Gill-cleaning mechanisms of a dendrobranchiate shrimp, Rimapenaeus similis (Decapoda, penaeidae): description and experimental testing of function.

Observations on functional morphology and results from experiments demonstrate that setiferous epipods compose the major gill-cleaning mechanism in a penaeoid shrimp, Rimapenaeus similis. Epipods on the second maxillipeds and on pereopods 1-3 are equipped with long setae bearing an array of digitate scale setules. These multidenticulate setae reach to most gills and are jostled among them during limb movements. Experiments were performed in which epipods were removed from the gill chamber on one side (experimental) but not the other (control); treated animals were exposed to fouling in a recirculating water system for 2 weeks. Particulate fouling, measured by reduction in relative gill transparency, was significantly greater on experimental than control gills. The pereopodal exopods, not previously implicated in gill cleaning in any decapod, were similarly identified as important gill-cleaning structures. Equipped with long multidenticulate setae like those on the epipods, exopods sweep back and forth over the gill filaments just under the gill cover, areas not reached by the epipods. Exopod-ablation experiments were conducted that showed that exopods prevent particulate fouling on gill surfaces over which they sweep. The similarity in action of the passive gill-cleaning system of R. similis to that of crayfish (Bauer [1998] Invert Biol 117:29-143) suggests the hypothesis that the epipodal and exopodal cleaning setae of R. similis are ineffective against epibionts. The reduction in epipodal and exopodal cleaning systems that occurs in the Penaeoidea is hypothesized to be compensated for by increased development of gill-cleaning setae on the branchiostegite, scaphognathite, or other structures.     

Flow visualisation and high speed video analysis of water jets in the snapping shrimp (Alpheus heterochaelis)

Snapping shrimp (Alpheus heterochaelis) produce a fast, well-focused water jet by rapid closure of their specialised snapper claw. As shown previously, water jets may injure the opponent in interspecific encounters (e.g. with small crabs) although no damage was observed in intraspecific encounters. For conspecific receivers the jet represents a potential hydrodynamic signal and can be analysed with the help of mechanosensory hairs. To gain more insight in the biophysical characteristics of the water jet we visualised and analysed jets of tethered snapping shrimp using standard and high speed video recordings. Water jet width increases with increasing distance from the snapper claw tip, and both width and distance increase with increasing snapper claw size. Water jet distances do not increase with increasing claw cocking duration (building up muscle tension) but medium cocking durations of about 550 ms result in longest distances. Mean water jet velocity is 6.5 m s⁻¹ shortly after claw closure but rapidly decreases subsequently. At the mean distance between snapping conspecifics (9 mm) water jet velocities produced by snapping shrimp with larger snapper claws are significantly higher than those of animals with smaller claws. Interestingly, males with equal snapper claw size as females produce significantly faster water jets. Accepted: 31 March 1999     

Fixation Biases towards the Index Finger in Almost-Natural Grasping

We use visual information to guide our grasping movements. When grasping an object with a precision grip, the two digits need to reach two different positions more or less simultaneously, but the eyes can only be directed to one position at a time. Several studies that have examined eye movements in grasping have found that people tend to direct their gaze near where their index finger will contact the object. Here we aimed at better understanding why people do so by asking participants to lift an object off a horizontal surface. They were to grasp the object with a precision grip while movements of their hand, eye and head were recorded. We confirmed that people tend to look closer to positions that a digit needs to reach more accurately. Moreover, we show that where they look as they reach for the object depends on where they were looking before, presumably because they try to minimize the time during which the eyes are moving so fast that no new visual information is acquired. Most importantly, we confirmed that people have a bias to direct gaze towards the index finger’s contact point rather than towards that of the thumb. In our study, this cannot be explained by the index finger contacting the object before the thumb. Instead, it appears to be because the index finger moves to a position that is hidden behind the object that is grasped, probably making this the place at which one is most likely to encounter unexpected problems that would benefit from visual guidance. However, this cannot explain the bias that was found in previous studies, where neither contact point was hidden, so it cannot be the only explanation for the bias.     

Hydrodynamics of larval settlement from a larva's point of view

Many benthic marine invertebrate animals release larvae that are dispersed by ocean currents. These larvae swim and can respond to environmental factors such as chemical cues. However, larvae are so small (generally 0.01-1 mm) that they are often assumed to be passive particles whose trajectories are determined by the motion of the water in which they are riding. Therefore, marine larvae are useful model organisms to study the more general question of how the locomotion of very small animals in complex, variable natural habitats is affected by the motion of the fluid (water or air) around them. Studying larval locomotion under conditions of water flow encountered in nature is challenging because measuring the behavior of an individual microscopic organism requires high magnification imaging that is difficult to do in the field. The purpose of this article is to synthesize in one place the various approaches that we have been using to address the technical challenges of studying the locomotion of microscopic larvae in realistic ambient flow. The steps in our process include: (1) measuring water flow in the field; (2) mimicking realistic water movement in laboratory flumes to measure larval scale fluctuations in velocity of flow and concentration of chemical cues; (3) mimicking fine scale temporal patterns of larval encounters with a dissolved chemical cue to record larval responses; (4) using individual-based models to put larvae back into the larger scale environmental flow to determine trajectories; and (5) mimicking fine scale spatial and temporal patterns of larval encounters with water velocities and shear to determine the instantaneous forces on larvae. We illustrate these techniques using examples from our ongoing research on the settlement of larvae onto fouling communities and from our published work on settlement of larvae onto coral reefs. These examples show that water velocities and concentrations of chemical cues encountered by microscopic organisms can fluctuate in fractions of a second and vary over scales of less than a millimeter.     

Scales of orientation, directed walks and movement path structure in sharks

1. Animal search patterns reflect sensory perception ranges combined with memory and knowledge of the surrounding environment. 2. Random walks are used when the locations of resources are unknown, whereas directed walks should be optimal when the location of favourable habitats is known. However, directed walks have been quantified for very few species. 3. We re-analysed tracking data from three shark species to determine whether they were using directed walks, and if so, over which spatial scales. Fractal analysis was used to quantify how movement structure varied with spatial scale and determine whether the sharks were using patches. 4. Tiger sharks performed directed walks at large spatial scales (at least 6-8 km). Thresher sharks also showed directed movement (at scales of 400-1900 m), and adult threshers were able to orient at greater scales than juveniles, which may suggest that learning improves the ability to perform directed walks. Blacktip reef sharks had small home ranges, high site fidelity and showed no evidence of oriented movements at large scales. 5. There were inter- and intraspecific differences in path structure and patch size, although most individuals showed scale-dependent movements. Furthermore, some individuals of each species performed movements similar to a correlated random walk. 6. Sharks can perform directed walks over large spatial scales, with scales of movements reflecting site fidelity and home range size. Understanding when and where directed walks occur is crucial for developing more accurate population-level dispersal models.     

The photophores of Meganyctiphanes norvegica (M. Sars) (Euphausiacea): mode of operation

The photophores of Meganyctiphanes were investigated with regard to the control of light production and with respect to their role in a hitherto unknown communication system using light flashes which became evident from observation of specialised signalling behaviour. To that purpose the light production was recorded during presentation of a range of stimuli delivered to the intact, tethered shrimp. Stimuli used were changes in ambient light, water turbulence, simulated predator approach and light flashes, as well as electric shocks and serotonin injections. Strong negative light gradients, exaggerating the natural sunset signal, reliably elicited light production, the peak of which lasted on average 2 min. In the late phase of this light production, low frequency water oscillations and turbulent flow (assumed intraspecific communication signals at close range) elicited transient increases in light production. Artificial light flashes presented to a group of shrimp evoked a signalling behaviour in which the animal points the light of its photophore beamers (positioned at the ventral side and normally directed downwards) for a fraction of a second at observers within the same depth level. The responses produced by the signalling behaviour indicate a fixed delay with respect to the triggering flash. Electric stimulation of the ventral nerve cord via implanted electrodes resulted in a strong light production with a latency of 160 ms. Injection of serotonin, resulting in haemolymph concentrations of 10^–5 M and higher, initiated increasingly strong and increasingly long-lasting continuous light production. Implications for the control of the photophores are discussed. Electronic Publication     

A single muscle moves a crustacean limb joint rhythmically by acting against a spring containing resilin

Background  

The beating or fanning movements of three pairs of maxilliped flagella in crabs and crayfish modify exhalent gill currents while drawing water over chemoreceptors on the head. They play an integral part both in signalling by distributing urine odours, and in active chemosensation.     

Gill chamber ventilation and scaphognathite movements in Crangon crangon (L.)

Buried Crangon crangon (L.) produce, anterior to their gill chambers, an inhalant and exhalant channel in the substratum. The combined shape and disposition of the maxillipeds effects separation of the inhalatory and exhalatory streams, and this channelling allows the maintenance of a posteroanteriorly directed flow of water over the gills of buried animals.Impedance techniques have been used to obtain qualitative data on the movements of scaphognathites. Reversals were identified and seen to occur particularly during burying and at the onset of walking. Reversals begin by the anterior region of the scaphognathite missing a beat, and end with the middle and anterior regions pausing.Analyses of movement patterns of individual scaphognathites show that a beat comprises equally-effective depression levation phases, but the timing sequence of the movements differs slightly from that reported in Carcinus maenas.     

Studies of the biology of the lesser weever fish Trachinus vipera Cuvier

The anatomical adaptations to a benthic mode of life are reviewed and discussed. The functioning of the pectoral, pelvic and anal fins in moving sand from underneath the animal, during burrowing, are described. Jets of water directed downwards from the gill chambers, produce a fluid sand-water mixture which facilitates the functioning of the pelvic fins, and the movement of the head into the sand. The direction of water movements associated with gill ventilation, which are propelled almost exclusively by the branchiostegal rays, are outlined. Inhalation is normally through the mouth, which is protected by papillae, from being blocked by sand. When the mouth is covered with sand, the water flow through it is reduced, and some inhalation occurs through the dorsal margin of the operculum, involving use of the supracliethrum.     

A review of long-distance movements by marine turtles, and the possible role of ocean currents

Sea turtle movements often occur in open‐sea unsheltered areas, and are therefore likely to be influenced by major oceanographic processes. Only recently has work started to examine the possible relationships of these movements with dynamic oceanic features, and consequently a clear picture of such interaction is only available in a few cases. Newborn sea turtles are thought to rely on oceanic currents to reach their pelagic nursery habitats. The actual extent and timing of these developmental migrations are known for only a few populations, but these movements probably last several years and range over thousands of km. Large juveniles that have been tracked during their pelagic stage were found to make long‐distance movements, sometimes swimming against the prevailing currents. Older juveniles of most species leave the pelagic habitat to recruit to neritic developmental habitats. This is a very poorly documented phase of the sea turtle life‐cycle, and the few available indications show that turtles may have to swim actively for enormous distances to counterbalance their previous drift with the current. The course and extent of adult postnesting migrations vary greatly among different turtle species, but two main patterns are evident. Some species, like green, hawksbill and loggerhead turtles, shuttle between the nesting beach and a specific feeding area used for the entire inter‐reproductive period. In these cases, individuals swim, rather than drift, to complete their journeys, with possible advection due to currents sometimes helping them to quickly reach their target, but sometimes providing navigational challenges. Other species such as the olive ridley and the leatherback turtle, leave the coastal nesting areas to reach the pelagic environment where they forage, and perform wandering movements. Major oceanographic processes (such as main currents and eddies) have been recently shown to have a remarkable influence on leatherback movements, making it questionable whether these journeys are to be considered migrations or, rather, prolonged stays in vast feeding areas.     

Dynamics of cannibalism in equal‐aged cohorts of Spodoptera frugiperda

1. Antagonistic interactions in herbivorous insects are often density‐dependent, so rates are predicted to vary dynamically over time as density changes. Fatal intraspecific interactions, especially cannibalism, occur between equal‐aged larvae in young first‐ and second‐instar Spodoptera frugiperda (J.E. Smith). 2. A cannibalism experiment was conducted, starting with seven different densities of neonate S. frugiperda larvae, each replicated 50 times. Larvae were examined daily for the duration of the first and second instars (7 days). Seven‐day mortality was density‐dependent. 3. A stochastic mathematical model was developed in which per‐capita mortality from antagonistic interactions among equal‐aged larvae varies dynamically as density changes. A maximum likelihood method was developed to estimate the conditional per‐capita mortality rate from antagonistic interactions given an intraspecific encounter. An alternative model with mean‐mortality from antagonistic interactions that depends only on the initial larval density was also developed. 4. The models were fitted to the experimental data, and compared using log‐likelihood. The dynamic model fitted the cannibalism data significantly better than the time‐averaged mortality model for all starting densities for the experimental data, implying that density‐dependent mortality varied dynamically over time even within short 7‐day periods. 5. The conditional per‐capita mortality rate from antagonistic interactions was also density‐dependent, possibly because encounters became more aggregated at higher density, or because the probability that a larva died from the interaction was higher at higher density, or both.     

Patch shape and orientation influences on seagrass epifauna are mediated by dispersal abilities

While there have been theoretical arguments supporting the importance of the shape and orientation of habitat patches for determining species abundances, there have been few empirical demonstrations that these processes actually operate. Instead, most field studies have focussed on the importance of patch area, isolation and edge effects. I demonstrate that passively dispersed seagrass epifauna respond to the shape and orientation of artificial seagrass patches when currents, the dispersal mechanism, are strong, but not when they are weak. Orientation is important because animals dispersing via tidally induced water currents move predominantly in a single direction, and thus patches oriented across the current intercept more potential colonists than do those patches oriented perpendicular to the current. Currents less affect taxa that actively disperse, or that are relatively sedentary. Fish species that tend to use intertidal areas at high tide, however, were more abundant in patches perpendicular to shore (and parallel to the current), presumably because these patches offer the greatest amount of edge to animals undergoing tidal migrations.     

Vergesellschaftung als Verhaltensbereicherung im Zoo – Soziale Interaktion und Raumnutzung bei Orang-Utans,Hulmans und Kurzkrallenottern in der Zoom Erlebniswelt Gelsenkirchen

Keeping different species together is getting more and more common in zoos, because it can entail several behavioural enrichment factors for the animals as long as the enclosure meets the requirements of all species and a suitable composition of animals is living in the mix. The aim of this study is to evaluate a community of Sumatran orangutans (Pongo abelii), northern plains grey langurs (Semnopithecus entellus) and Asian small-clawed otters (Aonyx cinerea) in the Zoological Garden of Gelsenkirchen.The behavioural monitoring of the animals reveals that the orangutans spend less time with feeding behaviour and locomotion than free ranging orangutans, whereas the percentage of resting periods is much higher. This difference is probably caused by the easy availability of food. The northern plains grey langurs show ethograms whose proportions are very similar to those of free ranging grey langurs in literature. In both species the juvenile individuals have higher activity levels than the adult animals.The analysis of the spatial use of the enclosure by the orangutans reveals that they use about half of the area and prefer places at the ground for resting. On an average the orangutans spend around 50% of the monitored situations on the ground which is a high level for an arboreal living species. The time spent on the ground is distinctly lower in orangutans reared by their parents than in hand reared individuals which are probably affected by humans. Another influencing point is the sex. Adult male orangutans spend more time on the ground. This is a fact which is also known from free ranging animals. Related to their environment free ranging northern plains grey langurs show a high adaptability and the individuals in the zoo use about 70% of the enclosure and use frequently all structuring elements like trees, roots and ropes. Like the orangutans the grey langurs have preferred areas in the enclosure where they spend their resting periods with grooming, lactating and the feeding periods. Generally, the Asian small-clawed otters spend less time in the enclosure than the monkeys and use less than one third of the area.Most of the time free ranging orangutans live solitarily and thus it was predictable that there is only a fraction of intraspecific interactions between orangutans whereas the percentage of interactions between langurs is higher. The northern plains grey langurs live in harem troops and social interactions are more than 40% of the zoo langur's ethogram on an average. Asian small-clawed otters are also highly social animals and intraspecific contacts take place very often.The analysis of the monitored interspecific interactions between all species reveals that the juvenile animals of both primate species have distinct more interspecific contacts than the adult animals. In most cases the interactions are positive or playful contacts. Especially between a 12-year-old female orang-utan and one of the juvenile langurs friendly interactions with body contact were monitored often. Between the adult animals agonistic contacts or submission are most common. The Asian small-clawed otters are very interested in the orangutans and often approach towards them to have a sniff or pull their hair.The community of all three species is quite harmonious and serves as behavioural enrichment for all individuals. For the visitors the mixed enclosure is a benefit as well.     

Dynamics of demographically open mutualists: immigration, intraspecific competition, and predation impact goby populations

Although it is now recognized that mutualistic species are common and can have stable populations, the forces controlling their persistence are poorly understood. To better understand the mechanisms that impact the stability of obligate mutualists, I conducted several field experiments within a sandy coral reef lagoon in Moorea, French Polynesia that manipulated densities of fish (gobies) that interact mutualistically with shrimp. Obligate, mutualistic partnerships of gobies and shrimp are common on Indo-Pacific coral reefs and have been shown previously to interact as follows: shrimp construct burrows in which both species reside, and gobies warn shrimp of predators through tactile communication. Augmentation of gobies by up to 100% above ambient densities within 9 m² plots produced no change in overall density of gobies or shrimp because gobies competed intraspecifically for a limited number of shrimp burrows and smaller gobies were outcompeted by larger individuals. I used predators to assess the impact of goby removal on the stability of goby and shrimp populations. First, although surveys taken throughout the lagoon revealed no relationship between goby and predator densities, predators correlated negatively with the proportion of adult gobies and positively with the proportion of small gobies paired with large shrimp. Second, experimental augmentation of predators resulted in a dramatic reduction of adult gobies within predator-addition plots, but had no impact on overall densities as immigrants rapidly replaced the missing adult gobies. Furthermore, goby turnover resulted in an increase in the proportion of small gobies paired with large shrimp because body sizes of gobies and shrimp in a burrow were similar prior to predator introduction, and predators apparently had a greater impact on gobies than shrimp. The mechanisms that prevent expansion (intraspecific competition) and collapse (immigration) of goby-shrimp populations likely contribute to local-scale stability of mutualistic populations in other terrestrial and aquatic environments.     

Seasonal and interannual variation in distribution and population abundance of the shrimp Crangon franciscorum in San Francisco Bay

Shrimp are an important component of the San Francisco Bay biota, both as predators on benthic fauna, and as a food source for predatory fish. Of three common species in the bay, Crangon franciscorum is the most abundant. The bay is predominantly a nursery area for maturing shrimp of this species. During the main reproductive period in the early spring, ovigerous females and planktonic larvae are in most years centered outside the bay in the nearshore ocean, although both are also present in the bay. Juveniles move into both the southern reach and the northern reach shortly after settling, and landward-flowing bottom currents are possibly instrumental in this migration. The seasonal cycle of shrimp abundance in the bay, dominated by this spring immigration of newly settled juveniles, is characterized by a progressive migration of the growing shrimp up the estuary coincident with upstream penetration of higher salinity water during summer. Differences in abundance and distribution between the years 1980, 1981, and 1982 suggest that the level of river discharge and accompanying salinity regime are important controlling factors in the distribution, recruitment levels, and subsequent survival and growth of C. franciscorum in the San Francisco Bay.     

Structure and function of the genital papillae in a tropical sand dollar, arachnoides placenta (L.) with a discussion on the adaptive significance of genital papillae in echinoids

The long genital papillae of the sand dollar, Arachnoides placenta (L.), are functionally adapted to discharge gametes into the water while the animal is buried below the surface of the sand. The adaptation not only increases the possibilities of fertilization and dispersal but also avoids the gametes being trapped in the mucus produced for feeding.It is suggested that echinoids which are mucus-ciliary feeders or which live in soft substrata are more likely to have genital papillae, and that selection for the development of papilla is more intense in males since the semen is more easily carried by ciliary currents than are the eggs; the fact that most echinoids do not have papillae is because they are inhabitants of habitats with swift currents and so there is no selective advantage for the developing of genital papillae.     

Physiological preparedness and performance of Atlantic salmon Salmo salar smolts in relation to behavioural salinity preferences and thresholds

This study investigated the relationships between behavioural responses of Atlantic salmon Salmo salar smolts to saltwater (SW) exposure and physiological characteristics of smolts in laboratory experiments. It concurrently described the behaviour of acoustically tagged smolts with respect to SW and tidal cycles during estuary migration. Salmo salar smolts increased their use of SW relative to fresh water (FW) from April to June in laboratory experiments. Mean preference for SW never exceeded 50% of time in any group. Preference for SW increased throughout the course of smolt development. Maximum continuous time spent in SW was positively related to gill Na⁺, K⁺‐ATPase (NKA) activity and osmoregulatory performance in full‐strength SW (measured as change in gill NKA activity and plasma osmolality). Smolts decreased depth upon reaching areas of the Penobscot Estuary where SW was present, and all fish became more surface oriented during passage from head of tide to the ocean. Acoustically tagged, migrating smolts with low gill NKA activity moved faster in FW reaches of the estuary than those with higher gill NKA activity. There was no difference in movement rate through SW reaches of the estuary based on gill NKA activity. Migrating fish moved with tidal flow during the passage of the lower estuary based on the observed patterns in both vertical and horizontal movements. The results indicate that smolts select low‐salinity water during estuary migration and use tidal currents to minimize energetic investment in seaward migration. Seasonal changes in osmoregulatory ability highlight the importance of the timing of stocking and estuary arrival.