Unravelling the Ecological Significance of Endogenous Rhythms in Intertidal Crabs

Organisms living along the shore are exposed to complex sets of environmental oscillations. In addition to solar (24.0 h) and lunar (24.8 h) cycles, local tides may reoccur on a 12.4 h schedule. Beyond daily routines, biweekly, monthly and annual rhythms may each have a significant impact on an animal's activity. For some time, it has been established firmly that intertidal crabs possess several internal biological clocks with distinctly different periods and properties. However, the versatility of these clocks has not been obvious. Crabs living in the littoral zone must adjust their internal chrono-meters to be synchronous with the specific temporal structure of the immediate habitat. Fine adjustments in their clocks will depend upon on a particular tide province and the location of their niche in the intertidal zone. Over a wide geographic range, the location of an intertidal habitat for one species may be in as many as four tidal provinces. Based on wave form and harmonic components, tide provinces are characterized as either a) semidiurnal, b) mixed, mainly semidiurnal, c) mixed mainly diurnal, or d) diurnal. Likewise, the primary frequency associated with an intertidal niche in each tide province may be augmented by diel (24 h) and semilunar (14 day) periods. In addition, supralittoral habitats may be influenced by monthly (28 day) and seasonal rhythms. Since some species live in several tidal provinces and different positions in the littoral zone, locomotor and larval release rhythms of intertidal crabs must naturally be adjusted to the timetable of the local habitat. Flexibility in ambulatory and egg hatching rhythms of crabs are discussed from this environmental perspective. The nature and location of the underlying circadian and tidal oscillators tracking these environmental rhythms are reviewed.     

TIDAL RHYTHMS: THE CLOCK CONTROL OF THE RHYTHMIC PHYSIOLOGY OF MARINE ORGANISMS

1. A great number of vital processes are rhythmic and the rhythms quite often persist in constant conditions. The best-known rhythms are circadian; much less is known about circalunadian rhythms, and this review was prepared in an attempt to rectify this deficiency. All through the article comparisons are drawn between circalunadian and circacian rhythms. 2. Activity rhythms. (a) The activity patterns of 28 intertidal animals are discussed. All describe a periodicity with a basic component of 24.8 hours, and this approximate period persists in the laboratory in constant light and temperature and in the absence of the tides. The duration of persistence ranges from a few cycles to months, and is a function of the species studied, the conditions imposed, and individual tenacity. (b) In those few cases where relatively long-term observations have been made, there is a trend for the period of the rhythm to become circatidal, or better, circalunadian. (c) The ‘desired’ phase relationship between rhythm and tidal cycle is species-specific. Geographical translocation experiments have shown that the phase is set by the local tides. (d) In some cases the amplitude of the persistent rhythm mimics the semidiurnal inequality of the tides. (e) In about a third of the species discussed, a circadian component has been found combined with the tidal component. Many of the other studies were of such short duration that a low-amplitude circadian component would have gone unnoticed. (f) The tidal rhythm is innate. However, the rhythm is (i) sometimes lacking in organisms living in non-tidal habitats, or (ii) fades after a spell of incarceration in constant conditions. Various treatments — some aperiodic — can induce the expression of the missing tidal rhythm. (g) In the green crab, removal of the eyestalks destroys the activity rhythm. 3. Vertical migration rhythms. (a) A rather surprisingly large number of intertidal animals have been found to undergo migration rhythms between the upper layers of the substratum and its surface. The movements are synchronized with the tides in nature, but most species have either been shown to be diurnal in constant conditions, or in cases where adequate testing has not been done, suspected of being so. (b) In only one species has confirming work shown that the fundamental frequency is truly tidal. This finding is especially important as it shows that tidal rhythms need only the single-cell level of organization for expression. Even at this level there appears to be a dictatorial override by a circadian clock. 4. Colour change. Low-amplitude tidal rhythms in colour change — superimposed on a more dominant circadian change — have been reported to be intrinsic in four species and inducible in a fifth. 5. Oxygen consumption. Tidal rhythms in oxygen consumption have been described for seven invertebrates and one alga; six of the species have superimposed solar-day rhythmic components also. 6. Translocation. A total of five geographical translocation experiments, in which the organisms were maintained in constant conditions throughout, have been tried. Unequivocally in one case, and possibly in a second, the test organisms rephased spontaneously to the times commensurate with local tidal conditions. In two other cases, the pretranslocation phase was retained. The fifth experiment has not been reproducible. 7. Determination of phase. (a) The tidal cycle on the home shoreline sets the phase of the inhabitant's rhythms. Even the location of a crab's burrow on the beach incline can play a determining role. (b) Paradoxically, the periodic wetting by inundation is not an important entraining factor for most intertidal organisms. Instead, the effective portions of the tidal cycle include one or more of the following. (i) Mechanical agitation, especially for animals living in an uprush zone where they are periodically subjected to the pounding surf, (ii) Temperature cycles, though they have not yet been systematically investigated, have very pronounced entraining roles in crabs. (iii) Pressure is probably not a generally important entraining agent for most intertidal organisms, but it is so for the green crab. (c) Light-dark cycles in general, whether daily or tidal in length, have no effect on the entrainment or phase setting of many tidal rhythms. There are two exceptions: (i) a 24-hour light-dark cycle is known to keep a tidal locomotor rhythm (one that becomes circalunadian in constant conditions) at a strict tidal frequency. (ii) In rhythms with both daily and tidal components, when the former is shifted by light stimuli, the latter is affected in a nearly identical manner. 8. Temperature. (a) The role of temperature on tidal rhythms is compared with its role on circadian rhythms. (b) The effects of different constant temperatures have so far been studied on only four tidal rhythms. All studies indicate a lack of any permanent change in period, which is not so with most circadian rhythms; the latter having temperature coefficients around 1.1. In two of the studies the rhythms under test temperatures were followed for less than a day, and a third study cannot be repeated. (c) Short exposure to very cold temperature pulses produced a response that may be interpreted as a temporary stoppage of the clock. Exposure to relatively less-cold pulses appear simply to reset the hands of the clock. The same responses have been demonstrated with circadian rhythms. (d) In the case of green crabs, which had become arrhythmic during prolongued captivity in the laboratory, a tidal rhythm could be reinitiated by a single short cold treatment. The cold pulse also set the phase of the rhythm. (e) A few superficial studies employing temperature steps or pulses have produced results which suggest that a phase-change sensitivity rhythm — just like that found associated with circadian rhythms — may underlie tidal rhythms. Certainly a determined search for this rhythm should be made in the near future. 9. Clock control of rhythms. (a) An argument is constructed claiming that tidal rhythms have a basic period of about 24–8 hours rather than the more expected tidal interval of 12.4 hours. In constant conditions, a circalunadian period is usually displayed. (b) After speculating that a frequency-transforming coupler may function between the clock and the overt rhythm, reasons are given that lead to the further speculation that both circadian and circalunadian rhythms could be generated by a single clock, via specific coupling mechanisms. (c) Two current hypotheses concerning the nature of the clockworks are reviewed and discussed. (d) Suggestions are made for future investigations.     

Endogenous rhythms of haemolymph flow and cardiac performance in the crab Cancer magister

Heart rate and arterial haemolymph flow rates were measured in freshly trapped Dungeness crabs, Cancer magister, using a pulsed-Doppler flowmeter. In the laboratory, freshly collected subtidal crabs exhibited endogenous rhythms in both cardiac function and haemolymph flow through one or more arterial systems, of both tidal and diurnal periodicity. The strongest tidal rhythms were recorded in the sternal and paired anterolateral arteries. These endogenous rhythms of selective tissue perfusion are related to an underlying locomotor activity, but may also be involved with hormonal transport or feeding. Changes in both heart rate and stroke volume were responsible for the increases in haemolymph flow rates. These rhythms were not entrained by aerial exposure, since confinement of arhythmic crabs in intertidal cages did not re-entrain an endogenous tidal rhythm. Endogenous locomotory rhythms are known to be controlled by neurohormones released in cycles from the sinus gland on the eyestalk. These hormones may also control the endogenous cardiovascular rhythms, since these were abolished after eyestalk ablation in freshly collected Cancer magister. These results suggest that hormones synthesized and released by the X-organ/sinus gland complex may, together with pericardial hormones, play a role in modulation of crustacean cardiovascular function.     

The intertidal mudflats of Barr Al Hikman,Sultanate of Oman,as feeding,reproduction and nursery grounds for brachyuran crabs

Brachyuran crabs are an important ecological and economical, yet often unstudied aspect of intertidal mudflats of the Arabian Peninsula. Here we provide baseline density estimates of crabs at the relatively pristine intertidal mudflats of Barr Al Hikman (Sultanate of Oman) and provide information on their life cycle and habitat preference. Across the winters of 2012–2015 crabs were sampled on a grid covering the entire intertidal depth gradient. 29 species were found and average densities varied between 12 and 54 crabs/m². Deposit-feeding and herbivorous crabs were the most abundant species across all winters. Size frequency data and the presence of ovigerous females show that most crabs species reproduce in the intertidal area. P. segnis, the most important crab for local fisheries, was found to use the intertidal area as a nursery ground. We analysed the relationships between the two most abundant crab species, Macrophthalmus sulcatus and Thalamita poissonii and the environmental variables: seagrass density, tidal elevation, median grain size and sediment depth using Random Forest models. The predictive capacity of the models and the relative importance of the environmental predictors varied between years, but crab densities in general were positively associated with seagrass density, presumably because seagrass offers feeding habitat.

     

Entrainment of the activity rhythm of the mole crab Emerita talpoida

The mole crab Emerita talpoida migrates with the tide in the swash zone of sand beaches. A circatidal rhythm in vertical swimming underlies movement, in which mature male crabs show peak swimming activity 1-2 h after the time of high tides at the collection site. In addition, there is a secondary rhythm in activity amplitude, in which crabs are maximally active following low amplitude high tides and minimally active following high amplitude high tides. The present study determined the phase response relationship for entrainment of the circatidal rhythm with mechanical agitation and whether the cycle in activity related to tidal amplitude could be entrained by a cycle in the duration of mechanical agitation at the times of consecutive high tides. After entrainment with mechanical agitation on an orbital shaker, activity of individual crabs was monitored in constant conditions with a video system and quantified as the number of ascents from the sand each 0.5 h. Mechanical agitation at the times of high tide, mid-ebb and low tide reset the timing of the circatidal rhythm according to the timing relationship to high tide. However, mechanical agitation during flood tide had no entrainment effect. In addition, a cycle in duration of mechanical agitation entrained the rhythm in activity amplitude associated with tidal amplitude. Both rhythms and entrainment effectiveness over the tidal cycle may function to reduce the likelihood of stranding above the swash zone.     

Go with the flow: tidal migration in marine animals

Many shallow-water organisms are subjected to regular, frequent and considerable changes in their environment imposed by the tides. Such changes range from complex alterations in local conditions, as in the intertidal zone, to simpler variations in current speed and direction offshore. To take advantage of the opportunities these changes provide, many animals undertake regular vertical and/or horizontal migratory movements in synchrony with the tidal cycle. Others use tidal currents selectively for transport to new habitats. Such movements vary in extent from a few mm to several km and may be modulated by the diel cycle of light intensity. This paper reviews the existence of tidal migration in marine animals from various taxa and habitats. Crustacean and fish examples illustrate the various functions of tidal migration, including transport, feeding, predator avoidance and reproduction. These functions are not necessarily mutually exclusive and may change throughout ontogeny. The mechanisms involved in migration are discussed in terms of direct reactions to environmental stimuli and the interaction of these responses with endogenous rhythms of activity and responsiveness.     

Tower construction by the manicure crab <Emphasis Type="Italic">Cleistostoma dilatatum</Emphasis> during dry periods on an intertidal mudflat

Animals living on upper intertidal mudflats experience habitat desiccation during neap tides when water does not flood the habitat. Individuals of the manicure crab Cleistostoma dilatatum construct cone-shaped towers at the entrance of their burrows, in which they remain during neap tides. These towers are the tallest known structures compared to body size built by crabs living on intertidal flats. The frequency of tower construction followed semilunar tidal cycles with most building done prior to neap tides when few crabs were active on the mudflat surface. Bigger crabs tended to make taller and wider towers with a wider pinhole on the top. These towers may regulate the microclimate in burrows.     

Activity rhythms and cyclical changes of microhabitat preferences in the intertidal pipefish Nerophis lumbriciformis (Pisces: Syngnathidae)

As an inhabitant of the intertidal zone, Nerophis lumbriciformis predictably should show a clear rhythmic activity, with its maximum closely correlated with the high tide period, as observed in several other rocky intertidal fish species. We investigated the rhythmic activity patterns of N. lumbriciformis and analysed to what extent specific substratum preferences may be linked to water level and, ultimately, to the particular activity rhythm patterns of the fish. To determine N. lumbriciformis substratum preferences, two different experiments were conducted. The first evaluated substratum preferences from among a group of the four most common substrates found in the sample area. The second experiment evaluated the fish's maintenance of that substratum preference, after alteration of the water level in a simulated ebb tide. From the available substrata, N. lumbriciformis displayed a clear preference for algae until the water level began to decrease. Then, the preference radically shifted from algae to boulders. N. lumbriciformis showed a clear rhythmic activity, affected by both tidal and circadian periodicity, producing asymmetrical activity peaks. Unlike the rhythms of other rocky intertidal fish species, the maximum activity peaks were not centred at the expected high tide period. The adaptive explanation for the apparently peculiar activity peaks appears to be related to the particular substratum preferences of N. lumbriciformis. The detected circatidal period seems, consequently, closely related to small migrations between substrata, whilst the observed increase in diurnal activity, independent of the tidal cycle, may be related to predatory activity, since N. lumbriciformis is a visual feeder. Electronic Publication     

Plasticity in an isopod's clockworks: Shaking shapes form and affects phase and frequency

The stimuli which normally synchronize the endogenous tidal rhythm of the isopodExcirolana chiltoni arise from turbulent waves moving across the beach. A phase-response curve for two-h pulses of similar stimuli has been derived from experiments in which individual isopods were treated with vigorous intermittent shaking in a flask of seawater. This response curve differs qualitatively from all results previously obtained by administering pulse stimuli to ordinary circadian rhythms: it is bimodal per circadian cycle, with two intervals of about 6 h duration, during which phase advance of up to 4 h results from treatment, separated by two other 6 h intervals during which phase delay of up to 3 h is evoked. This kind of responsiveness to entraining stimuli is of clear adaptive value for synchronization of a circadian rhythm to the mixed semi-diurnal tidal regime of the isopods' habitat.In addition to inducing phase shifts, this same treatment can strongly modify the persistent pattern of activity, and such effects also depend upon phase of treatment: when administered shortly before or shortly after onset of activity, shaking tends to increase the amount of activity in subsequent cycles; when administered in antiphase (6 to 18 h after activity onset), it tends to decrease the activity in the dominant activity peak, and to transform a unimodal pattern of activity into a bimodal pattern. Such induction of a persistent secondary peak of activity in a previously unimodal circadian pattern demonstrates a plasticity which has not been previously reported in those circadian rhythms which are synchronized to the day-night cycle.     

Rhythmic 24 h Variation of Core Body Temperature and Locomotor Activity in a Subterranean Rodent (Ctenomys aff. knighti), the Tuco-Tuco

The tuco-tuco Ctenomys aff. knighti is a subterranean rodent which inhabits a semi-arid area in Northwestern Argentina. Although they live in underground burrows where environmental cycles are attenuated, they display robust, 24 h locomotor activity rhythms that are synchronized by light/dark cycles, both in laboratory and field conditions. The underground environment also poses energetic challenges (e.g. high-energy demands of digging, hypoxia, high humidity, low food availability) that have motivated thermoregulation studies in several subterranean rodent species. By using chronobiological protocols, the present work aims to contribute towards these studies by exploring day-night variations of thermoregulatory functions in tuco-tucos, starting with body temperature and its temporal relationship to locomotor activity. Animals showed daily, 24 h body temperature rhythms that persisted even in constant darkness and temperature, synchronizing to a daily light/dark cycle, with highest values occurring during darkness hours. The range of oscillation of body temperature was slightly lower than those reported for similar-sized and dark-active rodents. Most rhythmic parameters, such as period and phase, did not change upon removal of the running wheel. Body temperature and locomotor activity rhythms were robustly associated in time. The former persisted even after removal of the acute effects of intense activity on body temperature by a statistical method. Finally, regression gradients between body temperature and activity were higher in the beginning of the night, suggesting day-night variation in thermal conductance and heat production. Consideration of these day-night variations in thermoregulatory processes is beneficial for further studies on thermoregulation and energetics of subterranean rodents.     

Recruitment of shore crabsCarcinus maenas on tidal flats: Mussel clumps as an important refuge for juveniles

During the late summer and early fall, juvenile shore crabs (Carcinus maenas L.) occurred in high abundances in mussel clumps scattered on tidal flats of the Wadden Sea. Abundances were much lower on bare tidal flats without mussel clumps and decreased substantially from July to November, whereas numbers in mussel clumps remained high. Large crabs left the tidal flats in early fall, whereas juveniles undertook tidal migrations only in the late fall. In March very few shore crabs were found in the intertidal area. The size of juvenile shore crabs living between mussels did not increase significantly during fall. On the bare tidal flats surrounding the mussels, a size increase was observed. Mussel beds and mussel clumps serve as a spatial refuge for the early benthic phases of juvenile shore crabs. Between mussels they can hide effectively from their epibenthic predators. Juvenile shore crabs do not leave the intertidal area and the mussel habitats before their major predators have left the area. Mussel clumps scattered over the tidal flats may be a critical refuge for juvenile shore crabs settling on tidal flats. Intensified efforts in mussel culturing in the European Wadden Sea during recent decades may have caused an increased abundance of mussel clumps on tidal flats, thus enhancing habitat availability for some mussel-clump inhabitants.     

Juvenile Chinese mitten crabs (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>) in the Thames estuary: distribution,movement and possible interactions with the native crab <Emphasis Type="Italic">Carcinus maenas</Emphasis>

The Chinese mitten crab, Eriocheir sinensis is a successful invader. Whilst non-breeding adult E. sinensis have been associated with the destruction of riverbanks, little is known about the ecology of this species in its invaded areas. This is especially true of the juveniles which are a key migratory stage. Intertidal surveys along the Thames estuary indicated an increase in the population since the 1990s. Juvenile E. sinensis were abundant in the sampled upper tidal region of the Thames, refuging under boulders in the intertidal at low tide. Seasonal differences in sampled populations were observed, with a significantly lower abundance of crabs found during winter compared to summer. Mark-recaptures indicated movement in the intertidal occurring during high tide, with an influx of new crabs evident after a single tidal cycle. Endogenous rhythms were also apparent, with peaks in activity occurring corresponding with night-time high tide conditions. E. sinensis juveniles can successfully exclude similar sized native Carcinus maenas from shelters in the laboratory, regardless of which species originally inhabited the shelter. This may have implications for native estuarine Carcinus populations, which can rely on such intertidal shelters for refuges.     

Environmental heterogeneity, spatial segregation of prey, and the utilization of southwest Atlantic mudflats by migratory shorebirds

We studied the relationships between the habitat use of migratory shorebirds and the spatial distributions of the Southwestern Atlantic Fiddler Crab Uca uruguayensis , polychaetes, sediment characteristics and tidal levels in the Río de La Plata estuary, Argentina, where U. uruguayensis is one of the most important intertidal species. Crabs have a well-defined patchy distribution that is segregated spatially from that of polychaetes. Crab density on the surface varied across the tidal cycle, reaching maximum values during low tide. Polychaete density decreased with depth but showed no change through the tidal cycle; however, given that sediment penetrability did change during the same period, their availability to probing shorebirds is expected to change. Habitat use by shorebirds followed the spatial distribution of prey; shorebirds that foraged on polychaetes (White-rumped Sandpiper Calidris fuscicollis , Two-banded Plover Charadrius falklandicus and Hudsonian Godwit Limosa haemastica ) focused their attention on the areas with the highest densities of polychaetes, whereas species that preyed mostly on crabs (Ruddy Turnstone Arenaria interpres , Whimbrel Numenius phaeopus and Grey Plover Pluvialis squatarola ) predominantly used areas with crabs. This segregation occurred particularly during low tide, a period in which polychaetes became fully available. Results show that the spatial and temporal heterogeneity of mudflats in relation to the types and availability of prey has a strong effect on shorebird habitat use.     

Endogenous rhythms and entrainment cues of larval activity in the horseshoe crab Limulus polyphemus

The American horseshoe crab, Limulus polyphemus (Linnaeus), typically inhabits estuaries and coastal areas with pronounced semi-diurnal and diurnal tides that are used to synchronize the timing of spawning, larval hatching, and emergence. Horseshoe crabs spawn in the intertidal zone of sandy beaches and larval emergence occurs when the larvae exit the sediments and enter the plankton. However, L. polyphemus populations also occur in areas that lack significant tidal changes and associated synchronization cues. Endogenous activity rhythms that match predictable environmental cycles may enable larval horseshoe crabs to time swimming activity to prevent stranding on the beach. To determine if L. polyphemus larvae possess a circatidal rhythm in vertical swimming, larvae collected from beach nests and the plankton were placed under constant conditions and their activity monitored for 72 h. Time-series analyses of the activity records revealed a circatidal rhythm with a free-running period of ≈ 12.5 h. Maximum swimming activity consistently occurred during the time of expected falling tides, which may serve to reduce the chance of larvae being stranded on the beach and aid in seaward transport by ebb currents (i.e., ebb-tide transport). To determine if agitation serves as the entrainment cue, larvae were shaken on a 12.4 h cycle to simulate conditions during high tide in areas with semi-diurnal tides. When placed under constant conditions, larval swimming increased near the expected times of agitation. Thus, endogenous rhythms of swimming activity of L. polyphemus larvae in both tidal and nontidal systems may help synchronize swimming activity with periods of high water and inundation.     

In situ monitoring of heart rates in shore crabs Carcinus maenas in two tidal estuaries: effects of physico-chemical parameters on tidal and diel rhythms

Heart rates were monitored in situ in the shore crab, Carcinus maenas, in relation to variations in depth, salinity, oxygen tension, temperature, light intensity and pH. Experiments were performed in the Looe Estuary, Cornwall, England and in Batson Creek in the Salcombe-Kingsbridge Estuary, Devon, England. Experiments in the Looe Estuary were conducted in the vicinity of a storm water storage discharge whereas the experiments in Batson Creek were performed on a clean site. Tidal rhythms in heart rates were commonly detected but diel rhythms in heart rate were also observed frequently. Both types of rhythm were more evident in animals from Batson Creek than from Looe. In Batson Creek, 12 out of 15 crabs expressed tidal rhythms in heart rate, whereas 6 out of 15 crabs expressed diel rhythms. In the two studies in the Looe Estuary, 6 out of 15 crabs and 3 out of 15 crabs expressed tidal and diel rhythm in heart rate, respectively. At both experimental sites, heart rates were positively correlated with increasing changes in depth and salinity, whereas heart rates were negatively correlated with light intensity. In addition, heart rates appeared to be positively correlated with increasing oxygen tension in the experiments performed in the Looe Estuary. The study suggests that depth and oxygen availability are more important to in situ heart rates in shore crabs within tidal estuaries than are salinity, light intensity and pH. Also, sewage discharge appears to cause an acute increase in heart rate, which may affect expression of biological rhythms in shore crabs.     

Quantitative tests of a dual circalunidian clock model for tidal rhythmicity in the sand beach isopod Cirolana cookii

In constant conditions (constant darkness [DD], 20°C), the sand beach isopod Cirolana cookii exhibits spontaneous rhythmic swimming activity with an average free-running period of 12.5h. The rhythms are seen as temporal adaptations to a complex intertidal environment. These results support a dual circalunidian clock model for tidal rhythms in which two components of the rhythm have characteristic periods and active phase lengths and are hypothesized to be controlled by separate circalunidian clocks. A quantitative model successfully simulates many of the properties of endogenous swimming rhythms of C. cookii, including free-running behavior, entrainment, and phase-response curves (PRCs). (Chronobiology International 17 (1), 29-41, 2000)     

Circalunidian clocks control tidal rhythms of locomotion in the American horseshoe crab,Limulus polyphemus

While many intertidal animals exhibit circatidal rhythms, the nature of the underlying endogenous clocks that control these rhythms has been controversial. In this study American horseshoe crabs, Limulus polyphemus, were used to test the circalunidian hypothesis by exposing them to four different tidal regimes. Overall, the results obtained support the circalunidian hypothesis: each of the twice-daily rhythms of activity appears to be controlled by a separate clock, each with an endogenous period of approximately 24.8 h. First, spontaneous “skipping” of one of the daily bouts was observed under several different conditions. Second, the presence of two bouts of activity/day, with different periods, was observed. Lastly, we were able to separately synchronize bouts of activity to two artificial tidal regimes with different periods. These results, taken together, argue in favor of two separate circalunidian clocks in Limulus, each of which controls one of the two bouts of their daily tidal activity rhythms.     

基于MAXENT模型评估北部湾潮间带中国鲎和圆尾鲎稚鲎的潜在地理分布及种群保育对策

鲎具有极高的经济价值和科研意义。近年来由于过度捕捞和栖息地受损等原因,亚洲鲎种群数量正急剧下降。鲎漫长的生命周期使得鲎资源的保护和增殖迫在眉睫。生态位模型已经广泛应用于物种的潜在地理分布预测。基于实地调研数据和公开发表的北部湾中国海域中国鲎和圆尾鲎地理分布数据,运用MAXENT模型得到中国鲎和圆尾鲎在广西北部湾(中国部分)的栖息地适宜度指数(Habitat suitability index, HSI),确定了这两种稚鲎在北部湾中国海域潜在适生区。模型分析结果表明,潮间带坡度和地形指数是影响中国鲎分布的主要环境因子,而潮间带底质的有机物含量和植被指数是影响圆尾鲎分布的主要环境因子,根据研究结果建议在两种稚鲎适生区建立保护区,进行人工放流稚鲎,加强对海草和红树林的生态建设,进而促进鲎资源种群恢复和发展。     

Entrainment of juvenile horseshoe crab activity to artificial tides

Juvenile American horseshoe crabs, Limulus polyphemus, express both daily and tidal rhythms. To determine if, and how, tidal cues influence the expression of these rhythms, we exposed 25 animals to artificial tides, and 17 to artificial tides with inundation, both with a 12:12 LD cycle. In the first experiment, 24% expressed daily rhythms of activity, 24% tidal rhythms, 12% a combination of the two, and the rest were arrhythmic. Under subsequent atidal conditions some expressed daily rhythms, but more were circatidal. In the second experiment, 6% expressed daily rhythms, 71% tidal, 12% a combination, and 12% were arrhythmic. Those expressing tidal rhythms were more active during flood/high tide, while daily animals tended to be nocturnal. Under subsequent constant conditions, the majority exhibited circatidal activity, with some expressing one activity bout per day. We conclude that juvenile horseshoe crabs entrain to artificial tides, with inundation cycles providing stronger cues than water depth changes.     

Shell utilization,predation pressure,and thermal stress in panamanian hermit crabs: An interoceanic comparison

Shell preference patterns of two common hermit crabs from hard bottom reef flats on the Caribbean coast of Panama are examined in relation to the predation pressures and physical stresses of their habitat. Clibanarius antillensis Stimpson lives in the high intertidal habitat and minimizes exposure to predators by seeking refuge during high tides. It prefers high-spired shells which maximize protection from thermal stress. Calcinus tibicen Herbst avoids tidal emersion and prefers low-spired shells which enhance resistance to the predators common on Caribbean reef flats.The results are compared with similar results from the tropical eastern Pacific Bay of Panama. Shell-crushing predation on Caribbean hermit crabs is suggested to differ quantitatively and qualitatively from predation on hermit crabs in the Bay of Panama. Predation on hermit crabs in the Bay of Panama is more intense and effects larger individuals than predation on Caribbean reef flat hermit crabs. In addition, shell-crushing predation on hermit crabs in the Bay of Panama is primarily from teleost fish predators (Diodon spp.), while predation on Caribbean hermit crabs is primarily by bottom-dwelling crustaceans.Differences in predation pressures and tidal regimes between the Caribbean and Pacific coasts of Panama are reflected in the shell preferences and behavior of hermit crabs from the two areas.