Atya Lanipes Research Articles

Description of the Early Larval Development in Freshwater Shrimp Atya lanipes Holthuis, 1963 (Decapoda: Caridea: Atyidae) from Puerto Rico

The family Atyidae is composed of species whose existence has been known since the seventeenth century. Widely found in the Caribbean, Atya lanipes is a freshwater scraper/filter feeder shrimp with an amphidromous complex life cycle. Hunte (1975) described the first larval (zoeal) stage of the species. However, no scientific study has described the early larval development of this species after the first stage. This study aimed to document the early larval development of Atya lanipes under laboratory conditions and compare its larval development with other previously described species of the Atyidae family. Larval development was recorded by taking daily photos and videos of larval (zoeal) growth using a stereo microscope. Larvae were also preserved in ethanol for further morphological analysis. The results revealed that the best conditions for Atya lanipes development were 30 ppm water salinity, constant gentle aeration, and 27 °C water temperature. Nine stages were identified for the description of the early larval development of Atya lanipes. Early larval stages differ primarily in interstage larval size, the appearance and development of the telson, appendage appearance, growth of antennae and antennules, and pigmentation. The present contribution represents the first study that describes the larval development of the Caribbean shrimp Atya lanipes.

Neurotoxicity and Oxidative Stress Development in Adult Atya lanipes Shrimp Exposed to Titanium Dioxide Nanoparticles.

Titanium dioxide is a type of nanoparticle that is composed of one titanium atom and two oxygen atoms. One of its physicochemical activities is photolysis, which produces different reactive oxygen species (ROS). Atya lanipes shrimp affect detrital processing and illustrate the potential importance of diversity and nutrient availability to the rest of the food web. It is essential in removing sediments, which have an important role in preventing eutrophication. This study aimed to determine the toxic effect of changes in behavior and levels of oxidative stress due to exposure to titanium dioxide nanoparticles in Atya lanipes and to determine the effective concentration (EC50) for behavioral variables. The concentrations of TiO2 NPs tested were 0.0, 0.50, 1.0, 2.0, and 3.0 mg/L with the positive controls given 100 µg/L of titanium and 3.0 mg/L of TiO2 NPs ± 100 µg/L of titanium. After 24 h of exposure, significant hypoactivity was documented. The EC50 was determined to be a concentration of 0.14 mg/L. After the exposure to 10 mg/L of TiO2 NPs, oxidative stress in gastrointestinal and nervous tissues was documented. The toxic effects of this emerging aquatic pollutant in acute exposure conditions were characterized by sublethal effects such as behavior changes and oxidative stress.

Titanium Oxide Nanoparticles as Emerging Aquatic Pollutants: An Evaluation of the Nanotoxicity in the Freshwater Shrimp Larvae Atya lanipes

Nanoparticles are man-made materials defined as materials smaller than 100 nm in at least one dimension. Titanium oxide nanoparticles are of great interest because of their extensive use in self-care products. There is a lack of nanotoxicological studies of TiO2 NPs in benthic organisms to have evidence about the effects of these pollutants in freshwater ecosystems. Atya lanipes is a scraper/filter that can provide a good nanotoxicological model. This study aims to determine how the TiO2 NPs can develop a toxic effect in the larvae of the Atya lanipes shrimp and to document lethal and sublethal effects after acute exposures to TiO2 NP suspensions of: 0.0, 1.0, 10.0, 50.0, 100.0, and 150.0 mg/L. The results show that early exposure to TiO2 NPs in Atya lanipes creates an increase in mortality at 48 and 72 h exposures, hypoactivity in movements, and morphological changes, such as less pigmentation and the presence of edema in exposed larvae. In conclusion, TiO2 NPs are toxic contaminants in the larval stage of the Atya lanipes. It is necessary to regulate these nanoparticles for purposes of the conservation of aquatic biodiversity, especially for freshwater shrimp larvae and likely many other larvae of filter-feeding species.

Effects of food supplies and water temperature on growth rates of two species of freshwater tropical shrimps

Summary Growth rates of individual freshwater shrimp of the species Atya lanipes and Xiphocaris elongata were measured in a second‐order stream in the Luquillo Experimental Forest in Puerto Rico over 10 years (1997–2007). Shrimp living at lower altitudes in warmer water and wider stream channels with more algal and detrital foods were predicted to grow and reproduce more rapidly. Shrimp were marked and recaptured periodically in pools located at three altitudes to determine whether temperature affected growth rates among individual A. lanipes and X. elongata. Mean annual water temperatures ranged from 20 to 24 °C with the uppermost pool being cooler than the lower pools. Mean annual growth rates for Atya and Xiphocaris were 0.27 and 0.1 mm carapace length, respectively, for all three populations. Differences in growth were partially influenced by how each species obtains its food. Atya is a filter feeder and scraper and has continuous access to suspended organic particles and biofilms. The slower growth rate for Xiphocaris elongata is most likely a result of the wider range in quality and accessibility of food resources. Differences in pool morphology and depths probably affected differences in food availability. Increased leaf litter retention in the deeper upper and lower pools probably increased shrimp growth rates, while washout of leaf litter from the relatively shallow, elongate mid‐altitude pool decreased Atya lanipes growth rates. These long‐lived, slow‐growing shrimp species transform a wide range of organic materials into their biomass. Because of the slow growth rates of these detritivores shrimp, tropical storms, hurricanes, droughts or other disturbances could have persistent, long‐term impacts on detrital processing and on the populations of their predators.

Effects of coupled natural and anthropogenic factors on the community structure of diadromous fish and shrimp species in tropical island streams

Summary 1. Overlapping river and road networks provide a framework for studying the complex interactions between natural and human systems, with river-road intersections as focal areas of study. Roads can alter the morphology of stream channels, pose barriers to freshwater fauna, provide easy access to streams for humans and non-native species and accelerate the expansion of urban development. 2. We determined what variables control the structure of diadromous fish and shrimp communities and assessed whether particular road crossings altered community structure in north-eastern Puerto Rico. We identified 24 sites that represented a range of river and road sizes across two catchments that drain El Yunque National Forest in Puerto Rico. 3. The location of natural barriers and the size of stream pools were the most important variables for predicting six of fifteen fish and shrimp distributions. Predatory fishes were predicted to be limited to areas in the river network below large, steep waterfalls, whereas adult shrimp Atya lanipes (Atyidae) were predicted to be present above these waterfalls. The fish Awaous banana was predicted to be present in pools >11.6 m wide, whereas the shrimp Xiphocaris elongata was predicted to be present in pools <10.4 m wide. The distributions of nine species were predicted poorly, but three of these species were common and three were rare. 4. Although urban and agricultural land covers were among the top three predictors of five species distributions, they were probably good predictors because they were correlated with the natural gradient. Further study is necessary to disentangle natural and anthropogenic gradients. 5. Road crossings, 10 of which were culverts, were not dispersal barriers for fishes or shrimps. On average, species were present both upstream and downstream from road crossings at 68% of sites where they occurred. Absences upstream or downstream from road crossings occurred at 16% of sites each and likely resulted from a failure to detect species. 6. Several existing features of these catchments and taxa may aid in fish and shrimp conservation. The headwaters are protected by management practices of El Yunque National Forest, connectivity within the river network has been maintained, and the diadromous life history of these organisms makes them resilient to pulsed disturbances.

Running the predator gauntlet: do freshwater shrimp (Atya lanipes) migrate above waterfalls to avoid fish predation?

Predation might cause the disparate distributions of fish and shrimp in Puerto Rican streams. Adult shrimp (Atya lanipes) are present only above waterfalls that pose barriers to predatory fishes. Because we have not found Atya in fish stomachs, we think that these amphidromous shrimp avoid predation by migrating to locations above waterfalls as post-larvae. We designed experiments in an artificial y-maze and in natural streams to test whether Atya avoid predatory fishes. In artificial streams, adult Atya avoided the scent of 2 fishes (Gobiomorus dormitor and Agonostomus monticola) regardless of whether the fish had recently been fed Atya, but they avoided the scent of Anguilla rostrata only when they had recently been fed Atya. Postlarval Atya avoided the scent of G. dormitor fed Atya and of crushed conspecifics. We then tested whether adult Atya in 4 natural headwater streams would redistribute themselves if exposed to the chemical scent or physical presence (visual, mechanical, and chemical cues) of G. dormitor. We expected Atya to move upstream from the manipulated pools, resulting in increased emigration from and lower Atya abundance in the pool. Atya relative abundance did not decline significantly in the pools when fish scent was added, indicating that chemical cues alone were not threatening enough to alter adult behavior in natural streams. The relative abundance of adults declined significantly in the pools when fish were added to in-stream cages, a result suggesting that addition of mechanical or visual cues to chemical cues signaled imminent danger and caused Atya to alter their behavior. Emigration increased, but lower activity levels also could have reduced abundances in pools. Our study provides mechanistic evidence that predatory fishes alter the behavior of postlarval and adult A. lanipes, and might explain why adult Atya are absent in reaches downstream of waterfalls.

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

Summary1. 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 &gt;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.

°Molecular evidence for sequential colonization and taxon cycling in freshwater decapod shrimps on a Caribbean island

Taxon cycling, i.e. sequential phases of expansions and contractions in species' distributions associated with ecological or morphological shifts, are postulated to characterize dynamic biogeographic histories in various island faunas. The Caribbean freshwater shrimp assemblage is mostly widespread and sympatric throughout the region, although one species (Atyidae: Atya lanipes) is geographically restricted and ecologically and morphologically differentiated from other Atya species. Using patterns of nucleotide variation at the COI mtDNA gene in five species of freshwater shrimp (A. lanipes, A. scabra, A. innocuous; Xiphocarididae: Xiphocaris elongata; Palaemonidae: Macrobrachium faustinum) from Puerto Rico, we expected to detect a signature of sequential colonization in these shrimp, consistent with the concept of taxon cycling, and expected that A. lanipes would be at a different taxon stage (i.e. an early stage species) to all other species. We also examined patterns of genetic population structure in each species expected with poor, intermediate and well-developed abilities for among-river dispersal. Population expansions were detected in all species, although the relative timing of the expansions varied among them. Assuming that population expansions followed colonization of Puerto Rico by freshwater shrimp, results bear the hallmarks of sequential colonization and taxon cycling in this fauna. A. lanipes had a star phylogeny, low mean pairwise nucleotide differences and recent (Holocene) estimates for an in situ population expansion in Puerto Rico, and it was inferred as an early stage species in the taxon cycle undergoing a secondary phase of expansion. All other species were inferred as late stage species undergoing regional population expansions, as their mean pairwise nucleotide differences were relatively high and phylogenetic patterns were more complex than A. lanipes. High rates of gene flow without isolation by distance among rivers were detected in all species, although results should be treated cautiously as some populations are unlikely to be in mutation-drift equilibrium. Nested clade analysis produced inconsistent results among species that all have high rates of gene flow and expanding populations.

Relative Importance of Bacteria and Fungi in a Tropical Headwater Stream: Leaf Decomposition and Invertebrate Feeding Preference

Bacteria and fungi provide critical links between leaf detritus and higher trophic levels in forested headwater food webs, but these links in tropical streams are not well understood. We compared the roles of bacteria and fungi in the leaf decomposition process and determining feeding preference for two species of freshwater shrimp found in the Luquillo Experimental Forest, Puerto Rico, using experimental microcosms. We first tested the effects of four treatments on decomposition rates for leaves from two common riparian species, Cecropia scheberiana (Moraceae) and Dacryodes excelsa (Burseraceae), in laboratory microcosms. Treatments were designed to alter the microbial community by minimizing the presence of bacteria or fungi. The fastest decay rate was the control treatment for D. excelsa where both bacteria and fungi were present (k = -0.0073 day(-1)) compared to the next fastest rate of k = -0.0063 day(-1) for the bacterial-conditioned D. excelsa leaves. The fastest decay rate for C. scheberiana was also the control treatment (k = -0.0035 day(-1)), while the next fastest rate was for fungal-conditioned leaves (k = -0.0029 day(-1)). The nonadditive effect for leaf decomposition rates observed in the control treatments where both fungi and bacteria were present indicate that bacteria and fungi perform different functions in processing leaf litter. Additionally, leaf types differed in microbial colonization patterns. We next tested feeding preference for leaf type and microbe treatment in microcosms using two species of freshwater shrimp: Xiphocaris elongata, a shredder, and Atya lanipes, a scraper/filterer. To estimate feeding preferences of individual shrimp, we measured change in leaf surface area and the amount of particles generated during 5-day trials in 16 different two-choice combinations. X. elongata preferred D. excelsa over C. scheberiana, and leaves with microbial conditioning over leaves without conditioning. There was no clear preference for fungal-conditioned leaves over bacterial-conditioned leaves. This lack of preference for which microbes were responsible for the conditioning demonstrates the importance of both bacterial and fungal resources in these tropical stream food web studies.

Effects of extreme low flows on freshwater shrimps in a perennial tropical stream

Summary Long‐term data on rainfall suggests that perennial rainforest streams rarely are subject to drying of riffles or pools in the wet, non‐seasonal Caribbean climate of Puerto Rico. Unusually low rainfall in 1994 caused some headwater riffles to dry out completely, resulting in isolated pools, reduced pool volumes and loss of access to microhabitats by benthic invertebrates. From 1992 to 1998, shrimp populations were sampled bimonthly using baited traps in six pools along 1200 m (from 305 to 480 m in altitude) of Quebrada Prieta, a second‐order headwater stream in the Luquillo Experimental Forest (Caribbean National Forest). Following contraction of the smaller and shallower pools in the most upstream section of the stream, mean densities of the dominant shrimp (Atya lanipes) increased from 22 to 75 shrimp m−2 of pool area during the 1994 drought year. A second common species (Xiphocaris elongata) increased from 5 to 14 shrimp m−2. A smaller percentage of adults of both species was gravid during the drought. Following the 1994 drought (1995–1998), densities of both shrimp species and reproductive activity of Atya returned to predrought (1990–1993) levels. However, the reproductive activity of Xiphocaris remained lower than in the predrought period. It is suggested that prolonged droughts, even in tropical rainforest biomes, may significantly alter aquatic communities through localised crowding effects resulting from habitat contraction, and lead to prolonged decreases in reproductive output. Consequently, major alterations in aquatic populations and communities would be predicted by current climate change scenarios of decreased total rainfall and increased variability.

Tubular localization of silent calcium channels in crustacean skeletal muscle fibers.

Ca2+-induced Ca2+ release (CICR) in the superficial abdominal flexor muscle of the crustacean Atya lanipes appears to be mediated by a local control mechanism similar to that of vertebrate cardiac muscle, but with an unusually high gain. Thus, Ca2+ influx increases sufficiently the local concentration of Ca2+ in the immediate vicinity of the sarcoplasmic reticulum Ca2+ release channels to trigger the highly amplified release of Ca2+ required for contraction, but is too low to generate a macroscopic inward current (i.e., the Ca2+ channels are silent). To determine the localization of the silent Ca2+ Channels, the mechanical, electrophysiological and ultrastructural properties of the muscle were examined before and after formamide treatment, a procedure that produces the disruption of transverse tubules of striated muscle. We found that tubular disruption decreased tension generation by about 90%; reduced inward current (measured as Vmax, the maximum rate of rise of Sr2+ action potentials) by about 80%; and decreased membrane capacitance by about 77%. The results suggest that ca. 80% of the silent Ca2+ channels are located in the tubular system. Thus, these studies provide further evidence to support the local control mechanism of CICR in crustacean skeletal muscle.

Freshwater Shrimp Effects on Detrital Processing and Nutrients in a Tropical Headwater Stream

In this paper, we report on a whole-pool manipulation of leaf litter decomposition in a tropical stream following a hurricane. The study was designed to distinguish how decapod species comprising two functional feeding guilds alter rates and magnitudes of leaf litter processing and nutrient release linking the detrital food web with the overall producer–consumer food web. Streams of the Luquillo Experimental Forest, Puerto Rico, are dominated numerically by two freshwater shrimp species (Atya lanipes and Xiphocaris elongata). To determine how these shrimp affected detrital processing following large leaf inputs associated with a hurricane, we manipulated the presence or absence of two species of shrimp in six fenced pools of a headwater stream with hurricane levels of Cecropia leaf litter over a 23-d period. The experiment was designed to determine how the two different shrimp affected: (1) the rate and amount of size fractionation of leaf material; (2) the localized nutrient concentrations in the pools; and (3) the rate of particulate export from the pools. Both shrimp species influenced detrital processing, but in fundamentally different ways. Xiphocaris shred intact, large leaves and converted them into fine, medium, and coarse particulates. Through this conversion process Xiphocaris increased the concentration and rate of downstream transport of suspended particulate organic matter. Xiphocaris also significantly increased the concentration of both total dissolved nitrogen and dissolved organic carbon, likely by changing the surface area to volume ratio of organic particles. Atya, a scraper/filter feeder, caused a slight increase in rates of leaf breakdown as compared to controls at the end of the experiment but filtered out fine organic particulates, resulting in less downstream export. Both decapod species affected detrital processing, but in fundamentally different ways, illustrating the potential importance guild diversity may have in altering both particulate and nutrient availability to the rest of the food web. In addition, these results suggest that the presence of both feeding guilds can significantly influence ecosystem responses to severe, large-scale disturbance events.

FRESHWATER SHRIMP EFFECTS ON DETRITAL PROCESSING AND NUTRIENTS IN A TROPICAL HEADWATER STREAM

In this paper, we report on a whole-pool manipulation of leaf litter decomposition in a tropical stream following a hurricane. The study was designed to distinguish how decapod species comprising two functional feeding guilds alter rates and magnitudes of leaf litter processing and nutrient release linking the detrital food web with the overall producer–consumer food web. Streams of the Luquillo Experimental Forest, Puerto Rico, are dominated numerically by two freshwater shrimp species (Atya lanipes and Xiphocaris elongata). To determine how these shrimp affected detrital processing following large leaf inputs associated with a hurricane, we manipulated the presence or absence of two species of shrimp in six fenced pools of a headwater stream with hurricane levels of Cecropia leaf litter over a 23-d period. The experiment was designed to determine how the two different shrimp affected: (1) the rate and amount of size fractionation of leaf material; (2) the localized nutrient concentrations in the pools; ...

Silent calcium channels in skeletal muscle fibers of the crustacean Atya lanipes.

The superficial (tonic) abdominal flexor muscles of Atya lanipes do not generate Ca(2+) action potentials when depolarized and have no detectable inward Ca(2+) current. These fibers, however, are strictly dependent on Ca(2+) influx for contraction, suggesting that they depend on Ca(2+)-induced Ca(2+) release for contractile activation. The nature of the communication between Ca(2+) channels in the sarcolemmal/tubular membrane and Ca(2+) release channels in the sarcoplasmic reticulum in this crustacean muscle was investigated. The effects of dihydropyridines on tension generation and the passive electrical response were examined in current-clamped fibers: Bay K 8644 enhanced tension about 100% but did not alter the passive electrical response; nifedipine inhibited tension by about 70%. Sr(2+) and Ba(2+) action potentials could be elicited in Ca(2+)-free solutions. The spikes generated by these divalent cations were abolished by nifedipine. As the Sr(2+) or Ba(2+) concentrations were increased, the amplitudes of the action potentials and their maximum rate of rise, V(max), increased and tended towards saturation. Three-microelectrode voltage-clamp experiments showed that even at high (138 mm) extracellular Ca(2+) concentration the channels were silent, i.e., no inward Ca(2+) current was detected. In Ca(2+)-free solutions, inward currents carried by 138 mm Sr(2+) or Ba(2+) were observed. The currents activated at voltages above -40 mV and peaked at about 0 mV. This voltage-activation profile and the sensitivity of the channels to dihydropyridines indicate that they resemble L-type Ca(2+) channels. Peak inward current density values were low, ca. -33 microA/cm(2) for Sr(2+) and -14 microA/cm(2) for Ba(2+), suggesting that Ca(2+) channels are present at a very low density. It is concluded that Ca(2+)-induced Ca(2+) release in this crustacean muscle operates with an unusually high gain: Ca(2+) influx through the silent Ca(2+) channels is too low to generate a macroscopic inward current, but increases sufficiently the local concentration of Ca(2+) in the immediate vicinity of the sarcoplasmic reticulum Ca(2+) release channels to trigger the highly amplified release of Ca(2+) required for tension generation.

On the relative importance of pool morphology and woody debris to distributions of shrimp in a Puerto Rican headwater stream

In this paper, we report the sizes and distributional orientation of woody debris in a headwater rainforest stream in the Luquillo Experimental Forest (LEF), Puerto Rico. We also provide results of a 4-month study of a wood addition experiment designed to increase cover for benthic macroinvertebrates (freshwater shrimp). We added branch-sized woody debris to 20 pools in three streams. We trapped four species of freshwater shrimp (two species of benthic detritivores and two predatory shrimp species) during each of the 4 months following wood additions. An analysis of pool morphology (maximum depth, surface area and volume) provided a useful predictor of shrimp abundances. In general, numbers of shrimps increased with sizes of stream pools. A repeated measures ANOVA demonstrated no effect of woody debris additions on total numbers of shrimp per pool area. Two detritivore species (Atya lanipes, a filter feeder and Xiphocaris elongata, a shredder) decreased in abundance with increased woody debris and there was no statistical relationship between woody debris additions and predators ( Macrobrachium carcinus and M. crenulatum). Small woody debris additions may have altered flow velocities that were important to filter-feeding Atya at the microhabitat scale, although the overall velocities within pools were not altered by wood additions. Lower numbers of Atya and Xiphocaris in two of the three streams may result from the occurrence of two predaceous fishes (American eel and mountain mullet) and more predatory Macrobrachium in these streams. One likely interpretation of the results of this study is that the stream pools in these study reaches had sufficient habitat structure provided by numerous rock crevices (among large rocks and boulders) to provide refuge from predators. Addition of woody debris did not add significantly to the existing structure. These results may not apply to stream channels with sand and gravel substrata where crevices and undercut banks are lacking and where woody debris often plays a major role by providing structure and refuge.

Distribution and Abundance of Tropical Freshwater Shrimp Along a Stream Corridor: Response to Disturbance

Different intensities and frequencies of disturbances can alter spatial and temporal patterns of abundances for coexisting species. We observed the effects of a high-flow event generated by Hurricane Hugo (19 September 1989) and belownormal stream flow during 1994 on freshwater shrimp populations in a tropical, montane stream. To determine if these different hydrologic regimes altered distributions of populations, we compared shrimp densities during three periods: pre-Hurricane Hugo (20 mo), post-Hurricane Hugo (50 mo), and low-flow (12 mo). There were significant differences in the relationships between locations of stream pools along an elevational gradient (300 to 470 m) and the abundances of two species of shrimp (Atya lanipes and Xiphocaris elongata) during these three periods. Atya increased in density with increasing elevation in a consistent fashion during all three periods. Densities also increased during the post-Hugo and low-flow periods relative to the pre-Hugo baseline. Xiphocaris increased in density with increasing elevation during the pre- and post-Hugo periods, but density decreased with elevation during the low-flow period. Palaemonid species of predatory shrimp, Macrobrachium carcinus and Macrobrachium crenulatum, consistently decreased in density with elevation during all three periods of observation. Multiple regression analyses demonstrated high predictability of Atya density distributions based on volume, pool depth, and coefficient of variation of pool depth during the pre-Hugo baseline observations. Xiphocaris densities were positively associated with pool width and negatively associated with the coefficient of variation of pool width in the pre-Hugo period. However, physical variables did not predict Atya or Xiphocaris densities during the post-Hugo period. During the dry period, there was a negative association between Atya densities and pool depth and width, and a positive association with the coefficient of variation of pool depth. During the dry period, Xiphocaris densities were best predicted as a function of maximum depth; there was a negative association with the coefficient of variation of pool width. Pre-Hugo Macrobrachium densities were negatively associated with the coefficient of variation of pool width, pool width-to-depth ratio, and elevation; in the post-Hugo and dry periods, Macrobrachium densities were best predicted by elevation. Mechanisms that likely cause these patterns of distribution include avoidance of predators coupled with active preference by prey species for pool habitats with low frequency of washout by storm flows, and with sufficient storage of food resources (microbially conditioned leaf detritus).

Responses of a Freshwater Shrimp to Chemical and Tactile Stimuli from a Large Decapod Predator

Montane, tropical streams in Puerto Rico are dominated by decapod crustaceans. The most ubiquitous freshwater shrimp, with a distribution ranging from mangrove swamps to the highest-elevation pools, is Atya lanipes, a scraper/filterer. A large predatory shrimp, Macrobrachium carcinus, is much less abundant but dominates the top of the stream foodweb. As part of our long-term ecological research, we have observed a general negative relationship between Atya abundance and the presence of the large predatory shrimp. In a series of laboratory and field experiments, we showed that this relationship was primarily due to avoidance responses by Atya to the predator species. Furthermore, our experiments showed that both chemical and mechanical/tactile cues of predator presence determined the among-pool distribution of Atya within streams. Although chemical cues alone elicited a relatively weak response, the combination of chemical and tactile cues resulted in very strong movement responses. If the prey were confined to pools with predatory shrimp, the prey significantly decreased their overall movement and spent substantially more time in crevices. If given the opportunity, prey shrimp emigrated out of areas that had predatory decapods.

Sulfhydryl alkylating agents induce calcium current in skeletal muscle fibers of a crustacean (Atya lanipes)

Voltage-clamp experiments using the three-microelectrode voltage clamp technique were performed on ventroabdominal flexor muscles of the crustacean Atya lanipes. Potassium and chloride currents were found to underlie the normal, passive response of the muscle. Blocking potassium currents with tetraethylammonium and replacing chloride ions with methanesulfonate did not unmask an inward current. By treating the muscle with the sulfhydryl-alkylating agent 4-cyclopentene-1,3-dione an inward current was detected. The current induced by the agent is carried by Ca2+, since it is abolished in Ca(2+)-free solutions. The induced Ca2+ current is detected at about -40 mV and reaches a mean maximum value of -78 microA/cm2 at ca. -10 mV. At this potential the time to peak is close to 15 msec. The induced Ca2+ current inactivated with 1-sec prepulses which did not elicit detectable Ca2+ current; the fitted hx curve had a midpoint of -38 mV and a steepness of 5.0 mV. Measurements of isometric tension were performed in small bundles of fibers, and the effects of the sulfhydryl-alkylating agents 4-cyclopentene-1,3-dione and N-ethylmaleimide were investigated. Tetanic tension was enhanced in a strictly Ca(2+)-dependent manner by 4-cyclopentene-1,3-dione. The amplitude of K+ contractures increased after treatment with N-ethylmaleimide. It is concluded that Ca2+ channels are made functional by the sulfhydryl-specific reagents and that the increase in tension is probably mediated by an increase in Ca2+ influx through the chemically induced Ca2+ channels.

Ultrastructural and mechanical properties of electrically inexcitable skeletal muscle fibers of the crustacean Atya lanipes

Examination of the ultrastructure and mechanical activation of the ventroabdominal flexor muscle of the freshwater crustacean Atya lanipes shows that the fibers are of the long sarcomere, tonic type. The fibers possess an ample and well-organized internal membrane system, with extensive regions of T SR dyad contacts near the ends of the A bands. An orbit of 10–12 thin filaments surrounds each thick filament. The lanthanum tracer method reveals a highly regular organization of the T-system, Z-tubules, and multiple internal clefts. Tension generation responds to extracellular potassium in a concentration dependent manner and is very slow. Mechanical activation is strictly dependent on extracellular Ca 2+, even though these muscle fibers do not generate Ca 2+ currents when depolarized. Tension development responds to caffeine and is also dependent on extracellular Na + suggesting that Ca 2+ release from the SR and Ca 2+ influx via the Na Ca exchanger intervene in mechanical activation.

Post-Hurricane Hugo Increases in Atyid Shrimp Abundances in a Puerto Rican Montane Stream

We report the first data on changes in tropical stream biota resulting from a major hurricane. Beginning in January 1989, we trapped freshwater shrimp, Atya lanipes, along 1200 m of a montane stream in the Luquillo Experimental Forest, Puerto Rico. Prior to Hurricane Hugo, shrimp densities were greater in the headwaters than at mid- and low elevation. In October 1989, one month after the hurricane, shrimp densities were reduced on average by 50 percent in the headwaters (apparently from washout) and increased by 80 percent at mid-elevation. From December 1989 to May 1990 overall shrimp densities increased rapidly to the highest abundances ever recorded. These densities most likely resulted from increased numbers of shrimp that migrated upstream from riverine pools; and, from increased availability of unusually abundant food resources (decomposing leaves and algae) that increased recruitment of these generalized consumers. Benthic communities in forested, headwater streams are likely to be resilient after intermediate levels of disturbance, because rapid debris-dam formation increases retention of food resources and reduces washout of invertebrate consumers. However, storms generating greater stream flow and/or less wind than Hurricane Hugo could cause extensive, longer-lasting decreases of benthic-dwelling shrimp because of greater washout of both consumers and food supplies.