Habitat use by endangered Japanese crayfish (<Emphasis Type="Italic">Cambaroides japonicus</Emphasis>) in low-gradient streams of southern Hokkaido,Japan: reach and microhabitat-scale analysis

The Japanese crayfish (Cambaroides japonicus), the only native crayfish in Japan, is endangered and has experienced rapid population declines. We surveyed the habitat requirements of Japanese crayfish at the reach and microhabitat scales in semi-natural low-gradient streams. Habitat use by Japanese crayfish differed between the spatial scales. Reach-scale analysis revealed that the bed slope was the only positive predictor of crayfish density. This finding indicates that relatively high-gradient reaches, such as headwater reaches, are an important habitat for the conservation of Japanese crayfish in low-gradient streams. Microhabitat-scale analysis showed that crayfish density was positively affected by substrate coarseness and the presence of instream vegetation cover (bank vegetation, woody debris, and leaf patches), whereas it was negatively affected by distance from the stream edge. Coarse substrates and vegetation cover may function as shelters from water flows and predators during low flow periods. The use of stream-edge areas may allow quick access to refugia and enable the crayfish to avoid unexpected flood disturbance and predation. These habitat characteristics should therefore be preserved for the conservation of Japanese crayfish, and scale-dependent habitat characteristics should be considered in future conservation plans.     

Light-dependent fluorescence in the coral Galaxea fascicularis

The concept of ecological stoichiometry has been useful for understanding nutrient dynamics in aquatic food webs; however, the majority of studies have focused on autotrophic systems, leaving detritus-based food webs largely understudied. In addition, most detritus-based studies have explored enrichment in high-gradient, low-nutrient systems, despite the fact that many of the streams most likely to face enrichment (those surrounded by agriculture) are low-gradient and contain inherently higher dissolved nutrient concentrations due to differences in soil type, geomorphology, and atmospheric deposition. Constraints on consumer growth due to consumer-resource imbalances have been documented in these low-nutrient streams, but the extent to which consumer growth may be limited in higher-nutrient, detritus-based streams is unknown. We investigated the impact of dissolved nutrients (N and P) on mayfly growth, using artificial streams simulating a high-nutrient detritus-based system. Mayflies were reared and sampled under two total nutrient concentrations, one meant to mimic a more natural undisturbed (ambient) watershed and one to mimic a disturbed (enriched) watershed. Under each of these conditions two N:P ratios (low and high) were tested. The low N:P treatments produced higher mayfly growth under both ambient and enriched conditions, showing that nutrient limitation can occur even in high-nutrient streams.     

A New Jamaican species of lisianthius (Gentianaceae)

Lisianthius adamsii, from the central and western parishes of Jamaica, is described as new. It is remarkable in having coriaceous leaves and ventricose corollas.     

Effect of environmental embryonic temperature on larval development of Macrobrachium rosenbergii (De Man)

Berried females of Macrobrachium rosenbergii (De Man) from Anuenue stock were allowed to incubate their eggs at three different temperatures (25,29, and 31°C). The newborn larvae were reared in the laboratory from hatch through completion of the metamorphosis to postlarva in 30 combinations of temperature (22–34° C) and salinity (0–34 ppt). Survival and stage attainment rates were observed. Multiple linear regression analysis and response surface methodology were used to estimate the response of larvae to these different temperature and salinity combinations. Dissimilarities in the response of zoeae from the three egg incubation temperatures were found. Larvae from eggs incubated at 25° C during embryonic development showed tolerance to a broader range of temperature and salinity conditions than those incubated at 29 or 31 °C. The response also changed with the ontogeny of the larvae. The zoeae are considered to have undergone acclimation during embryonic development, thus eliciting a different response.     

Delivery of hatching larvae to estuaries by an amphidromous river shrimp: tests of hypotheses based on larval moulting and distribution

1.  Amphidromous shrimps live and breed in freshwater rivers and streams, but their larvae require development in sea water. Larvae may hatch upstream and then drift to the sea, although in some species females have been reported to migrate to the coast before larvae are released. Here, we tested the relative importance of larval drift and female migration in Macrobrachium ohione (Decapoda: Palaemonidae) in a distributary of the Mississippi River in Louisiana, U.S.A.
2.  Newly hatched (stage-1) larvae are nonfeeding and will not moult to stage 2 (first feeding stage) without encountering salt water. A factorial experiment was conducted in the laboratory to test the effects on moulting to stage 2 of (i) time spent by stage-1 larvae in fresh water before (ii) exposure to and maintenance in water of different salinity. Larvae kept in fresh water for 1 or 3 days before a change to saline water at 6 or 10 ppt showed a greater frequency of moulting than those kept for longer (5 days) in fresh water or changed to less saline water (2 ppt). Non-moulting larvae died or were moribund within 11 days of hatching.
3.  The relative abundance of stage-1 larvae was measured with plankton tows at two locations in the river c. 150 km apart, one near the sea and one upstream. Larval abundances near the sea were significantly greater than those upstream.
4.  The results indicate that hatched larvae of M. ohione have a limited period in which to drift in fresh water before reaching water sufficiently saline to stimulate moulting to the first feeding stage. Female migrations may play an important role in delivering larvae of amphidromous species from large continental river systems in which distances to the sea are great, while larval drift alone may be sufficient in species living in short streams, like those found in many small mountainous tropical islands.     

Influence of freshwater flows on the distribution of eggs and larvae of black bream Acanthopagrus butcheri within a drought-affected estuary

This study tested the hypothesis that variable freshwater flow in the Gippsland Lakes, Australia, influences the location and extent of environmental conditions suitable for spawning and larval development of black bream, Acanthopagrus butcheri. Freshwater flow had a large influence on the salinity and level of stratification (difference between bottom and surface salinity) in the Gippsland Lakes. Freshwater flows throughout this study varied from no or low flows through to major flooding in 2007. Eggs of A. butcheri were found in similar concentrations throughout the lakes and rivers. More than 99% of larvae, however, were collected from within rivers, with very few larvae being collected from the lakes. A comparison of two spawning seasons revealed that the year with higher freshwater flows also had greater spawning activity and higher concentrations of larvae. Interestingly, there was a significant relationship between the distribution of eggs and larvae with the level of stratification. The highest concentrations of larvae occurred at sites with a difference in bottom and surface salinities of 15-20. This study demonstrates that despite A. butcheri spawning in the lakes and rivers, it is only locations with a halocline that function as larval nursery habitat.     

Drift ecology of western catostomid larvae with emphasis on Warner suckers, Catostomus warnerensis (Teleostei)

We studied drift ecology of Warner sucker, Catostomus warnerensis, larvae in streams of Warner Valley, Oregon in 1992 and 1993 in part to determine whether downstream transport of larvae limits juvenile recruitment. Variation in runoff for the two years was extreme, where peak discharge in 1992 was two orders of magnitude lower than peak discharge in 1993. We deployed drift nets in low-gradient stream sections that are modified by diversions. Low catches of drifting sucker larvae (zero in 1992 and two in 1993) indicate that losses into diversions were probably minimal. In 1992, we conducted in situ experiments to determine how sucker larvae behaved when exposed to high velocity current by placing individuals of various sizes in mid-channel and tracking drift responses by snorkeling. Even the smallest larvae used did not drift far (2.6 m) or for very long (0.97 min), as they resisted downstream transport by exploiting cover provided by aquatic plants and debris. We also recorded locations of larvae during a 24-hour cycle to determine whether positions occupied during the day changed at night. We found that groups were positioned closer to shore at night in 1993 than in 1992, suggesting that larvae reduced the likelihood of becoming entrained into swift current by moving away from mid-channel at night when visual orientation is impaired. These patterns of drift avoidance are different from behaviors of other western catostomid larvae, potentially because terminal lake habitat in Pleistocene basins such as the Warner Valley has been unreliable.     

Effect of desalination on the littoral tecturid gastropods in early development

The effect of decreased salinity on the limpetsCollisella dorsuosa, C. versicolor, C. radiata, andNotoacmea concinna (Mollusca, Gastropoda) is studied at early stages of development. It is shown that, for normal development from fertilization to the veliger stage, the salinity must be not less than 22–24‰. A decrease in salinity depressed the behavior of larvae. At earlier stages, the larvae were less sensitive to changes in salinity than at later stages. At the same time, veligers were better able to adapt to changes in salinity than trochophores. The vertical water distribution of larvae depended on their age. Two days after fertilization, the larvae sank down to start a benthic life. It is proposed that the narrow tolerance range of littoral limpet larvae to changes in salinity is connected with the fact that they spend a relatively small part of their life in plankton.     

Effects of Phytic Acid and Exercise on Some Serum Analytes in Rats Orally Exposed to Diets Supplemented with Cadmium

Cadmium is an environmental pollutant of increasing worldwide concern. It has been reported to be high in the soil where food crops are grown in some parishes of Jamaica. Surprisingly, no adverse effect of cadmium has been reported among the Jamaican population. However, phytic acid has also been shown to be high in some food crops grown in Jamaica. In this study, we evaluated the effects of phytic acid (1 %) and exercise on the metabolism of cadmium (5 mg cadmium/kg body weight) in rats. Five groups of rats were fed as follows: rats fed control diet, control diet supplemented with cadmium and subjected to exercise, control diet supplemented with phytic acid plus cadmium and subjected to exercise, control diet supplemented with cadmium plus phytic acid, and control diet supplemented with cadmium only. The animals were fed for 4 weeks and then sacrificed. Blood samples were collected for some biochemical assays. Percentage weight loss (28.42 %) was greatest in the group that had cadmium supplement only. The group fed control diet supplemented with cadmium only displayed increased liver enzymes and electrolytes except for the significant decrease in bicarbonate compared to other test groups. Similarly, blood urea nitrogen and uric acid were increased in the group fed cadmium supplement only compared to other test groups. Total cholesterol trended downwards in the test groups compared to control. These observations suggest that consumption of diet high in phytic acid with relatively high physical activity may be protective against the adverse effects of cadmium.     

A starvation test to determine optimal salinities for larval freshwater prawns,Macrobrachium rosenbergii

• 1.1. A starvation test was conducted in small beakers with stage 1 (S1) and stage 2 (S2) Macrobrachium rosenbergii larvae to determine optimal salinities.
• 2.2. Experiments were first performed with S2 larvae at 13 ppt to identify a suitable medium made with artificial sea salts.
• 3.3. A broad-range (0–35 ppt) and a subsequent narrow-range (9–16 ppt) salinity experiment with S2 larvae were used to identify 13 ppt as the optimal salinity, with 12 ppt as the next best; this agrees well with most previous estimates of optimal salinities for rearing larvae.
• 4.4. S1 larvae were also tested in a narrow-range salinity experiment but were not used further because, unlike starved S2 larvae, they molted during the experiment.
• 5.5. Identification of the optimal salinity was not affected by 50% daily water exchange or by bright light.
• 6.6. Exposure of larvae to three different salinities—7, 13 and 19 ppt—during S1 influenced the width of the optimal salinity range for S2 larvae.

     

ADAPTIVE COLORATION AND GENE FLOW AS A CONSTRAINT TO LOCAL ADAPTATION IN THE STREAMSIDE SALAMANDER,AMBYSTOMA BARBOURI

Predation is an important selective force that influences animal color patterns. Some larval populations of the streamside salamander, Ambystoma barbouri, inhabit streams with fish predators. Other larval salamanders are found in shallow, ephemeral streams that are predator-free. Quantitative melanophore cell counts and estimates of percent body area pigmented indicated that larval coloration is strongly correlated with stream type. Larvae that coexist with fish tend to be lighter than larvae from streams that are Ashless and ephemeral. Two approaches demonstrated that lightly pigmented salamander larvae better match the common background in relatively permanent streams and are less conspicuous to fish than dark larvae. First, using a model based on the spectral sensitivity of the fish and reflectance properties of salamanders and natural stream backgrounds, we showed that light larvae are three times more cryptic than dark larvae on rocks. Second, lighter larvae had higher survival than darker salamanders on rocks in a predator- choice experiment. It is not clear why larvae in ephemeral streams are darker. Larvae in ephemeral streams should be active to feed and develop rapidly and reach sufficient size to metamorphose before seasonal drying. Several hypotheses may explain why larvae tend to be darker in ephemeral streams, such as increased thermoregulatory ability, better screening of ultraviolet radiation (in these shallower streams), or better background matching to terrestrial predators. Among populations where salamander larvae coexist with fish, there are differences in relative crypsis. Larvae from populations with fish and relatively high gene flow from ephemeral populations (where larvae are dark) tend to be darker (with more melanophores) and more conspicuous to predators than those from more genetically isolated populations, where larvae are lighter and more cryptic. These differences illustrate the role of gene flow as a constraint to adaptive evolution.     

Predator–prey interactions in river networks: comparing shrimp spatial refugia in two drainage basins

1. Analysis of drainage networks provides a framework to evaluate the densities and distributions of prey species relative to locations of their predators. Upstream migration by diadromous shrimp (Atya lanipes and Xiphocaris elongata) during their life cycle provides access to headwater refugia from fish predation, which is intense in estuaries and coastal rivers. 2. We postulate that geomorphic barriers (such as large, steep waterfalls >3.5 m in height), can directly limit the distribution of predatory fishes and, indirectly, affect the densities of their prey (freshwater shrimps) in headwater streams. 3. We compared densities of shrimp in pools above and below waterfalls, in four headwater tributaries in two river basins of the Luquillo Mountains of northeastern Puerto Rico. We measured shrimp densities twice a year over 8 years (1998–2005) in Prieta, Toronja, Bisley 3 and Bisley 5 streams, which differ in drainage network positions relative to steep waterfalls in Río Espíritu Santo and Río Mameyes. 4. Predatory fishes are absent in the Prieta and Toronja pools and present in Bisely 3 and in lower Bisley 5 pools. Atya lanipes and X. elongata rarely occur in the Bisley streams where predatory fishes are present but these shrimps are abundant in Prieta and Toronja, streams lacking predatory fishes. 5. The mean carapace length of X. elongata is longer in pools where fish are present (Bisley 3 and lower Bisley 5) than in pools lacking fish (Prieta, Toronja, Upper Bisley 5). The increased body size is primarily due to significantly longer rostrums of individuals in stream reaches with fish (below waterfall barriers) than in those reaches lacking fish (above waterfall barriers). Rostrum length may be an adaptation to avoid predation by visually feeding fishes. 6. Atya lanipes and X. elongata distributions and densities were predicted primarily by drainage network position relative to the presence or absence of predatory fishes. High, steep waterfalls effectively impeded fish from moving upstream and created a spatial refuge. Xiphocaris elongata may rely on size refugia (longer rostrum) to minimize predation where spatial refugia are lacking.     

Landscape-Scale Hydrologic Characteristics Differentiate Patterns of Carbon Flow in Large-River Food Webs

Efforts to conserve, restore, or otherwise manage large rivers and the services they provide are hindered by limited understanding of the functional dynamics of these systems. This shortcoming is especially evident with regard to trophic structure and energy flow. We used natural abundances of carbon and nitrogen isotopes to examine patterns of material flow in ten large-river food webs characterized by different landscape-scale hydrologic characteristics (low-gradient river, high-gradient river, river stretches downstream of reservoirs, and reservoirs), and tested predictions from three ecosystem concepts commonly applied to large-rivers: The River Continuum Concept, The Flood Pulse Concept and the Riverine Productivity Model. Carbon derived from aquatic C₃ plants and phytoplankton were the dominant energy sources supporting secondary consumers across the ten large-river food webs examined, but relative contributions differed significantly among landscape types. For low-gradient river food webs, aquatic C₃ plants were the principal carbon source, contributing as much as 80% of carbon assimilated by top consumers, with phytoplankton secondarily important. The estimated relative importance of phytoplankton was greatest for food webs of reservoirs and river stretches downriver from impoundments, although aquatic C₃ plants contributed similar amounts in both landscape types. Highest 99th percentile source contribution estimates for C₄ plants and filamentous algae (both approximately 40%) were observed for high-gradient river food webs. Our results for low-gradient rivers supported predictions of the Flood Pulse Concept, whereas results for the three other landscape types supported the Riverine Productivity Model to varying degrees. Incorporation of landscape-scale hydrologic or geomorphic characteristics, such as river slope or floodplain width, may promote integration of fluvial ecosystem concepts. Expanding these models to include hydrologically impacted landscapes should lead to a more holistic understanding of ecosystem processes in large-river systems.     

Effects of Low Salinity on Adult Behavior and Larval Performance in the Intertidal Gastropod Crepipatella peruviana (Calyptraeidae)

Shallow-water coastal areas suffer frequent reductions in salinity due to heavy rains, potentially stressing the organisms found there, particularly the early stages of development (including pelagic larvae). Individual adults and newly hatched larvae of the gastropod Crepipatella peruviana were exposed to different levels of salinity stress (32(control), 25, 20 or 15), to quantify the immediate effects of exposure to low salinities on adult and larval behavior and on the physiological performance of the larvae. For adults we recorded the threshold salinity that initiates brood chamber isolation. For larvae, we measured the impact of reduced salinity on velar surface area, velum activity, swimming velocity, clearance rate (CR), oxygen consumption (OCR), and mortality (LC50); we also documented the impact of salinity discontinuities on the vertical distribution of veliger larvae in the water column. The results indicate that adults will completely isolate themselves from the external environment by clamping firmly against the substrate at salinities ≤24. Moreover, the newly hatched larvae showed increased mortality at lower salinities, while survivors showed decreased velum activity, decreased exposed velum surface area, and decreased mean swimming velocity. The clearance rates and oxygen consumption rates of stressed larvae were significantly lower than those of control individuals. Finally, salinity discontinuities affected the vertical distribution of larvae in the water column. Although adults can protect their embryos from low salinity stress until hatching, salinities <24 clearly affect survival, physiology and behavior in early larval life, which will substantially affect the fitness of the species under declining ambient salinities.     

马氏沼虾蚤状幼体发育特征及其与罗氏沼虾幼体的形态差异

马氏沼虾(Macrobrachium malcolmsonii)的幼体从孵化到变态为幼虾要经过多次蜕皮,每次蜕皮均伴随形态及附肢特征的改变。采用显微观测摄像法对马氏沼虾Z1-Z12各期幼体样本30尾进行了观察,测量结果表明Z2与Z1在复眼上存在较大差异,Z3与Z2、Z4与Z5的主要分期特征均位于尾节上,Z6的腹部出现腹足萌芽,Z7与Z8的形态差异在于第一触角内鞭和外鞭的附肢变化,判别Z9-Z12的主要分期依据为步足、腹足和额角形态。因此通过观察Z1-Z12幼体第1触角上的内鞭和外鞭,第2触角上的触角鞭和触角片,头胸甲上的额角、背刺、复眼和第1、2对步足,腹部的附肢、尾节和尾扇等形态变化,及测定各期幼体附肢的分节数和刚毛数等可量性状上的差异,可为马氏沼虾蚤状幼体分期提供生物学依据;此外还对马氏沼虾与罗氏沼虾幼体的形态差异进行了比较,通过测定幼体触角鞭的分节数量,为两种沼虾幼体之间的鉴别提供参考数据。       

瓯江口凤鲚鱼卵、仔稚鱼的时空分布及其与环境因子关系

为了解凤鲚(Coilia mystus)早期群体在瓯江口的时空分布及其影响因素, 研究根据瓯江口渔业资源调查数据, 利用两阶段广义加性模型(Two-stage GAM)分析影响凤鲚鱼卵仔稚鱼分布的环境因子。采用交叉验证评价模型的预测性能, 对2020年5—7月的凤鲚鱼卵、仔稚鱼出现率及资源量进行预测。结果表明: 5—7月是瓯江口凤鲚的主要繁殖期, 鱼卵主要分布在瓯江上游七都岛附近低盐水域, 仔稚鱼在瓯江水域均有分布, 瓯江下游灵昆岛近岸水域仔稚鱼的相对资源丰度高于瓯江上游。GAM模型分析表温、表盐是影响凤鲚早期群体分布的主要环境因子, 表盐对鱼卵的分布具有极显著影响(P<0.01), 适宜表盐在1以下, 适宜表温在24—26℃; 仔稚鱼对水温、盐度的适宜度宽于鱼卵, 表温对仔稚鱼的分布具有显著影响(P<0.05), 适宜表温为22—27℃, 适宜表盐为5—13。交叉验证分析认为模型对鱼卵的预测性能(AUC均值0.83)优于仔稚鱼(AUC均值0.71)。2020年预测结果显示, 凤鲚鱼卵的适宜栖息环境较窄, 主要集中在江心屿上游及灵昆岛下游附近水域, 仔稚鱼主要栖息在江心屿至灵昆岛水域。瓯江口作为凤鲚的主要栖息场所之一, 掌握凤鲚早期群体在瓯江口的分布特征及与环境因子间的关系, 能够为其产卵场栖息地的保护和资源养护管理提供科学依据。     

Distribution,body size,genetic structure and conservation of Siphlaenigma janae (Insecta: Ephemeroptera)

Distribution, body size and genetic structure of the poorly known New Zealand mayfly Siphlaenigma janae were investigated to improve understanding of its conservation status. It has now been recorded from 42 locations, 22 of which are reported for the first time. The distribution of S. janae extends from Northland to the central North Island, and also into the northwestern South Island. Population structure consisted of three distinct haplotype networks; two in the North Island and one in the South Island. Maximum uncorrected genetic distance was 6.1% but no strong evidence for the presence of sibling species was found. The specialist nymphs predominantly live in low-gradient first- and second-order forested streams, a habitat that is increasingly being threatened by land-use modification. We recommend S. janae remain classified as Nationally Vulnerable due to its unique taxonomic position, regionally distinct genetic structure and fragmented population that is likely experiencing ongoing decline.     

Density reductions by predatory trout increase adult size and fecundity of surviving caddisfly larvae in a detritus-based stream food web

1. In some situations, individuals surviving in environments where predation is intense can grow faster because the benefits of release from intraspecific competition outweigh costs associated with anti-predator responses. Whether these 'thinning' effects of predation occur in detritus-based food webs where resource renewal occurs independently of consumption by consumers was studied. We investigated how effects of predatory brown trout ( Salmo trutta ) on the larvae of the detritivorous stream caddisfly, Zelandopsyche ingens , influenced the size and fecundity of the caddisfly adults.
2. Trout substantially reduced the abundance of Z. ingens larvae, but adult male and female Z. ingens were significantly larger in trout streams compared to fishless streams. Females in trout streams had 33% more eggs than fishless stream females, and egg sizes were not significantly different. In mesocosms, Z. ingens larvae in low density treatments reflecting trout stream abundances grew significantly faster than larvae in high density treatments that were characteristic of fishless stream abundances. Non-lethal trout presence did not influence case building behaviour, feeding rates or growth or Z. ingens larvae, indicating non-lethal effects of predators were negligible.
3. Increased adult size and fecundity associated with trout stream individuals were probably a result of predator thinning of larval density indirectly releasing surviving Z. ingens from intraspecific competition. Thus, predator thinning did influence interactions between larvae in this detritus-based food web as larval growth was strongly density-dependent. However, extrapolating the total number of eggs potentially produced indicates the increased fecundity of females in trout streams would not compensate for losses of larvae to trout predation.     

Drift and settlement of stream insects in a complex hydraulic environment

1. The hydraulic and geomorphic characteristics of stream patches are often associated with distinctive assemblages or densities of stream invertebrates, and it is routinely presumed that these patterns reflect primarily species‐specific habitat requirements. An alternative hypothesis is that such patterns may be influenced by constraints on movement, such as the results of departure and settlement processes. We describe a manipulative experiment that examined how the hydraulic environments created by topographic bedforms influenced the drift behaviour and potential settlement sites for two species of mayfly (Baetis rhodani and Ecdyonurus torrentis). These species are common in the drift and often co‐occur in streams, but differ in their small‐scale distribution patterns, body shape and movement behaviour. 2. Flume experiments were carried out to determine how the hydraulic environments conditioned by a step bedform influence the behaviour of mayflies in the drift (swimming, posturing, tumbling), and the consequences of those behaviours (drift distance and time), compared to drift over a plane bed. The ramped step in the flume mimicked step bedforms that are common in coarse‐grained, high‐gradient streams. In contrast to the plane bed, a zone of recirculating flow was created downstream of the step, above which flow was faster and more turbulent. Uniform flows are used in most flume studies of drift; our approach is novel in recreating a complex hydraulic environment characteristic of stream channels. 3. Both species had some behavioural control over drift, and drift distances and times were shorter for live larvae than for dead larvae over the plane bed. The step had no impact on drift time or distance for live Baetis, but dead larvae were trapped in the flow separation eddy and drift time increased accordingly. Some Ecdyonurus also became trapped in the eddy, but live larvae drifted farther than dead larvae, and farther over the step than the plane bed. 4. Whilst in the drift, larvae altered their behaviour according to the ambient hydraulic environment, but in a species‐specific manner. Over the plane bed, Baetis had occasional swimming bursts, but primarily postured (maintained a stable body orientation), whereas Ecdyonurus spent roughly equal time posturing and swimming. In the more turbulent flows generated by the step, Baetis spent proportionately more time swimming, whereas Ecdyonurus spent more time posturing and often tumbling as body orientation became unstable. 5. In a high‐gradient stream, Baetis was more abundant close to steps than in plane bed patches with less complex flow, whereas the opposite pattern held for Ecdyonurus. Thus, the small‐scale distribution patterns of these species within streams correspond to their drift behaviours and ability to access various hydraulic patch types in our flume. These results are consistent with the hypothesis that constraints on movement and settlement may be important driver of distribution patterns within streams.