Population structure of two Antarctic ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, in winter sea ice

The spatial distribution and population structure of two dominant ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, were studied during winter at nine locations in east Antarctic fast ice. These species accounted for at least 90% of the total metazoan abundance at each location. Abundances were high, reaching 175 individuals l^-1 (190,000 m^-2) for D. glacialis and 660 l^-1 (901,000 m^-2) for P. antarctica. These abundances were probably partly supported by the high biomass of ice-algae (Pearson correlation coefficient, r=0.75), as indicated by chlorophyll-a concentrations (1.7-10.1 µg l^-1). The population structures of each species suggested very different life-history strategies. All developmental stages of D. glacialis were isolated from the ice cores, including females with egg sacs, supporting the hypothesis that this species reproduces in the sea ice during winter. This strategy might assist D. glacialis in leading a continually colonising existence, whereby it responds opportunistically to the availability of favourable habitat patches. The populations of P. antarctica were composed primarily of nauplii (>99%), consistent with past observations of a synchronised life cycle for this species. The strong coupling of the developmental cycle of P. antarctica to the growth and decay of sea ice suggests that local extinctions might occur in areas where ice break-out is unpredictable.     

Deposit-feeding amphipods (Monoporeia affinis) reduce the recruitment of copepod nauplii from benthic resting eggs in the northern Baltic Sea

We experimentally investigated the effect of different densities of the burrowing, deposit-feeding amphipod Monoporeia affinis on the recruitment of zooplankton from benthic resting eggs. Intact sediment cores with in situ density and species composition of zooplankton resting eggs and benthic fauna were collected in the northern Bothnian Sea, part of the Baltic Sea. We removed as many M. affinis as possible from the cores and then added different numbers of M. affinis to the cores to generate a range of densities. The cores were exposed to different densities of M. affinis for either 3 or 40 days, after which the hatched zooplankton was registered. One subset of the cores were initially incubated under low temperature (2–3 °C, to prevent hatching) for 37 days (the resting phase), to allow for effects of M. affinis on unhatched resting eggs. These cores were then incubated under higher temperature (13 °C) for 3 days (the hatching phase), to induce hatching and allow for effects on hatching or hatched specimens. In a second subset of cores with the same time and temperature schedule, the M. affinis density was experimentally reduced at the start of the hatching phase, to evaluate the effect of M. affinis during the hatching phase. To a third subset of cores, we immediately initiated the hatching phase, without an experimental resting phase, to evaluate the effects induced during the resting phase. The most common zooplankton species that hatched was Eurytemora affinis (Copepoda), followed by Bosmina longispina maritima (Cladocera). In all cores that were subjected to a resting phase, the numbers of hatched E. affinis were log-linearly negatively related to density of M. affinis. An increase of M. affinis density from 1,000 to 5,000 individuals m⁻², normal field densities, reduced the hatching by 60–70%. The negative impact was mainly exerted during the hatching phase, suggesting predation on, burial of or physical injury of hatching nauplii or eggs in a late development stage as likely mechanisms. Also, the number of B. longispina maritima that hatched was reduced by M. affinis during the hatching phase, but no clear relation to density of M. affinis could be identified. The results show that M. affinis can reduce recruitment to zooplankton from benthic resting eggs. Such impact by the benthos on resting stages of zooplankton is therefore a potentially significant link between the benthic and pelagic systems. Received: 10 August 2000 / Accepted: 13 November 2000     

Downward Migration of Dense Bottom Currents

The focus of this paper is on the dynamics of a dense current flowing along the continental slope, and the frictionally induced downward motion it experiences. In particular, the movement of the lateral boundaries where the isopycnals meet the bottom are considered. The current is taken to be wide compared to the Rossby radius, which is in accordance with observations and makes the dynamics quasigeostrophic. The time development of the plume thickness is studied, using three different parameterisations for the bottom friction.Independently of the choice of parameterisation, the following results are obtained: In the central part of the plume friction acts as a diffusive process to minimise the curvature of the upper surface of the plume. At the upper edge the plume quickly approaches a state with small slope, i.e. small geostrophic velocity and small frictionally induced downward flow. At the lower edge a tongue of dense water shoots out creating a widening layer. The thickness of the migrating layer is approximately equal to the boundary layer depth and its downward speed is comparable to the along-slope geostrophic velocity. The downslope end of the migrating layer may form a steep front which requires some precautions in the numerical procedure.     

Feeding in myodocopid ostracods: functional morphology and laboratory observations from videos

High-resolution videos, scanning electron microscopy and histology were used to study the feeding mechanism of myodocopid ostracods from the Pacific Coast of Japan, as exemplified by Vargula hilgendorfii (Müller, 1890) and a few other cypridinid species. Ostracods observed in the laboratory were attracted to a wide spectrum of natural food sources, behaving as predators of living prey (e.g. polychaete annelids), as opportunistic scavengers on dead animals (e.g. annelids, fishes, squid), and also consuming artificial food. Food sources may be detected by chemoreception. The fourth limb (endopodites with strong sclerotized setae) and the furcal lamellae (claws with teeth) act in coordination to abrade and eventually tear open the protective integument of living/dead prey such as annelids. The mandibular palps are used mainly to hold the food. Food sections and soft-body contents are transferred to the mouth by the fourth limb (endopodial “rake”) and fifth limb (exopodite with pectinate setae) and are passed to the oesophagus by the endites (mandibles, fourth and fifth limbs). Food is subsequently pumped up to the stomach by peristaltic contractions of the oesophagus (ring muscles) and stored in the stomach pouch. The upper lip of bioluminescent (V. hilgendorfii) and non-bioluminescent species of Cypridinidae often contact food, suggesting that some of the glands housed in this organ may emit digestive enzymes prior to ingestion. Ostracods are able to ingest massive quantities of food within a few minutes and to survive starvation for several weeks. In V. hilgendorfii, the midgut is a huge sac-like organ with no partition and is lined with a single layer of columnar epithelial cells. No differentiated hepatopancreas is present. The cypridinid produces a single faecal pellet wrapped in a thin reticulated, peritrophic membrane. Myodocopid ostracods exhibit a wide range of feeding strategies (detritus-feeding, comb-feeding, scavenging, predation, ectoparasitism) in both benthic and pelagic niches, and constitute a substantial source of food for many zooplankters. Adaptation of cypridinids to scavenging/predation is reflected in the morphology of their furcae, mandibles, fourth and fifth limbs, and their digestive system. Palaeontological data suggest that early Triassic cypridinids and possible late Ordovician myodocopids may have been carnivorous scavengers feeding on carcasses of cephalopods (ammonoids or orthoconic nautiloids), thus playing the same role of “recyclers” as modern representatives of the group. Received: 2 January 1998 / Accepted: 23 July 1998     

A correction to: biological and oceanographic insights from larval labrid (Pisces: Labridae) identification using mtDNA sequences

Previously published mtDNA cytochrome b sequences for Xyrichtys novacula (Linnaeus) and two morphologically distinct types of Xyrichtys larvae (Pisces: Labridae) included human cytochrome b sequence, presumably due to sample contamination and/or poor preservation of sample DNA. Those sequences had been used to identify the two types of Xyrichtys larvae as X. novacula, but owing to the contamination, the identifications are invalid. Fresh specimens were collected: X.␣martinicensis (Cuvier and Valenciennes), X. novacula, X. splendens (Castelnau), and Xyrichtys sp. larvae of each of the two morphotypes (dorsal-forward eye and ventral-forward eye). The cytochrome b fragment was amplified from each specimen using the polymerase chain reaction. Comparison of the sequences with human cytochrome b sequence confirmed that the new sequences were not contaminated. The interspecific differences in Xyrichtys sequences were less than previously reported, but still greater than observed in many fish genera. Distance and parsimony analyses indicated that X. novacula and X. martinicensis were more closely related to each other than to X. splendens. This conclusion differs from our previous conclusion that X. martinicensis and X. splendens were more closely related to each other than to X. novacula. Distance and parsimony analyses also demonstrated that both larval morphotypes were X. novacula and, thus, our previous conclusions regarding larval ecology and oceanographic transport remain unchanged. Received: 12 March 1997 / Accepted: 24 October 1997     

Short retention times of stomach temperature loggers in free-living seabirds: is there hope in the spring?

Stomach temperature loggers have proved useful for the study of feeding activity in free-living seabirds, but their usage has been restricted because they are frequently dislodged and lost during the seabirds' regular regurgitation of indigestible prey remains. In the present study we examine the incidence of spontaneous regurgitation (pellet production) in free-living seabirds, consider the effect this has on the likely retrieval of stomach temperature loggers and present a structural modification of the logger housings which leads to a much lower incidence of regurgitation. Systems were tested on albatrosses, cormorants, gannets and penguins. Received: 15 July 1997 / Accepted: 31 October 1997     

Uptake of arsenate, trimethylarsine oxide, and arsenobetaine by the shrimp Crangon crangon

Common shrimp, Crangon crangon (L.), were exposed to inorganic arsenic (arsenate), trimethylarsine oxide, or arsenobetaine in sea water (100 μg As l⁻¹) or in food (1 mg As g⁻¹ wet wt) for up to 24 d, followed by 16 d depuration in clean sea water with undosed food, in order to determine the efficiency of uptake and retention of the compounds. Accumulation of arsenic in the tail muscle, gills, midgut gland, exoskeleton, and remaining tissues was found to depend on the chemical form of the arsenic and the route of exposure. No arsenic was accumulated by C. crangon exposed to arsenate or trimethylarsine oxide in sea water. Shrimps exposed to waterborne arsenobetaine initially accumulated a small amount of arsenic in their tail muscle and gills. After 16 d, C. crangon fed arsenate, trimethylarsine oxide, or arsenobetaine had accumulated arsenic in their tail muscle to levels ∼2-, 2-, or 40-times, respectively, that of the control group. A roughly linear rate of accumulation was shown by shrimps fed trimethylarsine oxide or arsenobetaine, but C. crangon fed arsenate accumulated arsenic for 16 d, then lost arsenic such that their concentration on Day 24 was not significantly different from that of the control group. Patterns of arsenic accumulation in the gills of shrimps fed the compounds were similar to those seen in the tail muscle. On a whole animal basis, C. crangon retained ∼1.2% of the arsenate, 1.6% of the trimethylarsine oxide, and 42% of the arsenobetaine consumed over the first 16 d of exposure, with roughly half present in the tail muscle in each case. Data obtained support the view that the direct uptake of arsenobetaine from sea water does not make a significant contribution to the relatively high concentrations of this compound in marine crustaceans, and that food is the primary source. Naturally occurring arsenic compounds in C. crangon and possible transformations of the administered arsenic compounds were examined by high performance liquid chromatography using an inductively coupled plasma mass spectrometer as the arsenic-specific detector. Control C. crangon contained arsenobetaine as the major arsenic compound (>95% of total arsenic); tetramethylarsonium ion (0.7%) and an unknown arsenic compound (1.7%) were also present as minor constituents. Shrimp ingesting arsenobetaine accumulated it unchanged. Shrimp ingesting arsenate did not form methylated arsenic compounds; they appeared to contain their accumulated arsenic as unchanged arsenate only, although the possibility that some of the arsenic was reduced to arsenite could not be excluded. C. crangon ingesting trimethylarsine oxide biotransformed the compound predominantly to dimethylarsinate. Received: 9 October 1997 / Accepted: 11 February 1998     

Vulnerability of the copepod Acartia tonsa to predation by the scyphomedusa Chrysaora quinquecirrha : effect of prey size and behavior

Although scyphomedusae have received increased attention in recent years as important predators in coastal and estuarine environments, the factors affecting zooplankton prey vulnerability to these jellyfish remain poorly understood. Current models predicting feeding patterns of cruising entangling predators, such as Chrysaora quinquecirrha (Desor, 1948), fail to account for the selection of fast-escaping prey such as copepods. Nevertheless, our analysis of gastric contents of field-collected medusae showed that this scyphomedusa fed selectively on the calanoid copepod Acartia tonsa (Dana, 1846) and preferentially ingested adult over copepodite stages. We measured feeding rates in a planktonkreisel while simultaneously videotaping predator–prey interactions. C. quinquecirrha consumed adult A. tonsa ten times faster than copepodites. Differences in prey behavior, in the form of predator–prey encounter rates or post-encounter escape responses, could not account for the elevated feeding rates on adults. Prey size, however, had a dramatic impact on the vulnerability of copepods. In experiments using heat-killed prey, feeding rates on adults (1.5 times longer than copepodites) were 11 times higher than on copepodites. In comparison, medusae ingested heat-killed prey at only two to three times the rate of live prey. These results suggest that during scyphomedusan–copepod interactions, prey escape ability is important, but ultimately small size is a more effective refuge from predation. Received: 26 September 1997 / Accepted: 20 May 1998     

Seasonality of polypide recycling and sexual reproduction in some erect Antarctic bryozoans

The seasonality of polypide cycling has been investigated for three species of erect bryozoans from Antarctica: Isoseculiflustra rubefacta (Kluge, 1914), Nematoflustra flagellata (Waters, 1904) and Himantozoum antarcticum (Calvet, 1905). Approximately ten colonies of each species were collected monthly by SCUBA divers over a 14 mo period during 1992/1993, and the status of each individual zooid was classified as differentiating/regenerating, active (feeding autozooids), degenerate (brown body) or sexually reproductive (ovicells present, or zooid containing a larva). Polypide cycling in all three species was distinctly seasonal. New zooids formed at the growth margin and typically contained actively feeding polypides for ≃9 mo before these polypides degenerated into brown bodies in the austral winter (June). Very few polypides were active in the period from June to August, when water-column food levels were at their lowest; after this period new polypides differentiated. Individual zooids typically underwent a total of five (I. rubefacta and N. flagellata), or at least four (H. antarcticum) complete polypide cycles before becoming senescent. Polypide lifetimes generally became shorter as the age of the zooid increased. Sexual reproduction was also distinctly seasonal in these species, with bands of ovicells or sexually reproductive zooids being formed each year in late summer once a given colony had grown to a threshold size (or age). Larvae were then brooded for ≃10 mo before being released in January/February (N. flagellata) or February/March (H. antarcticum). The seasonal patterns of polypide cycling are related clearly to the variations in food availability, and these species appear to have the longest zooid lifetime (≃5 yr) and the slowest polypide cycling (once per year with polypide lifetimes up to 10 mo) reported for any bryozoan so far. Received: 7 May 1995 / Accepted: 19 November 1997     

Potential fluorosis problems around Chandidongri, Madhya Pradesh, India

The area around Chandidongri (latitude 21 deg 02 prime N and longitude 80 deg 37 prime E) is covered by granitic gneisses and amphibolites of the Amgaon Group and granites and rhyolites of the Dongargarh super group. The granites which cover the major portion of the area have been intruded into by a number of dolerite and epidiorite dykes and traversed by a few N–S trending shear zones. A well-known fluorite deposit is located along the Lamtidongri–Kotnapani shear zone. Besides this deposit, the authors have located a few occurrences of fluorite along other shear zones where epidiorite dykes are in association with granites. A number of cases of fluorosis have been reported from those parts of Rajasthan and Andhra Pradesh which are covered by granitic rocks and also from the eastern parts of Bhandara district which are adjacent to the area described here. The Chandidongri area is mainly covered by granitic rocks, which at places show the presence of fluorite mineralisation. Taking into consideration these facts, the authors have investigated the area to study the cases of fluorosis if any, and the impact of geology and fluorite mining on the water quality in this region. During the survey, we have not come across any cases of fluorosis in this region. The pH is more than 7 and the fluoride concentration is within the permissible limit.