Microhabitat associations of three species of Dryopoidea (Coleoptera) in an Ozark stream: a comparison of substrate, and simple and complex hydraulic characters

Microhabitat associations of adults and larvae of Stenelmis lateralis Sanderson, Optioservus sandersoni Collier (Coleoptera: Elmidae) and larvae of Psephenus herricki (DeKay) (Coleoptera: Psephenidae) were examined within riffles of an Ozark stream in Missouri. The invertebrate fauna was sampled from 150 quadrats, and each quadrat was characterized by a suite of substrate, simple hydraulic, and complex hydraulic variables. Larvae were assigned to size classes for analyses. MANOVAs were performed to determine if differences existed between sites of presence and absence for each of the three species separately. Subsequently, ANOVAs and Bonferroni sequential adjustments were performed to determine differences in each habitat variable that are related to presence or absence of each group. Multiple regression was performed to determine which habitat variables were important determinants of density for each taxon. Finally, discriminant function analyses were performed to determine the degree of overlap in microhabitat associations of each group. A clear distinction was seen between sites of occurrence and absence for P. herricki, and for larvae of each elmid species (MANOVAs). For P. herricki, current velocity and the complex hydraulic characteristics of Reynolds number and boundary Reynolds number were important in determining beetle presence. In contrast, substrate characteristics were major determinants of presence for larvae of O. sandersoni and S. lateralis. In general, the density of each of the three species was related to a different subset of abiotic variables. For each species, the most definitive microhabitat associations were seen in the early instars. Microhabitat associations for larvae of each elmid species were size class-specific. Inclusion of complex hydraulic characteristics along with substrate and simple hydraulic characteristics provided greater resolution of microhabitat associations.     

The water velocity preferences of Austrosimulium furiosum and Simulium ornatipes (Diptera: Simuliidae) larvae,and the implications for micro-habitat partitioning

The aquatic larvae of two simuliid species, Austrosimulium furiosum (Skuse) and Simulium ornatipes Skuse, which often occur together in Victorian streams, were shown in laboratory experiments to have preferences for different water velocities: larvae of A. furiosum preferred water velocities of 0.2–0.3 m s^–1, and S. ornatipes preferred water velocities of 0.9–1.3 m s ^–1 . Final instar larvae of both species selected slow water speeds of less than 0.25 m s ^–1 prior to pupation. Flow patterns around a cylinder in a laboratory stream were mapped, and the distribution of A. furiosum larvae within the wake, paired vortices and horseshoe vortex was recorded. Larvae selected areas with suitable water velocities and aligned with the flow, providing flow visualization of micro-current speed and direction. The potential for micro-habitat partitioning is discussed in relation to benthic sampling strategies.     

Life history and ecological role of the xylophagous aquatic beetle, Lara avara LeConte (Dryopoidea: Elmidae)

SUMMARY.

• 1 This study documents the life history of the xylophagous elmid beetle, Lara avara, and estimates its contribution to wood degradation in Oregon streams. The life cycle was found to be 4 to 6 or more years long, with all but 2–3 months of that spent in the larval stage.
• 2 Larvae grow through seven instars, taking about 1 year for instars 1–3, and from 3 to 5 or more years for instars 4–7.
• 3 Last-instar larvae leave the water to pupate. Adults live approximately 3 weeks and occur from May to August. The eggs are deposited on submerged wood.
• 4 Larvae probably obtain their nutrition by absorbing substances liberated into decaying wood by microbial activity. They do not produce their own cellulase, nor do they have a symbiotic gut flora similar to that of xylophagous cranefly (Tipulidae) larvae.
• 5 Faecal production by L. avara larvae averaged 13% dry body wt d^-1. This yields an estimate of faecal production of 1.6 g m^-2 y^-1 in Oregon Coast Range streams (about 0.3% y^-1 of wood standing crop).

     

Water fleas (Daphnia magna) provide a separate ventilatory mechanism for their brood

The planktonic filter feeder Daphnia magna depends on a steady oxygen supply by convection. In the ventral carapace chamber, this convection is established by the feeding current which is generated by the movement of the thoracic limbs. The present study revealed that this movement can cause an additional flow of medium which passes through the brood chamber of the animal. To visualise this current, ink or fluorescent microspheres were released by a microcapillary near the posterior opening of the brood chamber. The tracks of these probes were monitored by video microscopy. Digital motion analysis was used for the determination of flow velocity and flow rate. Ambient medium entered the brood chamber at the abdomen of the animal and moved then to the anterior end of the brood chamber before entering the ventral carapace chamber. Two horizontal lamellae, which are attached at both sides of the trunk and project laterally to contact the carapace walls, almost completely separate the dorsal brood chamber from the ventral carapace chamber. Water can only pass these barriers through small depressions in these lamellae at the level of the 3rd and 4th appendages. Female daphnids with embryos at late developmental stages showed more rapid water currents in the brood chamber than those with younger embryos. Moreover, animals showed higher flow rates when exposed to hypoxic conditions. As the oxygen uptake rate of older embryos is approximately three times higher than that of younger embryos, the enhanced brood chamber current could improve the oxygen availability for both the mother and its brood under conditions of reduced oxygen availability.List of symbols:a specific oxygen consumption rate (nmol mm-³ h-¹), K Krogh constant for oxygen diffusion (nmol mm-¹ h-¹ kPa-¹, P_c critical oxygen partial pressure (kPa), P o₂ oxygen partial pressure (kPa), r radius (mm), s distance (mm), t time (ms)     

Estimation of preferred water flow parameters for four species of Simulium (Diptera: Simuliidae) in small clear streams in South Africa

Blackfly larvae typically occur in fast-flowing riffle sections of rivers, with different blackfly species showing preferences for different hydraulic conditions. Very little quantitative data exist on hydraulic conditions linked to the blackfly species occurring in South African streams. Stones-in-current biotopes (i.e. fast riffle flows over cobbles) were sampled from four sites in three small clear streams in the Eastern and Western Cape provinces of South Africa. Mean water column velocities at each sampled stone were measured using a mini current meter, while flow velocities closer to the boundary layer where blackfly larvae occurred were estimated using indirect techniques (standard hemispheres and aerating tablets). Standard hemispheres were also used to calculate more complex hydraulic parameters such as Froude and Reynolds numbers. Four species of Simuliid were sampled in sufficient numbers to show trends in flow velocity preferences. Simulium impukane and S. rutherfoordi both occurred at their highest densities at velocities of 0.3m s^?1, while S. merops preferred velocities of 0.7m s^?1. Simulium nigritarse SL attained the highest densities of all the blackfly species sampled, and its relative abundances were greatest at velocities of 0.8–0.9m s^?1. Within the streams surveyed, all blackfly species occurred in subcritical-turbulent flows — based on a classification using Froude and Reynolds numbers — although two of the species were also found in high densities in supercritical flows where these existed at the sites. Local hydraulics within the stones-in-current biotope are complex, but in the absence of fine-scale equipment for measuring micro-velocities, standard hemispheres are a useful, cost-effective technique for the initial quantification of hydraulic parameters in small, clear streams. Such an approach facilitates further understanding of links between hydraulics and aquatic invertebrates in South African streams.     

Carapace movements aid air breathing in the semaphore crab,Heloecius cordiformis (Decapoda: Brachyura: Ocypodidae)

Summary While on land and recirculating branchial water the Australian semaphore crab Heloecius cordiformis (Decapoda: Ocypodidae), a semi-terrestrial airbreathing mangrove crab, sequentially depresses and elevates its carapace in a regular pump-like manner. The functional role of these carapace movements in aerial oxygen consumption is investigated. Carapace immobilisation (reversible and non-injurious) did not appear to affect branchial water circulation. In dry crabs (branchial water removed) carapace immobilisation had no effect on the rate of oxygen consumption (VO₂), heart rate or whole-body lactate (WBL) levels. In wet crabs (with branchial water) carapace immobilisation caused VO₂ to drop by 38% from 81 to 46 l O₂ · g^-1 · h^-1, heart rate to decline by 32%, from 2.5 to 1.7 Hz, and WBL levels to increase over 2.5-fold, from 0.27 to 0.67 mg · g^-1, after 3 h of carapace immobilisation. The (VO₂) of carapace-immobilised crabs with branchial water was similar to lung-occluded crabs with branchial water. Severe hypoxia induced physiological responses similar to those of carapace-immobilised crabs with branchial water. After 3 h of severe hypoxia, heart rate had declined by 80%, from 2.2 to 0.43 Hz, and the incidence of carapace pumping slowed by 85%, from 2.4 to 0.37 cycles · min^-1. It is concluded that in the absence of carapace movements branchial water in some way inteferes with lung ventilation. Under normal circumstances water circulation and lung ventilation are mutually exclusive processes (due to their singular dependence on the scaphognathites), yet in Heloecius these processes must be carried out simultaneously. Carapace movements may alleviate this conflict.Abbreviations FF, FR, SF, SR fast-forward, fast-reverse, slow-forward, slow-reverse scaphognathite pumping - MEA Milne Edwards aperture - VO₂ rate of oxygen consumption - WBL whole-body lactate     

Characteristics of nutrient dynamics in Lake Sagate,Japan, a shallow sand dune lake

We examined differences in digestibility and viability following gut passage through water penny larvae (Psephenus herricki) of Synedra ulna and Achnanthidium lanceoloatum, two common diatom taxa that differ in growth habit and autecological characteristics. Prior to the experiment, diatoms were cultured in Chu-10 media in petri plates to establish a monospecific biofilm to offer grazers. After collection, insects were left to clear their guts over night, allowed to graze for 3 hours on diatom biofilms, and then placed in vials over 1-mm mesh to defecate. Samples from source material and from insect feces were mounted in syrup media and the ratio of chloroplast-containing to empty diatom frustules was microscopically assessed. In addition, subsamples from source material and feces were sprayed onto agar plates prepared with Chu-10 and individual cells were mapped and tracked for 5 days to quantify reproduction. Cells of both S. ulna and A. lanceolatum taken from source material formed colonies on agar. Achnanthidium lanceolatum cells from insect feces also formed colonies, but with lower densities than those from source material. In contrast, none of the S. ulna cells tracked from fecal cultures formed colonies, and the percentage of S. ulna cells that were dead was significantly greater in feces relative to source material. Dead cell percentages of A. lanceolatum were also higher in feces relative to source material, but to a lesser degree than observed for S. ulna. These findings have potential implications for linking patterns of energy transfer in stream ecosystems and the structure and dynamics of benthic microalgal communities.     

Morphological features of the larvae of spider crab <Emphasis Type="Italic">Pugettia quadridens</Emphasis> (Decapoda: Majidae) from the Northwestern Sea of Japan

The entire cycle of larval development of the spider crab Pugettia quadridens (de Haan, 1850) (Decapoda: Majidae), widespread in Peter the Great Bay (Sea of Japan) is studied under the laboratory conditions. The development cycle of this species comprises prezoea, zoea I, zoea II, and megalopa. At a temperature of 18–20° C larval development took from 11 to 15 days. Zoea II is described in detail for the first time. Many morphological characters are found distinguishing zoea and megalopa of P. Quadridens in Russian waters from the larvae of this species in Japanese and Korean waters. Some characters of larvae are similar in P. Quadridens and the related species of the genus Pugettia. The larvae of P. Quadridens occur in the plankton of Vostok Bay from late June to late October with a density up to 5 ind/m³ at a surface water temperature of 13–21°C. They are easily distinguished from the other brachyuran larvae of this region by the absence of lateral spines on the carapace.Original Russian Text Copyright © 2004 by Biologiya Morya, Kornienko, Korn.     

Comparison of the cold hardiness of two larval Lepidoptera (Noctuidae)

Abstract Diapause larvae of the European corn borer (Ostrinia nubilalis (Hubn.)) and the related Mediterranean noctuid Sesamia cretica Led. possess sufficient supercooling ability to avoid freezing over their normal environmental temperature ranges. In progressive chilling experiments (10 days acclimation at each 5° step in the temperature range from 15 to ?5°C), mean supercooling points (measured at a cooling rate of 0.1°C min^?1) were lowered from ?20.4°C at 15°C to ?24.0°C at 5°C (lower lethal temperatures: c.?28°C) in O.nubilalis, compared with ?15.0 to ?17.2°C (lower lethal temperatures: ?15 to ?17°C respectively) in S.cretica. Concentrations of glycerol and trehalose determined by gas chromatography of whole body extracts were consistently higher in the former than in the latter species at both 15 and 5°C, and may be responsible for the deeper supercooling in O.nubilalis larvae. Acclimation to 5°C increased glycerol levels in O. nubilalis extracts compared with 15°C, and this was enhanced in larvae exposed for a further 10 days at each of 0 and ?5°C (glycerol being 438μmol ml^?1 body water). Haemolymph glycerol concentrations showed a similar pattern to whole body extracts in this species. Fat body glycogen was reduced during low temperature acclimation in both species. Body water contents did not change with acclimation in O.nubilalis, whilst S.cretica, containing significantly more water, lost c.7% during acclimation from 15 to 5°C. Haemolymph osmolalities increased during acclimation, especially in Ostrinia larvae, probably as a result of the accumulation of cryoprotectants. The majority of O.nubilalis larvae survived freezing under the conditions of the cooling experiments, whilst larvae of S.cretica did not, thereby confirming an element of freezing tolerance in the former.     

Larval Mirogrex terraesanctae (Cyprinidae) of Lake Kinneret (Israel): growth rate,plankton selectivities,consumption rates and interaction with rotifers

The rotifer Synchaeta pectinata dominated gut content of first feeding Mirogrex larvae (7 mm, 10 days age) and was a selected prey of neuston-caught larvae up to 15 mm TL. A negative L-value (linear index of selection) applied to predation on nauplii and copepodites by 7 and 8 mm larvae; nevertheless, caloric intake was dominated by copepods in 8–10 mm larvae. Neuston-caught larvae 13–20 mm TL fed selectively on Cladocera, especially Bosmina, and on the rotifer Asplanchna spp.Growth, estimated from otolith ring counts and from analysis of size distribution data, ranged from 3 to 7 mm mo^–1, with higher rates for early spawned larvae. When consumption as estimated from gut content, was compared to amounts of food required for growth, it appeared that the smallest larvae were underfed, while 13–16 mm fish obtained rations close to sufficiency.Rotifer standing stock biomass in Lake Kinneret has decreased in recent years, especially in winter, the spawning period of Mirogrex. Postulated causes are predation by an increasingly large population of Mirogrex larvae, and decrease of external supply. Larval distribution appeared to be linked to S. pectinata abundance; highest densities of both organisms occurred in the area of inflow from the Jordan and Golan streams. Larval food enrichment of inflow water by fish pond drainage might have caused observed increases in Mirogrex stock size since 1960.     

Carapace movements associated with ventilation and irrigation of the branchial chambers in the semaphore crab,Heloecius cordiformis (Decapoda: Brachyura: Ocypodidae)

Summary Carapace movements in crabs are briefly reviewed. While on land and recirculating branchial water, the Australian semaphore crab Heloecius cordiformis (Decapoda: Ocypodidae), a semi-terrestrial air-breathing mangrove crab, sequentially depresses and elevates its carapace relative to its thorax (0.5–1 mm excursion) in a regular pump-like manner. In quiescent crabs each carapace-pumping cycle lasts about 4 s; carapace depression takes 3 s and elevation 1 s. Carapace movements are brought about by pressures generated within the branchial chambers by the scaphognathites, probably in combination with carapace muscles. Carapace movements are associated with bilaterally synchronised scaphognathite activity. Unilateral scaphognathite activity was not observed. During normal forward recirculation of branchial water the scaphognathites beat at about 1.5 Hz (slow-forward pumping) and the lungs (epibranchial chambers) are not ventilated. In Heloecius, the lungs are not physically separated from the gills below by an anatomical barrier. Lung ventilation is accomplished during the following sequence of events: the carapace is lowered and the scaphognathites pump in a fast-forward mode at about 2.8 Hz. This activity preferentially pumps air out of the lungs and generates suction within the branchial chambers (4–10 cm H₂O below ambient) which draws water from external body surfaces into the hypobranchial space below and around the gills. At the end of the carapace's downward travel the scaphognathites switch from fast-forward to fastreverse beating at about 4 Hz. This pumps air into the lungs and the carapace elevates. As a result, during carapace elevation the water which had previously been drawn into the branchial chambers by fast-forward pumping activity is released and flows out between the legs and into the abdominosternal cavity. When the carapace reaches its original resting or up position the scaphognathites switch from fast-reverse to slowforward beating to re-establish water recirculation through the branchial chambers. This cycle is subsequently repeated. In stationary crabs, there are 2 carapace-pumping cycles per minute, increasing to 14 per minute in active crabs (walking). When water is absent, the lungs are preferentially ventilated by slow-reverse scaphognathite pumping activity. Carapace movements do not occur in the absence of branchial water. Carapace pumping is thought to provide a mechanism which permits the scaphognathites to ventilate the lungs in the presence of recirculating branchial water, without this water interfering with lung ventilation or being lost to the environment.Abbreviations FF, FR, SF, SR fast-forward, fast-reverse, slowforward, slow-reverse scaphognathite pumping - MEA Milne Edwards aperture     

Importance of Water Flow on Larval Growth and Pupation of Himalopsyche acharai, (Malicky and Chantaramongkol, 1989) (Trichoptera: Rhyacophilidae)

Larval mortality with growth and pupation rates of Himalopsyche acharai were compared between standing and flowing water in a laboratory from June to November 2004. In flowing water, 90% of larvae started pupal case building and 81.5% of them developed to pupae by October. In standing water, 92.6% died before pupation. The larvae reared in flowing water grew significantly faster than those in standing water. These results show why Himalopsyche acharai does not inhabit pools and slowly flowing streams.     

Effects of episodic acidification on macroinvertebrate assemblages in Swiss Alpine streams

1. Despite long‐standing ecotoxicological evidence that episodes of acidification in streams are important biologically, there is still uncertainty about their effects on invertebrate communities. We surveyed 20 streams in an acid sensitive Alpine area (Canton Ticino, Switzerland), where episodes are driven by snowmelt in spring and by rainstorms at other times of the year. Samples of water and macroinvertebrates were collected in pre‐event conditions (winter and summer) and during periods of high flow (spring and autumn). 2. Using pH, [Ca²⁺] and [Alⁿ⁺], streams were clustered into six acid–base groups that were either well buffered (groups 4–6), soft‐water with stable pH (group 3), or poorly buffered with low pH at high flow (groups 1 and 2). 3. Severe episodes occurred during snowmelt, when the group 1 streams became acidic with pH down to 5.0 and [Alⁿ⁺] up to 140 μg L^?1. pH declined to 6.2 in streams of group 2, but remained > 6.6 in groups 3–6. 4. Detrended canonical correspondence analysis showed that the streams sensitive to episodes (groups 1 and 2) had different invertebrate assemblages from well‐buffered sites (groups 4 and 5) or soft‐water stable streams (group 3), with faunal differences largest following spring snowmelt. Empididae, Isoperla rivulorum, Rhithrogena spp. and Baetis spp. were scarce in streams sensitive to episodes (groups 1 and 2). By contrast, Amphinemura sulcicollis was scarcer in hard‐water streams (groups 4–6). Taxonomic richness was lower in the episodic streams of group 1 than in other streams. 5. Together, these results indicate clear biological differences between acid‐sensitive streams with similar low‐flow chemistry but contrasting episode chemistry. Severe episodes of acidification appear to affect macroinvertebrate assemblages in streams in the southern Swiss Alps.     

Sharing morphospaces: early ontogenetic shape changes in two clingfish larvae (Pisces: Gobiesocidae) from the south‐east Pacific Ocean

Larval body shape changes and developmental timing were examined in two clingfish species from the south‐east Pacific Ocean, Gobiesox marmoratus and Sicyases sanguineus. Ontogenetic allometry showed no interspecific variation and <7 mm standard length (L_S) larvae of both species occupied similar morphospace, but larger G. marmoratus showed increased body depth while larvae of S. sanguineus developed a flattened head and maintained a hydrodynamic body. Estimated developmental timing suggests that larval body shape changes were faster in G. marmoratus than in S. sanguineus prior to settlement.     

The physiological response of larval <Emphasis Type="Italic">Chironomus riparius</Emphasis> (Meigen) to abrupt brackish water exposure

The physiological response of larval Chironomus riparius was examined following direct transfer from freshwater (FW) to brackish water (BW; 20% seawater). Endpoints of hydromineral status (hemolymph Na⁺, Cl⁻, and K⁺ levels, hemolymph pH, body water content, and whole body Na⁺/K⁺-ATPase and V-type H⁺-ATPase activity) were examined 1, 3, 5, 12 and 24 h following BW transfer. Larvae transferred from FW to FW served as a control. Hemolymph Na⁺ and Cl⁻ levels increased following BW transfer. Hemolymph pH was initially regulated, but significantly decreased after 24 h in BW. Changes in hemolymph ions were not caused by osmotic loss of water from the hemolymph, since larvae tightly regulated total body moisture content. Furthermore, salinity did not affect hemolymph K⁺. When larvae were transferred to BW, Na⁺/K⁺-ATPase (NKA) activity did not significantly alter relative to FW control animals. In contrast, V-type H⁺-ATPase (VA) activity in C. riparius significantly decreased in BW. In FW-reared C. riparius, whole body NKA and VA activities were equivalent. However, in the isolated gut with intact Malpighian tubules of FW-reared C. riparius, VA activity was significantly greater than whole body while NKA activity was equivalent. This suggested that gut and/or Malpighian tubule VA activity contributes significantly to whole body VA activity and that a decline in whole body VA activity in BW may be closely linked to alterations in the physiology of gut and Malpighian tubule tissue. Taken together, data indicate that VA is important for ion uptake in FW and that the NKA does not play a major role in regulating ion homeostasis when larvae are acutely exposed to BW.     

Flow velocity underpins microhabitat selection by gobies of the Australian Wet Tropics

1. Water flow is a critical driver of aquatic ecosystem health and function. Amid rising concerns over changing flow regimes, there is an urgent need to understand the functional mechanisms by which flow influences patterns of freshwater biodiversity. 2. We explored the functional link between flow velocity and microhabitat specialisation in a speciose group of freshwater gobies (comprising over half the total fish species richness) within insular streams of the Australian Wet Tropics under base flow conditions. We addressed two particular questions: (i) What is the relative selectivity of species towards streambed composition and water flow velocity? and (ii) Can patterns of microhabitat occupation be explained by differences in intrinsic flow performance among species? To answer these questions, we combined visual field observations of microhabitat use with flow tank assessments of flow speed performance. 3. Tropical freshwater gobies displayed strong specificity towards flow velocities, while being relatively non‐selective towards streambed composition. At opposite extremes of the spectrum, we found Sicyopterus lagocephalus occupying high‐flow (>1.0 ms^?1) microhabitats while Redigobius bikolanus selected slower‐flow (<0.05 ms^?1) areas. These patterns of microhabitat flow specificity were largely explained by the different abilities of species to swim and/or cling to the substratum under these different flow settings. 4. Our findings confirm suggestions that predictable base flows in tropical streams support habitat specialists, which include one species capable of occupying areas of extremely high flow that very few other fishes can withstand. 5. The functional link between flow and gobioid fish distribution patterns could occur throughout tropical streams of the Indo‐Pacific and Caribbean as a widespread phenomenon that may help inform stream flow management guidelines to maintain this substantial component of tropical freshwater biodiversity around the globe.     

Larger eggs at lower water temperature as a measure to assure effective hatchling size in the landlocked form of Ayu, <Emphasis Type="Italic">Plecoglossus altivelis</Emphasis>

Egg size is a critical life-history trait in which maternal investment is optimized to maximize maternal fitness. The adaptive significance of variable egg size among spawning groups of Ayu (Plecoglossus altivelis) landlocked in the Lake Biwa system was examined through field investigations and rearing experiments. Observed egg size variations were explained by the water temperature around spawning grounds established near the mouths of inlet streams. Two typical streams with different incubation temperatures showed similar maternal body sizes and hatchling sizes, but eggs attached to the stream bed were larger in the colder stream. An experiment that used eggs from a single clutch showed that a smaller hatchling size was obtained with a lower incubation temperature, indicating that the effect of differences in egg size on hatchling size can be canceled out by variations in incubation temperature. In general, larvae that are less than a certain threshold of effective body size are not expected to be assured of early success among conspecifics competing for foods. It is proposed that environments in which the incubation temperature varies favor variability in egg size to ensure that sufficient food is accessible to larvae.     

Survival of Heleomyza borealis (Diptera, Heleomyzidae) larvae down to − 60°C

Heleomyza borealis Boh. (Diptera, Heleomyzidae) overwinters as larvae in Arctic habitats, where they may experience winter temperatures below ? 15°C. The larvae freeze at c.? 7°C but in acclimation experiments 80% survived when exposed to ? 60°C. Of the larvae exposed to between ? 4 and ? 15°C, only 3% pupated. However, when cooled to ? 20°C this increased to 44%, with 4% emerging as adults. Larvae maintained at 5°C contained low levels of glycerol, sorbitol and trehalose, which did not increase with acclimation to low temperatures. However, levels of fructose increased from 6.1 μg mg^?1 fw in control animals to 17 μg mg^?1 fw when exposed to ? 2°C for 1 week. Larval body water (2.2 ± 0.1 g/g dw, mean ± SD, n = 100) and lipid content (0.22 ± 0.002 g/g fw, mean ± SE) showed no significant change during acclimation to low temperatures. Larvae maintained at a constant 5°C survived for over 18 months with little loss of body mass (from 7.5 ± 1.2 to 7.0 ± 1.2 mg fw, mean ± SD, n = 20), but none pupated. Heleomyza borealis larvae appear to feed and grow until they reach a body mass of about 7.5 mg and then become dormant. They remain in this state until they experience a low temperature stimulus (< ? 15°C) followed by a warm period (≈ 5°C). This ensures that the larvae pupate and adults emerge in early summer, allowing the maximum growing period before the following winter. Heleomyza borealis are adapted to survive long winters in a dormant larval state. They have a low metabolic rate, can conserve body water even at subzero temperatures but do not synthesize large quantities of cryoprotectants.     

Life history differences of Psilotreta locumtenens (Trichoptera: Odontoceridae) in two reaches of a mountain stream in Korea

The life history traits of the caddisfly, Psilotreta locumtenens Botosaneanu (Odontoceridae), were studied in two stream reaches with different thermal ranges (main and branch streams) of the Gapyeong Stream, a typical mountain stream located in the central Korean Peninsula. Psilotreta locumtenens larvae were quantitatively sampled monthly from November 2008 to July 2010, and biweekly during the emergence period (late April to early July), using a Surber sampler (30 × 30 cm). Adults were quantitatively sampled with a sweep net. Larval density in the main stream (324.21 ± 38.59 m^?2) was higher than that in the branch stream (60.48 ± 10.86 m^?2). The larvae hatched in the early summer and overwintered as 5th and 3rd instars in the main and branch streams, respectively. The emergence peak at the main stream was approximately 2 weeks earlier. The sex ratio at both sites was approximately 0.3. The life history in both streams was univoltine. Secondary production in the main stream was much higher than in the branch stream, owing to high larval densities, and the P/B ratios at the two sites were similar. This study demonstrated remarkable differences in larval growth patterns and emergence peaks in P. locumtenens between the two stream reaches due to differences in accumulated degree days (230.30 DD) and other phenological cues such as daily mean threshold water temperature (9°C) during the ascending phase, despite their relatively small mean annual water temperature difference of 0.58°C.     

Feeding dynamics and respiration of the bottom-dwelling caridean shrimp Nauticaris marionis Bate, 1888 (Crustacea: Decapoda) in the vicinity of Marion Island (Southern Ocean)

The feeding dynamics and oxygen uptake of the bottom-dwelling caridean shrimp Nauticaris marionis were studied during the April/May 1984, 1996 and 1997 cruises to Marion Island (Prince Edward Islands, Southern Ocean). N. marionis is thought to have an opportunistic feeding mode. Prey composition varied considerably between the years and sites investigated. Overall, benthic (mainly hydrozoans and bottom-dwelling polychaetes) and, at times, pelagic (largely euphausiids and copepods) prey items dominated in the stomachs of N. marionis both by occurrence and by volume. Generally, pelagic prey contributed more to the diets of smaller shrimps, while benthic prey was a more important component in the guts of larger specimens. Wet, dry and ash-free dry weight were determined for specimens used in respiration experiments. The respiration rates of N. marionis females with carapace length 6.6–11.1 mm ranged from 80 to 250 μl O₂ individual⁻¹ · h⁻¹, or from 0.588 to 2.756 μl O₂ · mg⁻¹ dry weight h⁻¹. Regression analyses showed highly significant correlations between oxygen consumption and carapace length for N. marionis. Daily ingestion rates estimated using an in situ gut content analysis technique (4.4% of body dry weight) and an energy budget approach (average 4.7% of body dry weight, range 2.0–7.5%) showed good agreement with each other. Accepted: 29 July 1998