Specialized pelagic feeding by Erpobdella montezuma (Hirudinea)

The pelagic amphipod, Hyalella montezuma comprised nearly 90% of the diet of the leech, Erpobdella montezuma in the near thermally constant environment of Montezuma Well, Arizona. This restricted diet, confirmed by both gut content and serological analyses, occurred even though other potential prey were abundant throughout the year in the pelagic column, littoral vegetation and sediments. Seasonal electivity indices for this pelagic prey species were ≥0.9. It is suggested that the highly predictable environmental conditions in Montezuma Well have contributed to this very close predator–prey relationship.     

Concomitant diel vertical migration of a predatory leech and its amphipod prey

SUMMARY.

• 1 The concomitant diet vertical migrations of the predaceous leech Erpobdella montezuma and its pelagic amphipod prey (Hyalella montezuma) were monitored for 1 year in Montezuma Well, Arizona, U.S.A.
• 2 High densities of H. montezuma occurred in the subsurface strata of the water column during the day, but a substantial portion of the population migrated into the surface 1 m at sunset. E. montezuma remained in the lower water strata during the day, but migrated vertically after sunset to exploit the high densities of H. montezuma near the surface. Densities of E. montezuma progressively increased in the upper strata of the water column after sunset as light energy fell below 1 μEin m^?2 s^?1.
• 3 It is suggested that the synchronized nocturnal migrations of the predator E. montezuma in relation to its prey H. montezuma, increases the foraging efficiency of the leech in the highly predictable environment of Montezuma Well.

     

The evolutionary importance of mechanoreception in three erpobdellid leech species

Summary The ability of an erpobdellid leech, Erpobdella montezuma, to discriminate between two congeneric amphipod prey (Hyalella montezuma and Hyalella azteca) through mechanoreception was examined. Both E. montezuma and H. montezuma, a pelagic filter feeder, are endemic to the near thermally constant environment of Montezuma Well, Arizona, and form a highly specialized predator-prey relationship. In contrast, H. azteca, a benthic detritivore, is widely distributed throughout North America. Erpobdella montezuma was highly responsive to the prey signals of the endemic H. montezuma and showed only a limited response to H. azteca. An inverse relationship occurred between the frequency of attack and size of the leech. The abilities of Erpobdella punctata and Nephelopsis obscura, neither found in Montezuma Well, to detect the signals of both amphipod prey were also examined. These species with non-specialized diets showed a low response to the signals of both prey species. This study demonstrates that very close predator-prey relationships can develop over evolutionary time in isolated aquatic systems through the use of mechanoreception.     

Prey location and prey choice by the freshwater leech Erpobdella octoculata using foraging kairomones

1. The freshwater leech, Erpobdella octoculata , is a generalist predator feeding on prey organisms such as Tubifex spp., Chironomus spp. and Asellus aquaticus. Using different experimental designs, we studied the use of foraging kairomones by the leeches for prey location.
2. Leeches were attracted to living as well as to freshly killed larvae of Chironomus sp., to Tubifex sp., and to A. aquaticus offered in Petri dishes covered with gauze. The leeches also reacted to an extract of macerated Tubifex sp. presented in agar.
3. Using ion exchange chromatography, the presence of histidine and glutamic acid was demonstrated in water contaminated by living larvae of Chironomus sp. and Tubifex sp. Agar blocks containing a synthetic mixture of these compounds at concentrations above 5 mg mL⁻¹ were attractive to the leeches.
4. Choice tests showed that leeches preferred chemical signals from Tubifex sp. over larvae of Chironomus sp. No difference was found between chemicals from Tubifex sp. and A. aquaticus , and A. aquaticus and larvae of Chironomus sp.
5. The results demonstrate that E. octoculata uses specific foraging kairomones in searching for prey and indicate that amino acids serve as foraging kairomones.     

Are the lake-dwelling leeches,Glossiphonia complanata (L.) and Helobdella stagnalis (L.), opportunistic predators on molluscs and do they partition this food resource?

SUMMARY. 1. Predation on the mollusc genera Potamopyrgus, Bithynia, Valvata, Lymnaea, Physa, Planorbis and Pisidium by the leeches Glossiphonia complanata (L.) and Helobdella stagnalis (L.) was examined in five eutrophic lakes at monthly intervals over one year using a serological technique.
2. Feeding intensity varied seasonally, being lowest in winter and highest in summer. Small, medium and large leeches preyed to the same extent on the several mollusc genera; this was true for both leech species. Qualitatively, the diet of both glossiphoniids was similar throughout the year. Snail genera were consumed in proportion to their relative abundance in the field. Quantitatively, Glossiphonia preyed more heavily on snails than did Helobdella.
3. It is concluded that both leech species are opportunistic predators.     

The affect of temperature on the metabolism and behaviour of an endemic amphipod, Hyalella montezuma, from Montezuma Well, Arizona, U.S.A.

1. Hyalella montezuma is endemic to Montezuma Well, Arizona, and is exposed to minimal diel and seasonal temperature fluctuations in the pelagic zone (21 ± 4 °C). Juvenile H . montezuma feed in the pelagic zone during the day and migrate into the littoral vegetation at night, while adults remain primarily in the littoral vegetation.
2. Oxygen consumption ( V O₂) of adult and juvenile H . montezuma was measured at 20, 25 and 30 °C. The V O₂ of both adult and juvenile H . montezuma increased with temperature. However, the V O₂ of juveniles was significantly greater than that of adults at all temperatures, with greatest divergence at 30 °C where mean juvenile V O₂ (6.31 μl mg^–1 dry weight (DW) h^–1) was almost twice that of adults (3.60 μl mg^–1 DW h^–1).
3. Survivorship of juveniles was significantly lower (54%) at 30 °C than at 27.5 °C (95%) after 4 h, whereas adults showed at least a 93% survivorship at both temperatures.
4. Our data suggest that temperature may have been the proximate cue that elicited the diel horizontal migration of juvenile H . montezuma in Montezuma Well, with the behaviour maintained and enhanced by intensive invertebrate predation in the pelagic and littoral zones.     

The affect of temperature on the metabolism and behaviour of an endemic amphipod, Hyalella montezuma, from Montezuma Well, Arizona, U.S.A.

1. Hyalella montezuma is endemic to Montezuma Well, Arizona, and is exposed to minimal diel and seasonal temperature fluctuations in the pelagic zone (21 ± 4 °C). Juvenile H . montezuma feed in the pelagic zone during the day and migrate into the littoral vegetation at night, while adults remain primarily in the littoral vegetation.
2. Oxygen consumption ( V O₂) of adult and juvenile H . montezuma was measured at 20, 25 and 30 °C. The V O₂ of both adult and juvenile H . montezuma increased with temperature. However, the V O₂ of juveniles was significantly greater than that of adults at all temperatures, with greatest divergence at 30 °C where mean juvenile V O₂ (6.31 μl mg^–1 dry weight (DW) h^–1) was almost twice that of adults (3.60 μl mg^–1 DW h^–1).
3. Survivorship of juveniles was significantly lower (54%) at 30 °C than at 27.5 °C (95%) after 4 h, whereas adults showed at least a 93% survivorship at both temperatures.
4. Our data suggest that temperature may have been the proximate cue that elicited the diel horizontal migration of juvenile H . montezuma in Montezuma Well, with the behaviour maintained and enhanced by intensive invertebrate predation in the pelagic and littoral zones.     

Varied diet and opportunistic foraging in the Ethiopian Bush-crow Zavattariornis stresemanni,an Endangered generalist

The Ethiopian Bush-crow Zavattariornis stresemanni is an endangered, co-operatively breeding southern Ethiopian endemic with a remarkably restricted range (c. 6 000 km²). The species’ range was recently found to be almost perfectly predicted by an envelope of cooler, drier and more seasonal climate than surrounding areas, but the proximate determinants of this range restriction remain unclear. We assessed whether specialisation in diet or foraging may restrict the range of the species by conducting foraging watches to determine prey composition, augmented by observations of opportunistic foraging techniques, and by comparing our results to previously published information on diet. Prey composition comprised a range of arthropods, such as insect larvae (62.7%), beetles (Coleoptera) (15.6%), and grasshoppers and crickets (Orthoptera) (11.8%). Prey was primarily obtained by pecks above ground (74.2%) but also frequently dug up (23.8%). Prey capture was most successful during pecks and we also found chicks were preferentially fed larger prey items over smaller ones by adults. We documented opportunistic behaviours such as nest-raiding and ox-pecking. Diet and foraging are varied and unspecialised, and therefore do not appear to explain the restricted range of the Ethiopian Bush-crow.     

Population dynamics and production of the pelagic amphipod Hyalella montezuma in a thermally constant system

SUMMARY. 1. The population dynamics and annual production of the multivoltine. pelagic amphipod Hyalella montezuma were studied over a 3-year period in the thermally constant environment of Montezuma Well. Arizona.
2. H. montezuma showed two maxima which coincided with spring and autumn phytoplankton maxima. Juveniles comprised over 85% of the population in the pelagic zone compared to 37% in the littoral vegetation and there were significantly more females in the littoral vegetation. It appears that juvenile and adult H. montezuma show different habitat preferences.
3. Total annual mean production for H. montezuma calculated by the size frequency method and adjusted for multiple generations, was 357 kg ha⁻¹ yr⁻¹, which is higher than single-species production estimates reported for most zoobenthic amphipods and multivoltine planktonic crustaceans. Average energy production for H. montezuma was 4640 kJ ha⁻¹ yr⁻¹ in the pelagic zone and 1072 kJ ha⁻¹ yr⁻¹ in the littoral vegetation.
4. Average cohort P / ratios for H. montezuma were higher in the pelagic zone (5.5) than in the litttoral vegetation (3.7). Juveniles had higher cohort P ratios than adults in the pelagic zone, while the reverse relationship was true in the littoral vegetation. We propose that different size-selective predators may contribute to the differences in P ratios for juveniles and adults in these two habitats.     

Predator–prey interactions: why do larger albatrosses eat bigger squid?

The relationship between predator sizes and prey sizes is well documented for terrestrial but rarely for marine ecosystems. We show that wandering albatrosses, the biggest albatross species, feed on larger cephalopod prey than those consumed by smaller albatrosses (grey-headed and black-browed albatrosses). This reflects differences in timing of breeding, foraging ecology and their feeding methods. Wandering albatrosses breed later in the year, during the austral winter, than smaller albatrosses (therefore catching older squid) and forage most of the year in Antarctic open waters, sub-Antarctic, subtropical and tropical waters, overlapping minimally with the smaller albatrosses' foraging range while breeding. Also, wandering albatrosses mostly scavenge whereas smaller albatrosses feed more on live prey. Prey ecology may also play a key role because many squid species might experience post-spawning mortality during the austral winter, becoming easily available to wandering albatrosses. Spawning in winter can be linked to predator avoidance (i.e. reduction in mortality in winter by avoiding pelagic predators) and would allow squid larvae to develop and take advantage of the high productivity (i.e. Antarctic phytoplankton bloom) in spring and at the beginning of summer. Thus, aspects of prey and predator ecology may combine to generate observed differences in prey size.     

The costs of becoming nocturnal: feeding efficiency in relation to light intensity in juvenile Atlantic Salmon

1. Most animals are active by day or by night, but not both; juvenile salmonids are unusual in that they switch from being predominantly diurnal for most of the year to being nocturnal in winter. They are visual foragers, and adaptations for high visual acuity at daytime light intensities are generally incompatible with sensitive night vision. Here we test whether juvenile Atlantic Salmon Salmo salar are able to maintain their efficiency of prey capture when switching between diurnal and nocturnal foraging.
2. By testing the ability of the fish to acquire drifting food items under a range of manipulated light intensities, we show that the foraging efficiency of juvenile salmon is high at light intensities down to those equivalent to dawn or dusk, but drops markedly at lower levels of illumination: even under the best night condition (full moon and clear sky), the feeding efficiency is only 35% of their diurnal efficiency, and fish will usually be feeding at less than 10% (whenever the moon is not full, skies are overcast or when in the shade of bankside trees). Fish were unable to feed on drifting prey when in complete darkness.
3. The ability of juvenile salmon to detect prey under different light intensities is similar to that of other planktivorous or drift-feeding species of fish; they thus appear to have no special adaptations for nocturnal foraging.
4. While winter drift abundance is slightly higher by night than by day, the difference is not enough to compensate for the loss in foraging efficiency. We suggest that juvenile salmon can nonetheless switch to nocturnal foraging in winter because their food requirements are low, many individuals adopting a strategy in which intake is suppressed to the minimum that ensures survival.     

Population dynamics and production of Jesogammarus annandalei, an endemic amphipod, in Lake Biwa, Japan

SUMMARY 1. Population dynamics and production of Jesogammarus annandalei , an endemic amphipod in Lake Biwa, were examined from April 1997 to June 1998. The life cycle of this species was 1 year with the new generation beginning in early autumn. They preferred low temperature (<12 °C) and their spatial distribution varied seasonally and accordingly.
2. In deep water, the abundance of J. annandalei ranged from 200 to 63 000 m⁻² and decreased towards summer and the biomass (0.01∼3.6 g C m⁻²) was on average comparable that of zooplankton. The density was much higher than that recorded by a study conducted 35 years ago.
3. Individual growth rate of this amphipod was high in winter and spring but decreased in summer. Annual production of J. annandalei (6.2 g C m⁻² year⁻¹) was only 2% of primary production but was at the higher end of the range reported for amphipods in oligo- and mesotrophic lakes.
4. These results are consistent with the view that Lake Biwa is becoming more eutrophic, with a consequent decrease in the abundance of predatory fish in the profundal zone.     

Discrete prey availability promotes foraging segregation and early divergence in Arctic charr, <Emphasis Type="Italic">Salvelinus alpinus</Emphasis>

Many animal species show individual foraging specialisms when potential prey requires prey-specific foraging strategies. Arctic charr are often found as benthic (macroinvertebrate) or pelagic (plankton) foraging specialists. Here, we tested specifically if given a choice of prey with different characteristics individuals would specialise in a single prey type and if individuals would chose prey based on their expressed trophic morphology, in a laboratory experiment and in a field observation. When offered a choice of benthic and pelagic prey most individuals (73%) showed that 100% fidelity to a single foraging source. Naïve individuals (not previously exposed to natural prey) with more robust head and mouth shape were more likely to forage on a benthic prey source (chironomids). In contrast, individuals with a more fusiform body, larger eye, but more slender head shape were more likely to specialise on pelagic prey (Artemia). Field observations of a natural population of Arctic charr from Loch Doine identified specialists foraging on either plankton or macrobenthos (on the basis of stomach contents) and some generalists. Morphological analysis showed that significant differences in shape reflecting recent foraging history. These results support the hypothesis that the availability of discrete, different prey types results in discrete foraging specialisms which in turn may result in the expression of discrete alternative phenotypes through subsequent plastic ontogenetic process. We conclude that this provides a partial explanation for why ecologically driven evolution processes are particularly prevalent in fishes from post-glacial lake systems.     

Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies ( Megarcys signata ) on fine sediment deposition and removal.
2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.
3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.
4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream-side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.
5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.
6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.     

Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies ( Megarcys signata ) on fine sediment deposition and removal.
2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.
3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.
4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream-side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.
5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.
6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.     

The recent evolutionary origin of the phenotypically novel amphipod Hyalella montezuma offers an ecological explanation for morphological stasis in a closely allied species complex

Numerous molecular studies have identified morphologically cryptic, freshwater invertebrate species, but have not suggested possible mechanisms for their phenotypic stasis. The amphipod crustacean genus Hyalella contains numerous morphologically cryptic species in the H. azteca complex, as well as a small number of morphologically very divergent, narrowly endemic taxa. One such taxon, Hyalella montezuma, is the sole planktonic filter-feeder within the North American amphipod fauna, and is known only from Montezuma Well, a fishless travertine spring mound in Arizona, USA. In this study, we conduct a phylogenetic analysis of mtDNA sequence data using likelihood, Bayesian and cladistic approaches to determine both the relationship of H. montezuma to the H. 'azteca' species complex, and to ascertain if its morphological and ecological differentiation have been comparatively recent. The results show that H. montezuma has a very close phylogenetic affiliation with one lineage in the H. azteca complex, indicating that its origin has been recent. We present evidence suggesting that fish predation is an important ecological factor, which constrains morphological and ecological diversification within the genus Hyalella, and that Montezuma Well has provided a relaxation on this constraint.     

Colony productivity and foundress behaviour of a native wasp versus an invasive social wasp

Abstract.  1. Colony productivity, prey utilisation, and foundress behaviour of a North American native wasp ( Polistes fuscatus ) versus an European invasive wasp ( Polistes dominulus ) were investigated in a controlled field experiment with optimal versus natural foraging conditions. Colonies with the optimal prey foraging conditions were provided with prey ad libitum within an enclosed area. The other colonies foraged in the adjacent field–woodland but had the same nest conditions as the other treatment.
2. When given prey ad libitum , both wasp species captured similar amounts of prey and the conversion to total offspring biomass was similar. But P. dominulus colonies produced 2.5 times the number of workers as P. fuscatus colonies, reflecting the smaller size of P. dominulus wasps.
3. Foundresses of P. dominulus were observed more often building or repairing the nest, thereby contributing to the production of colonies with twice as many cells as colonies of P. fuscatus . Foundresses of P. dominulus showed more acts of aggression toward workers than did P. fuscatus foundresses, which was not a function of adult density on the nest.
4. At the end of the experiment, P. dominulus colonies with optimal prey foraging conditions still had a high level of egg-laying and peaked in the number of pupae then, whereas egg-laying and the number of pupae per colony of the other treatments began to decline 2–3 weeks earlier. These results indicate that P. dominulus is more opportunistic than P. fuscatus , which may account in part for P. dominulus 's success as an introduced species in North America.     

Foraging conditions ‘at the end of the world’ in the context of long‐distance migration and population declines in red knots

The long‐distance migrant red knot (Calidris canutus ssp. rufa– Scolopacidae) alternates between the northern and southern ends of the New World, one of the longest yearly migrations of any bird and paradoxically overflying apparently suitable habitat at lower latitudes. This subspecies is sharply declining, with a major mortality event following 2000, attributed to commercial overharvesting of food resources at its Delaware Bay (USA) stop‐over site. A full understanding of this peculiar migrant requires an assessment of the foraging conditions at its southern hemisphere wintering sites. Here, for a major wintering site in Argentinean Tierra del Fuego (Río Grande), we describe and compare food abundance, diet and intake rates during January–February in 1995, 2000 and 2008. The two main prey types were the burrowing clam Darina solenoides and three species of epibenthic mussels Mytilidae. In the year 2000, food availability and intake rate were higher than those recorded at other sites used by knots anywhere else in the world, contributing to the explanation of why red knots carry out this impressive migration. Intake rate in 2008 on the two main prey types was dramatically reduced as a result of birds eating smaller prey and strongly increased human disturbance; the same year we also found a high prevalence of a digenean parasite in Darina. We suggest that during the strongly enhanced winter mortality in 2000, knots did not yet face ecological problems in their southernmost wintering area, consistent with the previous evidence that problems at northern stop‐overs negatively affected their numbers. However, in 2008 the ecological conditions at Río Grande were such that they would have facilitated a further decline, emphasizing the importance of a hemispheric approach to research and management.     

Changes in the Montagu’s Harrier Circus pygargus diet in Eastern Poland across decades promote insects and reptilians,but not birds and rodents

We investigated temporal changes in diet composition of the Montagu''s Harrier Circus pygargus breeding in natural habitat (calcareous peat bog) in SE Poland. We characterized diet composition in a three‐year period (2007–2009), based on pellet analyses. We investigated whether diet composition was affected by years or stage of breeding. We compared diet of the studied population between 2000s and 1990s and with other populations. We found that the food of the studied population was dominated by insects and mammals (by number) and mammals and birds (by biomass). Biomass and abundance of main prey items differed between studied years because of different air temperatures. We found some interannual differences in contribution of some prey items including higher number of thermophilic prey (insects and amphibians) in warmer years. Comparison of pellet composition in the 1990s and 2000s revealed significant increase in the abundance of thermophilic prey (insects and reptiles) and decrease of mammals including Microtus voles and birds. Those changes may be linked to habitat changes in areas neighboring peat bogs and climate change‐induced changes in prey communities. The studied population was able to respond to changes in foraging habitats and prey composition by opportunistic foraging on easily available prey. The diet of the studied population is the most similar to the geographically closest populations foraging in similar habitats and characterized by high contribution of insects.     

Reproduction and growth of the Arctic hyperiid amphipod Themisto libellula Mandt

Summary The hyperiid amphipod, Themisto libellula, is abundant in Arctic seas and is an important prey for fish, birds and mammals. Characterization of its life cycle has been hampered by the lack of winter collections. In this study, that portion of the hyperiid population inhabiting the upper 30 m of the water column in Frobisher Bay was sampled periodically in winter, spring and summer during five consecutive years. In one year the winter and spring collections included large numbers of ovigerous females. These samples provide new information about timing of reproduction, size at maturity, fecundity, embryonic development, hatching, release and rate of growth of T. libellula in the Arctic. The average length of the ovigerous females was 23.3 mm. This and the summer growth rate of about 6 mm/month indicates that they can reproduce when one year old. Eggs are laid prior to early February, and although some young may be released from the marsupium as early as mid-March, most are liberated during April and May. The fecundity is significantly correlated with female length and an average 23 mm long individual had 275 embryos. The size-specific fecundity is similar to that of hyperiids from temperate and subtropical waters.