Growth of the scyllarid lobsters Ibacus peronii and I. chacei

The growth rates of the morphologically similar scyllarid lobsters Ibacus peronii (Leach, 1815) and I. chacei (Brown and Holthuis, 1998) are described using data from a tag/recapture study and from tagged lobsters kept in captivity. Within particular size classes, we found no differences in moult increments between male and female I. peronii nor between male and female I. chacei. Small individuals of both species always had larger moult increments than larger individuals. For I. peronii, females moulted more frequently than males, and smaller size classes moulted more frequently than larger size classes. Female I. peronii therefore grew more quickly than males and reached their estimated size at sexual maturity (51 mm carapace length) after ∼2 yr. Moulting of I. peronii was seasonal, with most lobsters (96.3%) moulting between October and January. We found no differences in growth rates of I. peronii at two locations along the east coast of Australia: Coffs Harbour in New South Wales (30°18′S; 153°08′E), and Lakes Entrance in Victoria (37°53′S; 148°00′E). For I. chacei, we found no differences in the frequency of moulting between males and females and, because we also found no differences in the moult increments between males and females, the growth rates of both sexes were the same. Received: 14 August 1999 / Accepted: 20 January 2000     

Heat production in Saduria entomon (Isopoda) from the Gulf of Gdańsk during an experimental exposure to anoxic conditions

The benthic crustacean Saduria entomon occurs frequently in deeper parts of the Gulf of Gdańsk. It is one of a few species able to survive oxygen deficiencies in its natural environment. The anaerobic heat production of S. entomon during 40 h of anoxia was determined. Additionally, the effects of size, sex and activity of the organism on its heat production were investigated. Average heat production of S. entomon was 0.25 ± 0.16 mJ g⁻¹ wet wt s⁻¹ (n=55, avg. length 39 ± 6 mm). The amount of heat produced decreased with increasing body size. The heat production of S. entomon males was greater than that of females ( p<0.05). Specimens kept in chambers with glass beads and water were less active and had lower metabolic rates than those placed in chambers containing water only (more active). During 40 h of anoxia S. entomon gradually decreased its heat production to 5–16% of aerobic level, demonstrating the high adaptation of this species to changeable oxygen conditions in the Baltic Sea. Received: 31 July 1997 / Accepted: 21 January 1998     

Two distinct forms of the chitin-degrading enzyme N-acetyl-β-d-glucosaminidase in the Antarctic krill: specialists in digestion and moult

In the Antarctic krill Euphausia superba two forms of the chitinolytic enzyme N-acetyl-β-d-glucosaminidase (NAGase, EC 3.2.1.52) have been described, previously identified as NAGase B and NAGase C. Here, we demonstrate the organ-specific distribution and physiological relevance of both forms using a polyclonal antibody preparation which allows them to be distinguished immunologically. While NAGase B was localized in the integument and displayed a pattern of activity related to the moult cycle, the activity of NAGase C was independent of the moult cycle and was predominantly found in the gastrointestinal tract. Accordingly, NAGase B played a significant role in chitin degradation during the krill's moult, whereas NAGase C participated in the digestion of chitin-containing dietary components. Chromatographic elution profiles of isolated organs confirmed the immunological results by displaying characteristic organ-specific patterns in NAGase activity. The molecular characteristics of the moulting form, NAGase B, may further indicate a vesicular transport of moulting enzymes from the epidermis into the ecdysial space. Based on our results we develop a hypothesis explaining the concurrent processes of simultaneous chitin degradation and chitin synthesis occurring during moult. Received: 30 December 1998 / Accepted: 23 April 1999     

Does oxygen deficiency modify the functional response of Saduria entomon (Isopoda) to Bathyporeia pilosa (Amphipoda)?

The functional response of the predatory isopod Saduria entomon to the prey amphipod Bathyporeia pilosa was measured in normoxia (95% O₂ saturation), moderate hypoxia (45% O₂ saturation) and hypoxia (35% O₂ saturation) in aquarium experiments. The prey densities tested ranged from 400 to 8000 ind m⁻². Prey density influenced consumption rates of S. entomon in normoxia and 45% O₂ saturation, but there was no difference between consumption rates at these two oxygen levels. Nevertheless the form of functional response differed. In normoxia S. entomon showed a positively density-dependent functional response to B. pilosa, indicating a potentially stabilizing effect on the prey population. In moderate hypoxia the variance in consumption increased, decreasing the statistical power to distinguish between response models. The functional response of S. entomon in moderate hypoxia was best described with a density-independent response, characterized as destabilizing for the prey population. In hypoxia (35% O₂) predation by S. entomon did not respond to increasing prey density, as almost no amphipods were eaten at this oxygen level. The results are discussed in terms of the usability of theoretical models to examine predator–prey relationships in stressful environments. Received: 26 April 1997 / Accepted: 20 May 1997     

Effect of prey densities on cannibalism in Cape hake (Merluccius capensis) off Namibia

The preference of large (>60 cm total length) Cape hake (Merluccius capensis Castelnau, 1861) for small conspecifics under natural conditions was analyzed off southern Namibia (S.E. Atlantic) between 1983 and 1989. The results indicate that cannibalism is not related to the density of the small conspecifics nor to the density of alternative prey, but rather that large hake have a dietary preference for small conspecifics. The main consequence of such cannibalism is a lack of density-dependent regulation of the M. capensis population.     

Ovarian development and spawning in relation to the moult cycle in Northern krill, Meganyctiphanes norvegica (Crustacea: Euphausiacea), along a climatic gradient

Adaptive processes linked to reproduction were studied comparatively for three populations of Northern krill, Meganyctiphanes norvegica (M. Sars, 1857), sampled during winter and summer cruises in the Clyde Sea (W Scotland), the Kattegat (E Denmark), and the Ligurian Sea (NW Mediterranean). The aim was to investigate the functional relationship between egg production and moulting under contrasted climatic and environmental conditions. A staging system for female sexual development established for live krill was complemented by a histological study of the ovary at various developmental steps. During the reproductive season, all adult female krill were engaged in cyclical egg production. During experiments, female krill released one batch of mature oocytes in one or two spawning events. The ovary of postspawn female krill still contained developing oocytes for another egg batch. In the non-reproductive period, all female krill had a resting ovary. Ovarian structure and pattern of egg production were identical in the three populations, but seasonal timing of egg production was different. The model proposed for the Ligurian population of the annual cycle of ovarian development can be extended to the other two populations, taking into account the seasonal characteristics of each site. Random field samples were staged simultaneously for moult cycle and for sexual development. Moult stages and the seasonal variation of the intermoult period were studied for the Kattegat population using multi-year data and compared to data obtained during summer/winter cruises in the Clyde and the Ligurian Sea. At the three sites, intermoult period was shorter and temperature-dependent during the reproductive period, concurrent with the season of greatest food availability. During most of the year and the period of sexual rest, moulting activity was reduced. The relationship between spawning and the moult cycle was studied comparatively for the three populations. Eggs were released during the premoult phase of a “spawning moult cycle”, in one or two spawnings associated with apolysis and Moult Stage D1, respectively. Yolk accumulation for the next egg batch was completed during an alternating “vitellogenic moult cycle”. A model for the timing of cyclical egg production in relation to moulting, as proposed for the Kattegat, can be extended to the other populations, taking into account intermoult period variation with temperature. Temperature appeared to be the principal environmental factor controlling growth (through moulting) and egg production during the reproductive season, in connection with favourable trophic conditions. Received: 22 December 1997 / Accepted: 29 August 1998     

Life cycle of the copepod Calanus hyperboreus in the Greenland Sea

The seasonal ontogenetic migration of the Arctic copepod Calanus hyperboreus was described from surface-to-bottom hauls in the central Greenland Sea Gyre (GSG) and in the Westspitsbergen Current (WSC). All stages except females spent the winter below 500 m in the GSG and below 1000 m in the WSC. Seasonal ascent begins in April, and descent in July. For the C.␣hyperboreus population an active downward transport of 8.1 g m⁻² dry weight during 8 months of overwintering was estimated, similar to flux rates of particulate matter in sediment traps. Seasonal distribution of biomass was determined from weight measurements of single stages. Annual means varied from 4.0 to 9.2 g m⁻² in two different years in the GSG and were 1.1 in 1 year in the WSC. The life cycle in the Greenland Sea was reconstructed from field data on stage composition, vertical distribution, reproduction, and moult cycle phase from tooth development of CV. Laboratory experiments were conducted on the moulting of CIV and CV in fall. A 3-year (males) and 3- to 4-year (females) life cycle is proposed for the GSG and 2 to 3 years for the WSC. However, the small number of young larvae and the incomplete spring ascent by older copepodites observed in the WSC cast doubt on the reproductive success in the WSC. A suite of physiological strategies and adaptations performed by the developmental stages support survival of this species in harsh environments. Received: 25 January 1997 / Accepted: 11 February 1997     

Plasticity versus repeatability in seabird migratory behaviour

Pelagic seabird populations can use several discrete wintering areas, but it is unknown if individuals use the same wintering area year after year. This would have consequences for their population genetic structure and conservation. We here study the faithfulness of individuals to a moulting area within and among years in a small pelagic seabird, the Thin-billed prion, which moult their primary feathers during the early part of the non-breeding period. According to stable carbon isotope ratios (δ¹³C) of these feathers, 90% of Thin-billed prions moult in Antarctic and 10% in South American waters. Repeated samples from individuals in 2 or 3 years indicated that several birds changed between Antarctic and South American moulting areas or vice versa. However, individuals moulting in an area in one year were more likely to do so again. Four out of five adults maintained highly conserved δ¹³C over the extended moulting period. One bird, however, had systematic changes in δ¹³C indicating latitudinal movements between the two areas during moult. Thus, the present data show that this seabird species has a highly flexible migratory strategy, not only at the population level, but also at the individual level, enabling these seabirds to exploit a highly unpredictable environment.     

Bioerosion of coral reefs by two Hawaiian parrotfishes: species, size differences and fishery implications

Parrotfishes can be significant bioeroders and sediment producers on coral reefs. We quantified the bioerosion rates of two similarly sized Hawaiian parrotfishes with two different feeding modes (Scarus rubroviolaceus—a scraper and Chlorurus perspicillatus—an excavator). The results showed that feeding modes did not affect bioerosion rates but that bioerosion rates were size dependent, with largest individuals (S. rubroviolaceus 45–54 cm FL) bioeroding up to 380 ± 67 kg ind⁻¹ year⁻¹. The size for onset of bioerosion capabilities for both species was 15 cm. Grazing by the two species consumed 60% of the carbonate production of the fore reef area, suggesting that large parrotfishes in Hawaii are ecologically important bioeroders. As individual large S. rubroviolaceus contributed disproportionately more to bioerosion and sediment production than the equivalent biomass of smaller conspecifics, management strategies designed to retain normal reef bioerosion rates should seek to preserve the historical size structure of S. rubroviolaceus populations and to especially protect the larger size classes.     

Factors influencing cannibalism in the wolf spider Pardosa agrestis (Araneae, Lycosidae)

Cannibalistic tendencies are well known in spiders and may be a significant factor influencing population size. The wolf spider, Pardosa agrestis, is the dominant non-web-building spider in a wide range of central European agricultural habitats. Preliminary field observations indicated an extended reproductive period, which results in a very wide size distribution of juvenile instars. We hypothesised that if cannibalism is enhanced by differences in size, especially during periods when prey is scarce, these populations might be susceptible to cannibalism in an ecologically significant way. Laboratory studies were conducted on juvenile P. agrestis in arenas. We analysed the following specific aspects of cannibalism: (1) the effect of the weight ratio between the opponents; (2) the effect of weight per se, and (3) the role of hunger level in determining cannibalistic tendencies of spiders. The role of weight and hunger were analysed in separate experiments, in both cases by controlling for the other variable. The results showed that cannibalism was strongly positively correlated with both weight ratio and hunger, but absolute size/age of an individual could not predict the occurrence of a cannibalistic event. These experiments generated the plausible hypothesis that cannibalism might be an important phenomenon in the regulation of real populations, which should be tested specifically in future field experiments. Received: 15 December 1997 / Accepted after revision: 10 October 1998     

The behavioral ecology of cannibalism in cane toads (<Emphasis Type="Italic">Bufo marinus</Emphasis>)

Laboratory studies show that predatory cane toads (Bufo marinus) exhibit specialized toe-luring behavior that attracts smaller conspecifics, but field surveys of toad diet rarely record cannibalism. Our data resolve this paradox, showing that cannibalism is common under specific ecological conditions. In the wet–dry tropics of Australia, desiccation risk constrains recently metamorphosed toads to the edges of the natal pond. Juvenile toads large enough to consume their smaller conspecifics switch to a primarily cannibalistic diet (67% of prey biomass in stomachs of larger toads). Cannibalistic attack was triggered by prey movement, and (perhaps as an adaptive response to this threat) small (edible-sized) toads were virtually immobile at night (when cannibals were active). Smaller metamorphs were consumed more frequently than were larger conspecifics. The switch from insectivory to cannibalism reflects the high dry season densities of small conspecifics (in turn, due to desiccation-imposed constraints to dispersal) and the scarcity of alternative (insect) prey during dry weather. Our study pond (102 m in circumference) supported >400 juvenile toads, which consumed many metamorphs over the course of our study. Toads appear to be low-quality food items for other toads; in laboratory trials, juvenile toads that fed only on conspecifics grew less rapidly than those that ate an equivalent mass of insects. This effect was not due to parotoid gland toxins per se. Thus, cane toads switch to intensive cannibalism only when seasonal precipitation regimes increase encounter rates between large and small toads, while simultaneously reducing the availability of alternative prey.     

Behavioural response to gradually declining oxygen concentration by Baltic Sea macrobenthic crustaceans

Behavioural responses to decreasing oxygen concentrations were studied in the amphipods Monoporeia affinis (Lindström) and Pontoporeia femorata Krøyer and the isopod Saduria entomon (L.), all benthic macrofauna species from the Baltic Sea. Infrared time-lapse video recording was used. Seven oxygen concentrations were studied, starting near saturation (about 90%), followed by about 65, 49, 33, 16, 8 and 5% oxygen saturation, with each oxygen level maintained for 24?h. M. affinis was more active than P. femorata. The duration of swimming activity of M. affinis was shorter at ≤33% oxygen saturation compared with at the two highest levels. The duration of swimming by P. femorata was longest at 90, 65 and 5% oxygen saturation. For both amphipods the frequency of swimming activity was highest at 5% oxygen saturation. Above 7% oxygen saturation, S. entomon spent most of its time in the sediment, whereas at levels ≤7% it was most often found on the sediment surface. Both amphipods were predominantly night active (nocturnal), whereas no diurnal variation in behaviour was detected for S. entomon. It is suggested that the amphipods minimise the costs associated with obtaining oxygen as well as their risk of predation by moving little on or above the sediment until a lower critical oxygen level is reached. Below this level, they show an oxygen-seeking behaviour. S. entomon also hid in the sediment for as long as possible, suddenly coming to the surface once the oxygen concentration became intolerably low. Its behaviour is also indicative of a trade-off between predation risk and obtaining sufficient oxygen. It is suggested that the change in behaviour shown at about 33% oxygen saturation could lead to a long-term alteration in community structure in the deeper parts of the northern Baltic proper, whereas the change at about 8% oxygen saturation reflects a more acute, short-term response.     

Intraspecific mimicry and status signals in juvenile African penguins

Summary Observations of resting groups of African penguins Spheniscus demersus on land showed that adults were more aggressive towards juveniles than towards adults. Head coloration was important in triggering this aggression. Adults probably discriminate against juveniles to exclude inexperienced birds from co-operative feeding groups and thus maximize their own energetic returns. There was a disproportionately low frequency of penguin groups at sea containing both adult and juvenile birds. Almost a quarter of juvenile penguins moulted a variable amount of their heads into adult plumage at sea. This reduced the amount of aggression received in proportion to the degree of head moult. Head moult probably occurs in only the fittest juveniles. Birds which moult can join adult feeding groups where communal feeding enhances success. The costs of moulting at sea prevent the occurrence of head moulting in the entire population of juveniles.     

Moult in relation to some aspects of reproduction and growth in swarms of Antarctic krill,Euphausia superba

In a 14-d period 38 swarms of Antarctic krill,Euphausia superba Dana, were sampled in an area 55.5 x 55.5 km to the southwest of Elephant Island, at the north of the South Shetland Islands. Moult stage, maturity stage, and size of ca. 100 krill from each swarm were measured. Each of the characteristics varied greatly between swarms. Moulting krill were found in most swarms, but in one swarm all of the krill were just about to moult. Ways in which moulting may act as a possible sorting mechanism are discussed. Data for all the analysed krill (ca. 3000 specimens) were used to investigate the interdependence of moult rate, sexual maturation and growth. While all immature krill moulted at approximately the saine rate in the study, there were significant differences in the moulting rates of mature male and female krill. Gravid female krill continued to moult, although less frequently than mature males. As a consequence males had to attach spermatophores to females after each moult. It is likely that variation in moult rate of females and males was related to the energy expenditure required for ovary development in females and spermatophore production and searching behaviour in males. Spawning and moulting were only partly coupled in gravid females. Spawning appeared to take place predominantly during Moult Stage D₂. There was no evidence of intermoult growth by intersegmental dilation.     

Glycogen storage in relation to the moult cycle in the two crustaceansEmerita asiatica andLigia exotica

InEmerita asiatica, the quantitative fluctuations in the glycogen content of the hepatopancreas were markedly related to the moult cycle. The glycogen content of the hepatopancreas was maximum during premoult stages. It has been suggested that reserve glycogen, in addition to meeting general metabolic needs, may be especially involved in the genesis of the sugar precursors of chitin. The marked fall in the hepatopancreas glycogen values following moulting is probably due to its utilization in chitin synthesis during and immediately following moulting. InE. asiatica, the storage of glycogen was found to be meagre in muscles, and muscle glycogen had no bearing on chitin synthesis during moulting. InLigia exotica, glycogen is stored to a lesser extent than inE. asiatica, and was found in connective tissue and muscles. Significant fluctuations were noted in glycogen values correlated with the phases in the moult cycle when chitin synthesis took place. InL. exotica, the hepatopancreas does not serve as a storage depot for glycogen related to chitin synthesis.     

The selective advantages of cannibalism in a Neotropical mosquito

We investigated the benefits of larval cannibalism in the Neotropical mosquito Trichoprosopon digitatum. The clutch size of the mosquito in the field was strongly correlated with adult female size, indicating a fitness advantage to being large. In controlled laboratory experiments, we compared the survivorship and eventual adult sizes of larvae that were given the opportunity to cannibalise conspecifics throughout their lifetimes with the survivorship and adult sizes of larvae that were prevented from cannibalising. Since the benefits of cannibalism are likely to depend on the context in which it occurs, the experiment was conducted at two levels of alternative food availability. When food availability was high most larvae survived to adulthood, females cannibalised more than males and there was no measurable advantage to cannibalism in terms of survival rate, emergence time or adult size. Larvae were significantly more cannibalistic when food availability was lower, although under these conditions no larvae survived to adulthood. Nevertheless, under low food an important fitness benefit to cannibalism was revealed: individuals which had the opportunity to cannibalise survived significantly longer as larvae than those which did not. This increased longevity is likely to provide an important advantage to mosquito larvae when they wait for the input of unpredictable food sources. Received: 7 October 1995/Accepted after revision: 13 April 1996     

Alarm signal response in the brittle star Amphiura filiformis

 Flat fishes, such as the dab Limanda limanda, commonly prey on arms of Amphiura filiformis. We demonstrate in flume experiments that A. filiformis showed a more or less simultaneous response to an up-stream predation by dab on conspecifics: the arms, which are stretched up into the water column when filter feeding, responded by bending down to the sediment surface, and some arms submerged, at least partly, into the sediment. We interpret this as an alarm response. A similar, but significantly weaker response in A. filiformis was also recorded when dab ate Amphiura chiajei. Homogenates of the two Amphiura species and arm pinching – to simulate partial predation – elicited a similar response in A. filiformis, the arms bent to the sediment surface. In contrast, however, the behavioural response of the arms to the homogenates was to protrude further out of the sediment and sweep the surface vigorously. We interpret this as a feeding behaviour. Response to arm pinching was less clear. The ecological consequences and origin of alarm signal response in A. filiformis are discussed. Received: 8 February 1999 / Accepted: 16 July 1999     

Costs of overlapping reproduction and moult in passerine birds: an experiment with the pied flycatcher

The process of moult in birds requires the investment of substantial amounts of energy and nutrients in feather production and attendant processes. Flight performance may be reduced during the moult of the wing feathers, and moulting birds may suffer from an increased predation risk. These factors may explain why the moult is usually separated in time from other energetically demanding processes, such as reproduction. In this study, we investigated the importance of the temporal separation of moult from breeding activities in the pied flycatcher Ficedula hypoleuca. We induced a moult-breeding overlap by removing the two innermost primaries on both wings, thus imitating the natural loss of these feathers during the initial stage of the moult. The experiment probably did not stimulate feather regrowth, but may have reduced flight performance. Just before fledging of the young, manipulated males and females, as well as the fledglings, had a lower body condition than control birds. The return rate of adult males in the year following the manipulation was significantly lower for males in the manipulated group than for control males. The return rate of females was lower than that of males, but there was no difference between the two female groups. The number of nestlings recruited in the year after the manipulation was significantly lower for the group with an induced moult-breeding overlap than for the control group. The reproductive performance in recruited and returning old females was independent of the manipulation, and in returning adults and recruits the feather quality in terms of number of barbs/mm was independent of the manipulation. These results suggest that a moult-breeding overlap induces fitness costs in terms of reduced recruitment rate and adult survival in a long-distance migrant. Received: 17 September 1997 / Accepted after revision: 23 February 1998     

Endogenous control of timing of metamorphosis in megalopae of the shore crab Carcinus maenas

Batches of hundreds of freshly collected megalopae of the shore crab Carcinus maenas (L.) showed persistent circatidal rhythms of moulting to the juvenile crab stage when maintained in constant laboratory conditions. Peaks of moulting occurred around expected times of high tides, with few megalopae moulting at other times. In larvae collected offshore, the highest tidally-timed peak of metamorphosis occurred during the second to fifth expected times of high tide, and metamorphosis of 50% of each batch took about 22 h or longer. In contrast, in larvae collected at the water's edge, 70% metamorphosed during the first expected episode of high tide, within 6 to 8 h after collection. However, although inshore megalopae moulted before offshore ones, the tidal timing of moulting remained unaltered whether megalopae were collected at neaps or springs, from the water's edge or farther offshore, in the presence or absence of natural substratum, and under various light–dark and salinity regimes. Metamorphosis of C. maenas megalopae around the times of high tides may enhance settlement into the upper intertidal zone. Early juveniles of the crab apparently prefer that zone as they are most abundant there and, unlike adults, do not undertake up-and-down-shore migration with tides. The present finding demonstrates, for the first time, endogenous physiological timing of circatidal periodicity in the metamorphic moult of crab megalopae, suggesting that endogenous factors, as well as exogenous ones should be taken into account in considering the process of settlement by crab megalopae. Received: 21 February 1996 / Accepted: 27 November 1996     

On the timing of moulting processes in reproductively active Northern krill Meganyctiphanes norvegica

The interactions between moult phasing, growth and environmental cues in Northern krill (Meganyctiphanes norvegica) were examined through analysing populations at seasonal, weekly, and daily timescales. The analyses were carried out on resident populations of krill found in three different neritic locations that experience similar environmental signals (the Clyde Sea, Scotland; the Kattegat, Denmark; Gullmarsfjord, Sweden). Seasonal analyses were carried out on the Clyde Sea population and showed that moulting frequency increased significantly moving from winter to summer. The proportion of moulting females in summer samples was often more than double the proportion of moulting males, suggesting that females had a comparatively shorter intermoult period (IMP). Weekly samples taken from the Kattegat showed a similar pattern. However, although the difference between the proportion of female and male moulters was significant in one week, it was not another, mainly because of the variability in the proportion of female moulters. Such variability in females was equally evident in the daily samples taken at Gullmarsfjord. It suggests that females have a shorter IMP (12.5 days) than males (18.4 days) and are more likely to moult in synchrony. Nevertheless, the daily samples revealed that males are also capable of moult synchronisation, although less frequently than females. Shortened IMPs in females were not a result of the abbreviation of specific moult stages. Accordingly, reproductive activity did not alter the course of the normal moult cycle. There was no significant difference between the total body lengths of males and females indicating that females achieve the same levels of growth despite moulting more frequently and having to provision the energy-rich ovaries. This is in contrast to most other crustaceans where the energy costs of reproduction reduce female growth. The fact that females were less abundant than males, probably by suffering a greater level of mortality, suggests that different behavioural strategies, particularly vertical migration regimes, were adopted by each sex to maximise growth and reproduction.