Adaptation for accuracy or for precision? Diel emergence timing of the intertidal insect Pontomyia oceana (Chironomidae)

Synchronization of reproduction is obviously under strong selection, but it is not always clear whether the selection is for accuracy of timing, that is, minimal deviation from the environmental optimum, or alternatively, for precision of timing, that is, least extent of spread from the population mean. The former is usually adaptive for exploiting certain environmental conditions, whereas the fitness gain of the latter is usually associated with co-occurrence with conspecifics. We studied the intertidal midge Pontomyia oceana, which has a life cycle of about 30 or 45 days culminating in a highly synchronous swarming of the adult stage that lasts only about 1–2 h. We found that the external proximate factors, controlling their diel swarming, comprise two cues, that is, sunrise and sunset. These two cues, however, are not in existence to improve the accuracy of timing, as their diel swarming times differ widely, with respect to light condition or to tidal condition, in different evenings. Rather, the function is to improve the precision of timing. During the metabolic processes in preparation for emergence, waiting for the cues reduced variation in the population. The extent of dispersion in emergence (swarming) time in an evening is much smaller, when two cues are used compared with the inferred one-cue situations. The short adult life of the marine midges must have rendered mate-finding a stringent selective force despite their apparent high densities. The same principle, that is, using multiple cues to improve precision of timing, may be applicable to many long-lived, free-spawning species, for example, corals, where gametes can fertilize effectively only within a short time after release.     

Diel,lunar and seasonal rhythms in the reproduction of two tropical damselfishes: Pomacentrus flavicauda and P. wardi

It has been hypothesized that site-attached coral reef fishes breed during periods when their offspring will be rapidly flushed from the reef environment, but within the season when these propagules are least likely to be forever lost from the local populations. Daily, monthly and annual rhythms were observed in the reproduction between November 1978 and February 1980 of two tropical damselfishes, Pomacentrus flavicauda and P. wardi, from the southern Great Barrier Reef, Australia. These data support the first part of the hypothesis, but not the explanation for seasonality. When breeding, females of both species spawned demersal eggs into the care of males during a brief period at first light. After several days, larvae hatched from these eggs during another brief period after sunset. An experiment confirmed that the change from light to dark triggered the hatchings and that the emerging larvae were photopositive. Eggs of P. flavicauda usually hatched on the evening of their fourth day in the nest; eggs of P. wardi hatched on their fifth day. A small proportion of eggs required longer incubation. Hatching success was 90%. When breeding, both species spawned in bi-weekly cycles synchronised with the phases of the moon. The largest clutches, as well as the most clutches, were laid at the peaks of these cycles. In the case of P. flavicauda, maximum hatchings occurred on days during which spring high tides fell near sunset (the cue for hatching to begin). It is likely that these hatchlings were rapidly flushed off the reef by the ebbing tides. The hatching cycles of P. wardi consistently peaked 4 d earlier in the lunar month. Consequently, hatchlings of this species remained in shallow water at least one tidal cycle longer than those of P. flavicauda. This difference may be a consequence of the different habitat specialisations of the two species. Both species bred seasonally during the warmest, calmest months of the year. This is not the period of least current flow on the Great Barrier Reef. The observed seasonality may reflect the best times for larval survival in the open sea rather than an adaptation that limits the loss of offspring from the parent populations.     

Diel vertical migration of the marine copepod<Emphasis Type="Italic"> Calanopia americana</Emphasis>. I. Twilight DVM and its relationship to the diel light cycle

Marine copepods commonly exhibit vertical movements in the water column over the diel cycle, termed diel vertical migration (DVM), with the most common pattern being an ascent in the water column to minimum depth around sunset and descent to maximum depth around sunrise. The present study characterized the DVM pattern of the pontellid copepod Calanopia americana Dahl in the Newport River estuary (North Carolina, USA, in July 2003). The estuary is shallow and well-mixed, and the study site (34°43N; 76°40W), 1.5 km inside the estuary entrance, is unusual in lying within a gyre where tidal currents are always in the seaward direction. Changes in C. americana vertical abundance were related to spectrally relevant changes in light throughout the diel cycle. Simultaneous measurements of light and zooplankton abundance near the surface (0.5 m depth) and near the bottom (0.5 m above bottom) were made over one 4-h period and two 3-day periods during different phases of the tide. These observations suggest that C. americana undertook twilight DVM in the Newport River estuary; an ascent to the surface occurred at sunset, followed by a descent to near the bottom around midnight, with a second ascent to the surface and then descent to near bottom at sunrise. DVM in C. americana was independent of the tidal cycle, with the initial ascent in the water column at sunset possibly associated with relative rates of irradiance change. Copepod vertical movements were consistent with a night-active endogenous rhythm, and appeared independent of the abundance of predatory chaetognaths, Sagitta spp. In DVM studies with migrators like C. americana that are broadly sensitive to visible wavelengths of light, measuring photosynthetically active radiation may be a reasonable alternative to measuring light in a spectrally relevant photometric unit.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00227-005-1569-x.Communicated by J.P. Grassle, New Brunswick     

Recruitment,growth, and diel vertical migration of Euphausia pacifica in a temperate fjord

The pelagic crustacean Euphausia pacifica Hansen was sampled with a multiple-sample 1.0 m² Tucker trawl and a multiple-sample 1.0 m² vertical net in Dabob Bay, Washington on 17 dates between May 1985 and October 1987. Size (stage) structure and abundance of the population were determined for each date, while vertical distribution and diel migration were determined for 13 dates. Although internannual variability in both timing and magnitude of events occurred, consistent patterns were discernable. The population produced a large pulse of larvae (2 to 5 mm) in late spring of each year, apparently in response to the vernal phytoplankton bloom. Much lower abundances of larvae occurred during summer and autumn of each year, and larvae were completely absent during winter. Recruitment to the juvenile (6 to 9 mm) and adult (10 mm) stages was strongest during the summer, with abundances of these individuals peaking in summer and autumn. Individual growth rates, determined by modal progression analysis, were calculated for E. pacifica. Rates ranged from zero for some adult cohorts during the winter to 0.12 mm d^-1 for larvae during spring. The latter are among the highest ever reported for this species in the field. The vertical distributions and diel vertical migrations (DVM) of E. pacifica varied seasonally and between size (stage) classes. At night, all size classes were distributed in the surface layer (upper 25 m) irrespective of season or year. During the day, the larger/older stages were always distributed at middepths (50 to 125 m). In contrast, the daytime distribution of the larvae was more variable, being concentrated at the surface during spring and early summer of 1985, and at increasing depths later in the summer and autumn of 1985 and again in spring of 1986. This resulted in invariant DVM in the juveniles and adults, but variable DVM in the larvae, the latter of which is hypothesized to be a response to variable abundances of zooplanktivorous fish.     

Diel and seasonal changes in the distribution of fish on a southeast Brazil sandy beach

The effect of diel and seasonal changes in the distribution of fishes on a subtropical sandy beach on the southeastern coast of Brazil were studied. Seine netting was carried out on both seasonal and diel scales between July 1998 and June 1999. A total of 46 fish species was recorded, six being numerically dominant: Anchoa tricolor, Gerres aprion, Harengula clupeola, Atherinella brasiliensis, Mugil liza and Diapterus rhombeus. Seasonal changes in abundance of dominant species were detected. Species dominant in winter were Anchoa tricolor, H. clupeola and Atherinella brasiliensis; in spring, Anchoa tricolor and G. aprion; in summer G. aprion and D. rhombeus; and in autumn M. liza and H. clupeola. Overall, diel patterns did not reveal any significant trends; however, if we consider each season separately, an increase in A. tricolor abundance was recorded during the day in winter/spring, being replaced at night by H. clupeola in winter, and by G. aprion in spring. Increases in number of individuals and biomass at sunset, and decreases during the night were recorded. The winter/spring inshore/offshore movements at diel scale performed by the three most abundant species demonstrate that diel fluctuation acts more at a species-specific level than at a structural one; in summer there was no evidence of diel movements due to a heavy influx of G. aprion and D. rhombeus, which use the area throughout day and night, increasing overall abundance. Seasonal movements seems to be related to ontogenetic change in species, while diel movements in the fish assemblage seem to be more related to physiological requirements, such feeding activity of each particular species, than to physico-chemical conditions.Communicated by O. Kinne, Oldendorf/Luhe     

Rhythms in larval release of the sublittoral crab Neopanope sayi and the supralittoral crab Sesarma cinereum (Decapoda: Brachyura)

Ovigerous females of the subtidal xanthid crab Neopanope sayi (Smith) and the high intertidal grapsid crab Sesarma cinereum (Bosc) were collected during the summers of 1986 and 1987 in the Beaufort, North Carolina (USA), area and brought into the laboratory, where rhythms in larval release were monitored. When crabs with late-stage embryos were put under a 14 h light:10 h dark cycle in an otherwise constant-environment room, an apparent tidal rhythm in release of larvae was observed for both species, with N. sayi releasing near the time of day and night high tides, and S. cinereum releasing around the time of night high tides. The time of sunset relative to high tide was a complicating factor, since larval release for both species was often concentrated around sunset when evening high tides fell several hours before sunset. When a group of N. sayi and S. cinereum were brought into the laboratory and placed under constant lowlevel light for 5 d, the release rhythm of the population persisted, thus implying that the rhythm is endogenous. Larval release near the time of high tide and often at night is common among brachyurans living in tidal areas, regardless of specific adult habitat, suggesting a common functional advantage. Possibilities include transport of larvae from areas where predation and the likelihood of stranding and exposure to low-salinity waters are high, as well as a reduced probability of predation on adult females. Results of the present study suggest that the importance of release after darkness may increase with increasing tidal height of the adult.     

Variability in abundances of fishes associated with seagrass habitats in relation to diets of predatory fishes

The spatial, diel and tidal variability in the abundance of piscivorous fishes and their teleost prey, and the dietary composition of predatory fishes were investigated in beds of Heterozostera tasmanica within Port Phillip Bay, Australia, from September 1997 to February 1998. Predatory and prey fish assemblages were sampled from beds of H. tasmanica at three locations during each combination of diel (day and night) and tidal (high and low) cycles. Pelagic and benthic crustaceans represented >60% by abundance of the diets of all predatory fishes. Seven species, 54% of all predatory fishes, were piscivorous. These piscivores consumed individuals from seven families, 36.8% of the fish families being associated with seagrass. Western Australian salmon, Arripis truttacea (Arripidae) (n = 174) and yank flathead, Platycephalus speculator (Platycephalidae) (n = 46) were the most abundant piscivores. A. truttacea consumed larval/post-larval atherinids, gobiids and sillaginids. P. speculator consumed late-juvenile/adult atherinids, clinids and gobiids. While the abundances of piscivores varied between locations (P < 0.001) and diel periods (P = 0.028), the relative differences in piscivore abundance between sites and diel periods were not consistent between tides. The abundances of A. truttacea varied in a complex way amongst sites, diel period and tidal cycle, as shown by a three-way interaction between these factors (P = 0.026). Only during diurnal periods at St. Leonards was the abundance of A. truttacea significantly higher during high than low tides (P < 0.001). During the other diel periods at each site, the abundance of A. truttacea did not vary. P. speculator was significantly more abundant nocturnally (P = 0.017). The abundance of small (prey) fishes varied significantly amongst sites (P < 0.001). During the day, the abundance of small fishes did not vary between high and low tides (P = 0.185), but their nocturnal abundance was greater during low tide (P < 0.001). Atherinids (n = 1732) and sillaginids (n = 1623) were the most abundant families of small fishes. Atherinids were significantly more abundant nocturnally (P = 0.005) and during low tides (P = 0.029), and varied significantly amongst sites (P < 0.001). Sillaginids varied significantly only amongst sites (P < 0.001). Seagrass beds provide a foraging habitat for a diverse assemblage of predatory fishes, many of which are piscivorous. Anti-predator behaviour and amongst-location variability in abundances of piscivorous fishes may explain some of the diel and tidal, and broad-scale spatial patterns in small-fish abundances. Received: 23 July 1999 / Accepted: 18 January 2000     

Timing of eclosion in marine insects on Mediterranean shores – studies on Clunio mediterraneus, C. ponticus and Thalassomyia frauenfeldi (Diptera: Chironomidae)

In three Clunio stocks, two from the Mediterranean Sea (Banyuls, Rovinj) and one from the Black Sea (Sosopol), an identical circasemilunar eclosion pattern in combination with a circadian eclosion time of day was evoked in breeding experiments under an artificial moonlight cycle (four nights of 0.3 lux every 30 d, 12 h light:12 h dark). The eclosion peaks occurred on days with artificial moonlight and approximately 2 weeks later, and with regard to the time of day between mid-night and lights-on. In spite of the weak tidal amplitudes on Mediterranean shores, extreme low water levels occur when a nocturnal low-water time coincides on days around full and new moon (spring-tide situation) with winds from land to sea. Field observations at Banyuls and Rovinj agree with the experimental results. In contrast to Clunio spp. adults (life span only a few hours), Thalassomyia frauenfeldi adults (life span up to several days in the laboratory) demonstrated an unsynchronized eclosion pattern in the experiments (no lunar concentration, wide diurnal eclosion gate). Although inhabiting the same intertidal area, T. frauenfeldi walking on exposed intertidal substrates can delay egg-deposition for days and can probably wait for favorable low water levels. The taxonomical status of European Clunio populations is reviewed; minute morphological distinctions are presented for C. mediterraneus in the otherwise morphologically uniform, but ecophysiologically differentiated genus. Received: 15 May 1997 / Accepted: 16 June 1997     

Seasonal stresses shift optimal intertidal algal habitats

We studied how the growth, reproduction, and survival of a common intertidal rockweed (Fucus distichus) varied across its tidal elevation at 14 sites around San Juan Island, Washington, USA in spring–summer and fall-winter seasons. We also measured a suite of environmental factors including temperature, light, emersion time, slope, fetch, and herbivory. To interpret the response of Fucus we included measurements of phlorotannins and carbon storage compounds (mannitol, laminarin). Growth and reproduction exhibited parallel patterns across tidal zones and sites. Tidal zone was a significant source of variation for many Fucus response variables, whereas variation between sites was high but not generally a significant factor explaining Fucus growth and physiology. Unexpectedly, the tidal zone in which Fucus achieved its highest growth and reproduction switched between seasons. High zone thalli grew and reproduced better than Mid zone thalli in fall but not in spring. This result can be explained by different combinations of factors influencing Fucus in each season. In spring, longer emersion times due to daytime low tides resulted in lower growth rates higher on the shore, likely due to carbon limitation. In fall during nighttime low tides, emersion and carbon limitation stresses were minimal. Overall, fall growth was lower than spring growth, but low fall light was not responsible. Instead, warmer average fall temperatures in the High zone apparently favored growth and reproduction relative to the Mid zone. In contrast, Mid zone thalli were subjected to more intense herbivory and hydrodynamic stress associated with wave exposure and steep substrata during the fall. At least for some seaweeds, living in the presumably more stressful high zone can actually confer higher integrated performance.     

Circatidal swimming activity rhythm in a subtidal cumacean Dimorphostylis asiatica (Crustacea)

Dimorphostylis asiatica, a cumacean crustacea inhabiting the sublittoral line of the Seto Inland Sea, shows a rhythmic pattern of swimming activity coinciding with daily and tidal components in the field. This activity pattern was bimodal at first, i.e., circatidal activity coinciding with high tides at their habitat. The mean free-running period (i.e., bitidal interval) of this endogenous rhythm was 23.1 h at 10°C, which was significantly shorter than the environmental tidal cycle. In most cases (90% of the records) this bimodal activity became unimodal within 10 d. The unimodal period was 24 to 27.5 h, which is markedly longer than the preceding bimodal period. The bimodal pattern observed in the present study was not observed in the field. The difference between field and laboratory activity patterns can be explained in terms of direct response of the bimodal circatidal rhythm to the day-night cycle in the field. Field observations were made and laboratory experiments conducted between 1988 and 1992.     

Contrasting diel patterns in vertical movement and locomotor activity of whale sharks at Ningaloo Reef

Activity patterns of animals often relate to environmental variables such as food availability and predation pressure. Technological advances are providing us with new tools to monitor and better understand these activity patterns. We used animal-attached data loggers recording acceleration and depth to compare activity patterns and vertical habitat use of whale sharks (Rhincodon typus) at Ningaloo Reef, Western Australia. Whale sharks showed a moderate reverse diel vertical migration but exhibited a clear crepuscular pattern in locomotory activity. Peak activity occurred at sunset, whereas vertical movement peaked prior to this. Typical ram surface filter feeding could be identified and occurred primarily during sunset and the first hours of night. At such times, direct observations indicated whale sharks were feeding on tropical krill swarms. Kinematic analysis of postural data and data from vertical movement suggests that whale sharks at Ningaloo spend ~8 min per day actively ram surface filter feeding. Considering the high biomass present in krill schools, it is estimated that whale sharks at Ningaloo have a similar energy intake as those at other aggregation sites. Diel patterns in activity and diving behaviour suggest that whale sharks have tuned their diving behaviour in anticipation of the formation of these high-density patches which appear to only be periodically, but predictably available at sunset. Our results confirm that diel patterns in vertical habitat selection and vertical movements do not necessarily reflect patterns in activity and foraging behaviour. Direct quantification of activity and behaviour is required in gaining accurate representation of diel activity patterns.     

Comparative life-history study on sympatric hyperiid amphipods (<Emphasis Type="Italic">Themisto pacifica</Emphasis> and <Emphasis Type="Italic">T</Emphasis>. <Emphasis Type="Italic">japonica</Emphasis>) in the Oyashio region,western North Pacific

Life-history features of the sympatric amphipods Themisto pacifica and T. japonica in the western North Pacific were analyzed based on seasonal field samples collected from July 1996 through July 1998, and data from laboratory rearing experiments. T. pacfica occurred throughout the year, with populations peaking from spring to summer. In contrast, T. japonica were rare from autumn to early winter, but became abundant in late winter to spring. Mature T. pacifica females and juveniles occurred together throughout the year, indicating year-round reproduction. Mature T. japonica females were observed only in spring, and juveniles occurred irregularly in small numbers, suggesting limited, early-spring reproduction in this study area. Size composition analysis of T. pacifica identified a total of eight cohorts over the 2 years of the study. Due to the smaller sample size and rarity of mature females (>9.6 mm) and males (>7.1 mm), cohort analyses of T. japonica were not comparable. Laboratory rearing of specimens at 2°C, 5°C, 8°C and 12°C revealed that a linear equation best expressed body length growth by T. pacifica, while a logistic equation best expressed body length growth by T. japoncia. Combining these laboratory-derived growth patterns with maturity sizes of wild specimens, the minimum and maximum generation times of females at a temperature range of 2–12°C were computed as 32 days (12°C) and 224 days (2°C), respectively, for T. pacifica, and 66 days (12°C) and 358 days (2°C), respectively, for T. japonica. The numbers of eggs or juveniles in females marsupia increased with female body length and ranged from 23 to 64 for T. pacifica and from 152 to 601 for T. japonica. Taking into account the number of mature female instars, lifetime fecundities were estimated as 342 eggs for T. pacifica and 1195 eggs for T. japonica. Possible mechanisms for the coexistence of these two amphipods in the Oyashio region are also discussed.Communicated by O. Kinne, Oldendorf/Luhe     

Seasonal rhythm in a Mediterranean coastal fish community as monitored by a cabled observatory

To adequately describe seasonal rhythms in habitat utilization by fish communities, observations are needed that occur at a relatively high frequency and over large temporal windows. For the first time, images collected from a cabled video-observatory (the western Mediterranean expandable SEAfloor OBservatory; OBSEA) were utilized to examine habitat utilization by coastal fishes. Over the course of a year, hourly digital images of the water column and an artificial reef were obtained during daylight hours. For each image, the total number of species, an estimate of the abundance of individuals, and the Shannon Diversity Index were quantified. A total of 22 fish species commonly associated with nearshore western Mediterranean habitats were identified, and significant spatial (water column vs. artificial reef), seasonal (spring, summer, autumn, and winter), and daily (morning, midday, and sunset) differences in habitat utilization occurred. Four species (Diplodus vulgaris, Diplodus cervinus, Diplodus sargus, and Scorpaena porcus) were associated with the artificial reef, while one species (Pagrus pagrus) chiefly occurred in the water column. Chromis chromis and Diplodus annularis occurred at the site more frequently in the winter and autumn, respectively, while 14 other species utilized the site more frequently in either the spring (8 spp) or summer (6 spp). In addition, Dentex dentex and Spicara maena occurred more frequently at sunset, while D. cervinus displayed a crepuscular rhythm (occurring more frequently in the morning and at sunset). Species diversity was highest in the summer and lowest in the winter. If not taken into account when planning in situ sampling, such seasonal and/or diel differences may lead to spurious estimates of population sizes and biodiversity. We suggest that cabled video-observatories offer a non-invasive and reliable technology for faunistic sampling and population assessment in coastal water of the Mediterranean and likely elsewhere.     

Effects of plant size and distribution on the numbers of invertebrate species and individuals inhabiting the brown alga Pelvetia fastigiata

The invertebrates living on specimens of the brown alga Pelvetia fastigiata, growing in the intertidal zone at La Jolla, California (USA) in November 1975 were enumerated. Within 7 collecting sites, larger plants generally shelter more animal species and individuals than smaller plants. The number of species on a given plant can be described as an equilibrium number; numbers of species and individuals can become similar on defaunated and transplanted algae of comparable sizes placed at the same experimental site. Such equilibria are site-specific because the colonization, immigration and loss rates of animals differ among the sites studied. Relationships between plant size and the number of animal species and individuals on P. fastigiata also differ among the collecting sites. The between-site differences are related to tidal level, to habitat diversity and to habitat patch-size. Small isolated plants without epiphytes shelter few species. The faunas of larger isolated plants at mid-tide levels generally include many thallus-dwelling, tubiculous, vagrant and epiphyte-dwelling species, but few such species commonly inhabit plants within aggregations of P. fastigiata. Within aggregations, the plants host relatively few epiphytes and thus lack habitat diversity, and the net emigration rates of many animals including epiphyte-dwellers are relatively high. Plants within aggregations, however, usually shelter more animal indivuduals than isolated plants. Thus, faunal diversity is reduced, not increased within the largest patches of P. fastigiata.     

Behavioral responses of larvae of the crab Rhithropanopeus harrisii (Brachyura: Xanthidae) during diel vertical migration

Larval Rhithropanopeus harrisii (Gould) show nocturnal vertical migration. Larval behavioral responses to different rates of increase and decrease in light intensity were measured in an apparatus with a natural angular light distribution. A central objective was to establish whether phototaxis actually participates in vertical migration. At sunset the level of light adaptation controlled the readiness of the larvae to migrate, while an ascent was initiated by a preductable relative decrease in intensity (e.g. 4.0x10^-3s^-1). Rates of relative decrease around sunset would evoke continuous upward swimming. Gravity was the orienting cue and there was no change in swimming speed during the ascent. At sunrise, the larval descent was initiated by exposure to an absolute light intensity of about 0.23 log unit above the lower visual threshold. Light served as an orienting cue, as larvae descended by a negative phototaxis. Thus, phototaxis is not a laboratory artifact and does participate in vertical migration. A consideration of behavioral responses of other crustacean zooplankton indicates there is considerable variation in the initiating and directing cues for vertical movements. The variety of behavioral responses of R. harrisii suggests that a synthesis of hypotheses about migration may provide the proper basis for explaining the mechanisms underlying diel vertical migration.     

Comparative study of crustacean larval photoresponses

Ovigerous females of four brachyuran (Cancer gracilis, Lophopanopeus bellus bellus, Hemigrapsus oregonensis and Scyra acutiforns) and two anomuran (Pagurus beringanus and P. granosimanus) species, which live as adults in coastal areas, were collected near Friday Harbor, Washington, USA, in 1985, and spectral sensitivity and phototactic pattern of their larvae were measured. Responses were compared with previous measurements on estuarine species to determine whether responsiveness varies with adult habitat. Estuarine and coastal species have similar photoresponses. Spectral sensitivity of the test brachyran species had two maxima, one near 400 to 420 nm and another around 500 nm. The anomuran species were similar, but had an additional peak in the region of 580 to 620 nm. This sensitivity is adapted to daytime light conditions in the adult environment, and available spectra at the time of larval movement during diel vertical migration. Upon light adaptation and stimulation with a narrow light field, all six species showed positive phototaxis to high light intensities and a pronounced negative response to low intensities. Only the negative response will occur in natural underwater light conditions, and it is part of a predator-avoidance shadow-response which operates in areas of higher light intensity. The same phototactic pattern is observed in all species when darkadapted except H. oregonensis. If nocturnal vertical migration occurs, this negative response may be responsible for the descent at sunrise and depth maintenance during the day.     

Reintroduction of eelgrass (Zostera marina) in the Dutch Wadden Sea; review of research and suggestions for management measures

Eelgrass (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal flats. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Reintroduction of eelgrass ( Zostera marina ) in the Dutch Wadden Sea; Review of research and suggestions for management measures

Eelgras (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal falts. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Activity rhythms of two cirolanid isopods from an exposed microtidal sandy beach in Uruguay

Activity rhythms of two cirolanid isopods, Excirolana armata and Excirolana braziliensis, were studied based on both seasonal field observations and laboratory experiments, at an exposed microtidal sandy beach in Uruguay. The natural emergence patterns of both species were observed in the field for 1 year, twice in each season, and correlated to sea level, expected tidal cycles and diel cycles. Laboratory experiments were carried out in order to detect endogenous rhythms of activity and observe how emergence of both species was affected by changes in light and/or sediment thixotropy. We also compared behavioral strategies of sympatric species that occupy different beach levels. Sea level (and thus swash zone position) during field sampling did not follow expected tidal cycles for most sampling occasions. E. armata was observed in activity most of the time, but activity only correlated with sea level on three out of eight occasions, and only once was correlated to expected tidal cycle. Laboratory results showed that emergence under constant conditions was rare; changes in sediment thixotropy stimulated emergence, but the response was not cyclical; light had little effect on this response. On the other hand, E. braziliensis was fairly scarce in the water column, but swimming individuals were observed always during the night. They displayed an endogenous circadian activity pattern in the laboratory which augmented in response to changes in sediment thixotropy. The natural light/dark cycle modulated both spontaneous and response emergence by increasing day/night differences in activity. In this study E. armata, a midlittoral species more exposed to sea level variations, seemed to rely entirely on different physical and/or biological cues to trigger emergence at the appropriate time. E. braziliensis, found mostly in the upper intertidal zone, emerged in a circadian rhythm, which was stimulated by changes in sediment thixotropy and reinforced by light cycles. The results of this study led us to conclude that on microtidal, unpredictable beaches, local physical and biological factors can combine to determine different activity strategies in organisms from different intertidal levels. Received: 23 March 2000 / Accepted: 30 August 2000