer, while the less abundant grazer (A. typa) reached its peak densities in winter. Interestingly, the area of kelp damaged by grazers was highest in autumn and early winter, suggesting that grazing impacts accumulate during periods of low kelp growth, which can thus be considered as ‘vestiges of herbivory past.’ Among the factors determining the observed seasonal patterns, strong variability of environmental conditions, reproductive cycles of associated fauna, and predation by fishes vary in importance. Our results suggest that during spring and early summer, bottom‐up processes shape the community structure of organisms inhabiting large perennial seaweeds, whereas during late summer and autumn, top‐down processes are more important.     

Studies on the nutrition and feeding preferences of Aplysia: Development of an artificial diet

An artificial diet made up principally of chemicals set in agar with a small amount of water extract of the green alga Ulva fasciata Delile has given good growth and spawn production in the sea hare Aplysia dactylomela Rang. The potential use of such a diet in the study of nutrition of sea hares and of other marine invertebrate herbivores is discussed and its possible rôle in identifying the characteristics of seaweeds which govern natural food choices in these animals is considered.     

The autumn mesozooplankton community at South Georgia: biomass, population structure and vertical distribution

Mesozooplankton were sampled at shelf and oceanic stations close to South Georgia, South Atlantic during austral autumn 2004 with a Longhurst Hardy Plankton Recorder. Onshelf biomass ranged from 2.18 to 5.75 g DM m^?2 (0–200 m) and was dominated by the small euphausiid Thysanöessa spp. At the oceanic stations (10.57–14.71 g DM m^?2, 0–1,000 m) large calanoids, principally Rhincalanus gigas comprised ～47–52% of biomass. Here Calanus simillimus was still active and reproducing in surface waters (0–11.2 eggs fem day^?1) but R. gigas and Calanoides acutus were largely resident in the warm deep water and undergoing their seasonal descent. A comparison with spring and summer data indicated increased abundance and biomass from spring through to summer followed by a decline towards autumn particularly over the shelf. Autumn values in oceanic waters differed little from summer. Mesozooplankton biomass in the surface 200 m of the oceanic stations as a proportion of that found in the top 1,000 m ranged from 63 to 78% of the total in spring and 62–73% in summer, but was only 23–29% of the total in this study, following redistribution down the water column.     

Temporal and morphological factors related to mineral composition in Myriophyllum heterophyllum michx

The emergent floral stem, apex, sub-apex, mid-stem, lower stem, and roots of Myriophyllum heterophyllum Michx were analyzed for ash, P, Na, K, Ca, Mg, Fe, Mn, Zn, Cu and Pb. Samples were collected 14 times from June 1979 through July 1980 from Lake Winnipesaukee, New Hampshire, U.S.A.The plant segments sampled differed significantly in mineral content, but seasonal pulses for each mineral were usually in synchrony between the different segments sampled. Sodium (range, 0.8–2.7% dry wt.) and potassium (range, 1.4–3.3% dry wt.) were the dominant elements in the submersed stem. The dominant element in the floral spike was Ca (range, 2.4–4.1% dry wt.) and in the roots was Fe (range, 2.8–8.0% dry wt.). Sodium concentrations were greater in the submersed stem (range, 0.8–2.7% dry wt.), relative to the roots (range, 0.5–0.9% dry wt.) and flora stem (range, 0.3–1.1% dry wt.). Lead concentrated in the roots significantly more (range, 0.0040–0.0115% dry wt.), than in other plant parts (range, 0–0.0037% dry wt.).The submersed stem had spring or summer peaks in ash, P, Na, K, Ca, Mg, and Zn content, whereas Fe and Mn exhibited late winter to early spring peaks. No evidence of nutrient storage during the non-growing season was evident in the segments sampled. Explanations for the seasonal and morphological differences observed are presented.     

GRAZING OF FRESHWATER RHODOPHYTA1

Grazing of freshwater Rhodophyta was examined from three perspective: observation of algal fragments in gut contents of herbivores collecte from streams with populations of red algae, measurement of ignestion rates of three inveratebrates in single diet and choice experiments and analysis of potential food value of the rhodophytes by determining protein, carobohydrate, lipid and estimated caloric contents. The algal thallus forms studied were Audouinella violacea (Kütz.) Hamel, which consists of simple filaments growing in dense tufts, Batrachospermum virgatum (Kütz.) Sirod., which is a densely branched mucilaginous filament, and Tuomeya americanna (Harv.) Papenfuss, which is a branched cartilaginous psedoparenchyma. The thirteen invertebrate taxa containing pieces of one or more of these algae in theri guts included two amphipods, two mayfly larvae, three caddisfly larvae, one beetle larva, four chironomids and one snail. The frequencies with which the Rhodophyta were found in the gut contents were as follows: Audouinella 31%, Tuomeya 38%, Batrachospermum 46% and “chantransia stages” 54%. In all of the individual and choice experiments, Audouinella was ingested at the greates rate, followed by Batrachospermum and then Tuomeya, regardless, of the grazer. Audouinella had the highest protein and lipid contents while Tuomeya displayed the opposite trent. All three rhodophytes had very large calofic values (5.0–6.8 kcal μg^?1 dry weight) due to their high lipid and low ash contents. Therefore, preferential grazing of Audouinella appears to be bases on its simple thallus and high protein contents rather than its calorific value.     

Seasonal flexibility in organ size in the Andean lizard Liolaemus moradoensis

The understanding of animal functioning in fluctuating environments is a major goal of physiological and evolutionary ecology. In temperate terrestrial habitats, one of the most pervasive changes in environmental conditions is that associated with the seasonal change along the year. In this study, we describe the pattern of seasonal variation in the size of nine internal organs in the lizard Liolaemus moradoensis from the Andes Mountains of Central Chile. We observed that the size of digestive organs was greater during summer in comparison to other seasons. Dry masses of liver and fat bodies reached maximum values during summer and minimum during spring. We suspect that lowest spring values are related with build‐up costs of energetically expensive organs (e.g., digestive, muscle mass) at the end of the hibernation period. Dry mass of the heart and lungs did not show a clear pattern of variation, suggesting that cardiac and pulmonary performance were maintained throughout the year. The dry mass of kidneys was greater during winter than during summer, a result observed in other hibernating lizards but for which there is no clear explanation. Finally, the dry mass of testes showed a maximum value during autumn and a progressive reduction toward summer, indicating that reproduction occurs during autumn. When represented in a bivariate space, acquisition (digestive), distribution (heart, lungs and kidneys), storage (liver and fat bodies), and expenditure (testes) organs generate four clusters. In general terms, observed seasonal pattern of change in organ size is in agreement with those reported for other lizard species that inhabit highly fluctuating environments. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Calorific values of the soft parts of the tellinid bivalve Macoma balthica (L.) as determined by two methods

Calorific values of the ash-free dried soft parts of Macoma balthica (L.), sampled monthly during 1 year at a tidal flat station in the Dutch Wadden Sea, have been estimated by two methods. The indirect method started from an analysis of the biochemical composition and used conversion factors for the various components to calculate the calorific value of the total tissue. The direct method employed a micro-bomb calorimeter and a furnace for separate ash determinations.The indirect method yielded estimates that were consistently higher than those from the direct one. Two sources, explaining most of the difference, could be traced, viz. incomplete drying of the samples used in the calorimeter and a too high conversion factor for weight to calories applied conventionally for lipids. This conversion factor was found to depend on the method of extraction of the lipids. but was always significantly below the commonly used 9.3 to 9.7 cal · mg^?1.The best estimates for the annual mean of the calorific value at complete combustion of the ash-free and well-dried soft parts of Macoma were 5.47 for the direct and 5.59 cal·mg^?1 ash-free dry wt for the indirect method. Both values are higher than estimates for Macoma published earlier by various authors. Calorific values at physiological oxidation amount to only about 4.6 cal·mg^?1 or less, due to incomplete protein oxidation.For this reason, it is concluded that biologically significant calorimetry cannot dispense with biochemical analysis. On the other hand, calculation of calorific values from biochemical composition should be accompanied by some calorimetry, as the conversion factor for the lipid fraction, at least, may deviate significantly from commonly applied values.     

Morphology and life history of Scytosiphon canaliculatus comb. nov. (Scytosiphonales,Phaeophyceae) from Japan*

Morphology of field material and life history in culture were studied in Scytosiphon canaliculatus (Setchell et Gardner) comb. nov. from northern Japan. Erect gametophytes of S. canaliculatus are cylindrical, tubular, up to 7 mm wide and 40 cm long, and without regular constrictions. S. canaliculatus has pronounced anisogamy and ascocysts accompanied with plurilocular gametangia. The life history of S. canaliculatus showed an alternation between erect gametophytes and crustose prostrate sporophytes bearing unilocular sporangia. Since field sporophytes of S. canaliculatus were found to be identical with Hapterophycus canaliculatus Setchell et Gardner (Ralfsiaceae, Phaeophyceae), it is proposed to transfer H. canaliculatus to the genus Scytosiphon. In the field, gametophytes with plurilocular gametangia appeared in spring and disappeared in summer. Sporophytes with unilocular sporangia were collected in late autumn and winter. Unilocular sporangia were produced at 15°C in short-day culture conditions and unispores developed into erect gametophytes at 5–15°C. It is suggested that the seasonal Occurrence of gametophytes in the field is due to the seasonal formation of unilocular sporangia, which is regulated by temperature and photoperiod.     

Strong seasonal patterns of DOC release by a temperate seaweed community: Implications for the coastal ocean carbon cycle

Release of dissolved organic carbon (DOC) by seaweed underpins the microbial food web and is crucial for the coastal ocean carbon cycle. However, we know relatively little of seasonal DOC release patterns in temperate regions of the southern hemisphere. Strong seasonal changes in inorganic nitrogen availability, irradiance, and temperature regulate the growth of seaweeds on temperate reefs and influence DOC release. We seasonally surveyed and sampled seaweed at Coal Point, Tasmania, over 1 year. Dominant species with or without carbon dioxide (CO₂) concentrating mechanisms (CCMs) were collected for laboratory experiments to determine seasonal rates of DOC release. During spring and summer, substantial DOC release (10.06–33.54 μmol C · g DW⁻¹ · h⁻¹) was observed for all species, between 3 and 27 times greater than during autumn and winter. Our results suggest that inorganic carbon (C_i) uptake strategy does not regulate DOC release. Seasonal patterns of DOC release were likely a result of photosynthetic overflow during periods of high gross photosynthesis indicated by variations in tissue C:N ratios. For each season, we calculated a reef-scale net DOC release for seaweed at Coal Point of 7.84–12.9 g C · m⁻² · d⁻¹ in spring and summer, which was ~16 times greater than in autumn and winter (0.2–1.0 g C · m⁻² · d⁻¹). Phyllospora comosa, which dominated the biomass, contributed the most DOC to the coastal ocean, up to ~14 times more than Ecklonia radiata and the understory assemblage combined. Reef-scale DOC release was driven by seasonal changes in seaweed physiology rather than seaweed biomass.     

Seasonal variation in the energy content of benthic macroinvertebrates of Lake Nainital,U.P., India

The paper discusses seasonal variation in the energy contents of four macrobenthic invertebrates of Lake Nainital during 1977–78. The energy values varied from 16971–19437 J/g dry weight in Tubifex tubifex, 16 511–20 231 J/g dry weight in Glossiphonia weberi, 19 019–25 289 J/g dry weight in Chironomus plumosus and 19 583–20 549 J/g dry weight in Viviparus bengalensis. The former two genera exhibited highest energy contents during summer, whereas the latter two revealed highest values during winter. On mean annual basis, the highest values were recorded for C. plumosus and lowest for T. tubifex. In V. bengalensis, variation occurred in ash fraction (%) and in energy values (Joule per gram dry weight) but not in energy values of organic fraction (Joule per gram ash-free dry weight), while the other three genera displayed variation in all three variables.     

Annual changes in the concentration of minerals and organic compounds of Quercus suber leaves

Mature leaves are the primary source of sugars, which give rise to many secondary metabolites required for plant survival under adverse conditions. In order to study the interaction of field‐grown cork oak (Quercus suber L.) with the environment, we investigated the seasonal variation of minerals and organic metabolites in the leaves, using inductively coupled plasma atomic emission spectrometry, elemental analysis and nuclear magnetic resonance spectrometry. Statistical analysis showed that the data strongly correlated with seasonal climate and were divided in three groups corresponding to: (1) spring‐early summer, (2) summer and (3) autumn‐winter. The concentration of N, P, K and leaf ash content were highest in spring (recently formed leaves), reached the minimum during the hot and dry summer and increased slightly during the rainy period of autumn‐winter. Conversely, Na, Mg and Ca concentrations were lowest in spring‐early summer and increased during summer and autumn‐winter, the Ca concentration increasing five‐fold. Two cyclitol derivatives, quinic acid and quercitol were the major organic metabolites of the leaves. Their concentration along the season followed opposite trends. While quinic acid predominated during spring‐early summer, when it contributed 12% to the leaf osmotic potential, quercitol was predominant during autumn‐winter, when its contribution to leaf osmotic potential was about 10%. This different preponderance of the two compounds is expressed by the quercitol/quinic acid ratio, which can be as low as 0.2 in early summer and as high as 9 in winter. Sucrose and glucose concentrations also increased during autumn‐winter. Evidence for the quercitol protective role in plants during stress is discussed, and on the basis of structural similarity, it is suggested that quinic acid could have an identical importance, with a protective role against heat and high irradiance. It is concluded that the marked changes in Q. suber leaf composition throughout the year could have important implications in the plant capacity to endure climatic stress.     

Distribution and leaf growth of Thalassodendron pachyrhizum den Hartog in Southern Western Australia

The seagrass Thalassodendron pachyrhizum den Hartog grows on limestone reef platforms. Monthly leaf biomass was measured over 2 years and showed a strong seasonal variation with maximum biomass of 500 g m⁻². This seagrass loses all its leaves except for a bud and this characteristic was used to obtain a conservative estimate of productivity by change in standing stock. Leaf growth during the growing season was 6.6 mg Cg⁻¹ day⁻¹. Leaf length frequencies showed that new leaves formed during autumn (March–April). They grew from autumn until spring (November) and began to senesce in summer, followed by leaf fall in late summer (February–March).The growth of rhizome shoots “invading” free substratum space and the growth of new stems was measured for a 300-day period; about 9 leaves were produced in this period.     

Geographic variation in feeding preference of a generalist herbivore: the importance of seaweed chemical defenses

The ecological impacts of generalist herbivores depend on feeding preferences, which can vary across and within herbivore species. Among mesoherbivores, geographic variation in host use can occur because host plants have a more restricted geographic distribution than does the herbivore, or there is local evolution in host preference, or both. We tested the role of local evolution using the marine amphipod Ampithoe longimana by rearing multiple amphipod populations from three regions (subtropical Florida, warm-temperate North Carolina and cold-temperate New England) and assaying their feeding preferences toward ten seaweeds that occur in some but not all regions. Six of the ten seaweeds produce anti-herbivore secondary metabolites, and we detected geographic variation in feeding preference toward five (Dictyota menstrualis, Dictyota ciliolata, Fucus distichus, Chondrus crispus and Padina gymnospora, but not Caulerpa sertularioides). Amphipod populations that co-occur with a chemically-rich seaweed tended to have stronger feeding preferences for that seaweed, relative to populations that do not co-occur with the seaweed. A direct test indicated that geographic variation in feeding preference toward one seaweed (D. ciliolata) is mediated by feeding tolerance for lipophilic secondary metabolites. Among the four seaweeds that produce no known secondary metabolites (Acanthophora, Ectocarpus, Gracilaria and Hincksia/Feldmannia spp.), we detected no geographic variation in feeding preference. Thus, populations are more likely to evolve greater feeding preferences for local hosts when those hosts produce secondary metabolites. Microevolution of feeding behaviors of generalist marine consumers likely depends on the availability and identity of local hosts and the strength of their chemical defenses.