How a subsocial intertidal beetle,Bledius spectabilis,prevents flooding and anoxia in its burrow

Summary All stages of Bledius spectabilis Kratz (Staphylinidae) dig wine-bottle shaped burrows in the intertidal Salicornia zone. The adult female lays her eggs around her burrow and, by remaining with them, prevents both flooding by the tide and anoxia. Flooding is prevented by an ever-ready burrow which exploits a surface tension effect and by blocking the burrow once the tide comes in. The burrow has a narrow neck (about 2 mm in diameter) leading to a living chamber 5 mm in diameter. Tides over artificial tubes in agar showed that a critical minimum neck diameter of 2–3 mm prevented sudden flooding, giving time to block the neck with mud. Blocking took about four minutes. Ensuring the burrow is reopened at each low tide is a vital role of the brooding female in anaerobic or impermeable soils. Field models of burrows became anoxic in 4 days, much less than the 4 week long egg stage. A lab model system with anaerobic agar and calculations of oxygen uptake and diffusion supported this conclusion. Mortality of orphaned larvae may be much lower, however, in burrows within large Bledius aggregations because of mitigating good drainage and soil aeration: larval mortality from physical causes in these burrows did not increase over 3 weeks but without the mother, 14% of the eggs were attacked by mould, and two burrows were taken over by a predatory carabid, Dichierotrichus gustavi, and all larvae eaten. The surface tension effects of small air-filled openings may be used by small air-breathing animals in many intertidal habitats. Like Bledius, other intertidal animals, including large ones, may block their burrows at high tide to keep them full of air. Maternal care, in particular the combination of behaviours which protect the brood from the tide and anoxia, enables this airbreathing insect to colonise the inhospitable habitat of the intertidal saltmarsh.     

Développement larvaire de deux Tellinacea,Scrobicularia plana (Semelidae) et Donax vittatus (Donacidae)

Scrobicularia plana Da Costa and Donax vittatus L. were reared in the laboratory through settlement. Fertilizable eggs were obtained by perfusing the ovary with 5% 0.1 M ammonium hydroxyde in sea water. S. plana eggs have a thick chorion, inside which the early larval stages develop; they hatch as straight-hinge larvae more than 60 h after fertilization. This brood protection is considered to be an adaptation to osmotic pressure changes and pollution in the environment. D. vittatus eggs have a very thin chorion and are unprotected. Further development is planktotrophic and very similar for the two species. Under laboratory conditions, the pediveliger stage is attained 3 weeks after fertilization and settlement occurs 1 week thereafter. S. plana spat stop growing until a suitable substratum is available. Meanwhile they undergo a byssus drifting important for postlarval dispersion. Post larvae resume growth as soon as a small quantity of fine sand is added to the rearing jar. The exhalant siphon is developed first, when the post larvae reach a length of 600 m; the inhalant siphon is formed later, at a length of approximately 900 m. Evolution from the larval hinge to the juvenile hinge stage occurs sooner in S. plana than in D. vittatus. Comparison of laboratory larval development with field development indicates that spawning occurs in June and August for S. plana in North Wales (UK).     

Effects of temperature and salinity on larval development ofNecora puber (Brachyura: Portunidae)

Temperature and salinity affected both length of larval development and mortality inNecora puber collected in the Ría de A Coruña during December 1984 and January 1985. Development time decreased considerably with increased temperature. This decrease was sharper when temperature increased from 15° to 20°C than when it increased from 20° to 25°C. At 35S, average development took 48, 32 and 28 d at 15°, 20° and 25°C, respectively. At the three salinities tested (25, 30 and 35), larval development was completed only at 15°C, at 20°C/30 and 35S, and at 25°C/35S. Development times at 15° and 20°C were highly significantly different at both 35 and 30S (P 0.01). However, there were no significant differences between development times at 20° and 25°C (P > 0.05). Within any one specific temperature series, no significant difference was observed between the salinity values tested (P > 0.05). The duration of each of the five zoeal stages was similar within each and the same temperature/salinity combination, whereas the duration of the megalop was twice as long as any of the zoeal stages. The combination of the lowest temperature (15°C) and the highest salinity (35) tested resulted in the greatest larval survival of 28%. Highest mortality occurred at 25°C, at which temperature development was completed only at 35S. A sharp drop in larval survival was observed in the transition period Zoea V — megalop in all combinations of temperature and salinity tested. Within the limits of tolerance to temperature and salinity, the former effected more pronounced differences in the duration of larval development, while salinity appeared to constitute a limiting factor for survival.     

Dominance of larval forms in euphausiid (Crustacea:Eucarida) ontogenesis

Variability and dominance of Euphausia superba Dana larval forms (Furcilia-I and Furcilia-II stages) in the plankton of the Scotia and Weddell Seas have been investigated. New forms (1, 2, 3 and 14) have been recorded. The dominant forms of Furcilia-I stage are proposed to represent the initial dominants. In accordance with the initial dominance, five types of larval form combinations have been established. During the initial dominance change — from 5 to 2, the larval form number increases. This phenomenon is due to parallel development of euphauiid larvae; it results from a developmental pathway change in the main larval mass. Such deviations are reflected in an initial dominance change. In each pathway, the larvae pass through an unequal number of moults before reaching Form 5 (in accordance with the proposed moult scheme of Furcilia-I stage larvae). In the case of initial dominance of less-developed forms, the presence of different parallel pathways increases. As a result, the total number of larval forms present in the plankton changes, causing increased variability of the total number of larval forms. Each type of larval form combination is the result of summation of all larval forms, which derive from different developmental pathways of the larvae in different localities. The most usual developmental pathway for Euphausia superba is through Form 5, which may, thus, be regarded as the main developmental pathway for this species. This pathway probably corresponds to optimal environment conditions for larval growth and morphogenesis with maximal synchronisation of both processes. Consequently, it may be suggested that changes in developmental pathways of larvae are connected with suboptimal environmental conditions.     

Morphological and histological changes during the growth and starvation of herring and plaice larvae

Reared herring (Clupea harengus L.) and plaice (Pleuronectes platessa L.) were examined for morphological and histological changes during growth and starvation. The growth rate of herring larvae of 0.22 mm/day was less than that reported for wild stock, but this difference was attributed to survival of runts in laboratory. Larval plaice had a growth rate of 0.16 mm/day. The relative condition factor (antilogarithm of intercept of length-weight line) was used to assess condition throughout the larval stages. Starvation resulted in a progressive collapse of the larval body, especially of the ventral body surface around the pectoral girdle of both species (assessed by the pectoral angle) and of the spacing between the organs of the head in herring. There was a breakdown of the herring gut with decreases in epithelial cell height and catabolism of the connective tissue coat and a marked reduction in the transverse sectional area of the plaice liver. The changes in the pectoral angle in both herring and plaice and the eye height to head height ratio in herring should be useful to fishery biologists for assessing nutritional condition, even on board ship.     

Kin preference behavior in Bufo boreas tadpoles

Summary The ontogeny of sibling recognition behavior was studied in the laboratory in tadpoles of the western toad (Bufo boreas boreas) to test the hypothesis that Bufo tadpoles associate with siblings and to compare this behavior with two species of anurans previously studied. Tadpoles reared exclusively with sibs demonstrated a preference to associate with sibs over non-sibs both early and late in development but tadpoles reared with sibs and non-sibs (mixed rearing groups) exhibited no preference. Larvae that developed a preference for sibs after being reared with them for 75 days lost this preference following exposure to a mixed group for 2 to 6 days. Additionally, larvae reared in a mixed group did not develop a preference for the familiar mixed group nor was a preference exhibited by individual larvae that were reared exclusively with 5 non-sibs. These results suggest that larvae learn by prior association to discriminate sibs from non-sibs and that preferences are rapidly modifiable following exposure to non-sibs. However, the latter two experiments suggest that social preferences are not totally labile and are not based entirely on familiarity. Field observations of larval activity, aggregation behavior, and dispersal patterns in B. boreas indicate that tadpoles mix with non-sibs from early larval stages throughout development. Because even short term exposure to nonsibs resulted in a loss of sib preference in our experiments, we suggest that a kin selection interpretation of B. boreas aggregation behavior is not parsimonious. Results differ with those of other studies of sib recognition in larval anurans.     

Larval development in the Antarctic nemertean Parborlasia corrugatus (Heteronemertea: Lineidae)

Embryonic and larval development were followed from fertilisation to settlement in the Antarctic heteronemertean Parborlasia corrugatus (McIntosh, 1876). The first cleavage occurred 10 to 15 h after fertilisation, and the second at 17 h. Larvae hatched at the gastrula stage, between 170 and 200 h post-fertilisation, and were 150 m in diameter. Early larval stages aggregated in dense groups near the surface of incubation vessels and were positively phototactic. Early pilidium larvae were recognisable 435 h post-fertilisation. They were 155×152 m in size, and possessed a complete apical tuft of cilia and a full marginal band of locomotory cilia. At this stage, the gust was visible through the body wall, and the mouth was open and was 40 m in diameter. Late pilidia, 222×193 m in size, were helmet-shaped. They had an apical tuft over 100 m long, and possessed a lobed marginal band of locomotory cilia. Pilidia were observed aggregating close to the bottom of incubation vessels 1200 to 1350 h (50 to 56 d) after fertilisation, and this was interpreted as settlement behaviour. At this stage, the apical tuft had been lost and they were highly contractile, being capable of compressing their bodies. However, neither developing juveniles within the larval envelope nor hatched juveniles were observed. Pilidia consumed the microalgae Tetraselmis suecica, Thalassiosira pseudonana and Isochrysis galbana. They also fed on particulate organic material < 1 m in size, as shown by the presence of material in the guts of larvae offered filtered extracts of algal cultures. There was some indication that larvae could use dissolved organic material, since pilidia held in seawater with organic material removed did not survive as long as those in filtered seawater or in filtered water with added amino acids. However, the only larvae to exhibit settlement behaviour in the feeding experiments were those offered Tetraselmis succica and Thalassiosira pseudonana, and these required a longer development time to reach this stage than pilidia in the standard cultures, where a mixed algal diet was offered.     

Survival of communally reared larval and juvenile lobsters,Homarus americanus

Survival of individually reared larval and juvenile stage lobsters, Homarus americanus (Milne-Edwards), was significantly higher than in corresponding groups of communally reared individuals. Among communally reared lobsters, the mortality rate was highest in the second-stage larvae and then progressively decreased in the later stages. The relationship between survival and duration of molt period of each life-cycle stage indicates that asynchronous molting in the groups of communally reared lobsters is a contributing factor to the higher mortality rate. The molting and mortality curves of communally held lobsters reared from the first larval to first or second juvenile stage showed best cross correlation at 0- or 1-day time lag. The decreased mortality rate observed in the later larval and juvenile stages appears to have resulted from the establishment of new behavior patterns. Group interactions which are influenced by numerous extrinsic and intrinsic factors lead to higher mortality rate (cannibalism) among communally reared lobsters.     

Effect of intraspecific density on life history traits and population growth rate of Neanthes arenaceodentata (Polychaeta: Nereidae) in the laboratory

The effects of intraspecific density on life history traits and population dynamics of the nereid polychaete Neanthes arenaceodentata Moore were assessed in a laboratory experiment. Survival, growth, and fecundity were measured for one generation of worms at densities of 40, 80 and 160 worms per 840 cm² (1x, 2x, and 4x treatments, respectively). Density did not affect size (prior to pairing), percentage of worms paired, time to pairing, or size of mature paired males. Density did have a significant negative effect on survival, size of mature paired females, time to spawning, percentage of females that reproduced, and number of eggs per reproducing female. As density increased, mean survival was 90.0, 80.8 and 74.0%; mean size of mature females was 52.2, 49.2 and 48.1 segments; mean time to spawning was 100.6, 102.4 and 109.4 d; and mean fecundity was 881, 622, and 598 eggs per female, for 1x, 2x and 4x treatments, respectively. Increased density reduced the potential population growth rate, ; for a given rate of larval survivorship, was lower in the 2x and 4x treatments than the 1x treatment. Analysis of sensitivity of to changes in survivorship indicated that population growth rate at the highest density was sensitive to both changes in larval survivorship and the probability of producing a successful brood, although at low densities, was sensitive only to changes in larval survivorship. We attribute these density effects to aggressive bahavioral interactions between the worms, primarily the adults. This experiment identifies key life history traits that could be measured in future experiments to test population level responses of N. arenaceodentata populations to pollutants, both in the laboratory and in the field.Contribution No. 820 of the U.S. Environmental Protection Agency, Environmental Research Laboratory and Contribution No. 189 of the Marine Sciences Institute, University of Connecticut     

Studies on the nutritional requirements of the larval stages of Penaeus japonicus using microencapsulated diets

Fatty acid biosynthesis in the larval stages of Penaeus japonicus Bate was examined by feeding microencapsulated diets containing (1-¹⁴C) palmitic acid, and fat-free diets supplemented with defined fatty acids. Highest larval growth rates were achieved on diets containing Tapes philippinarum lipid and, when defined fatty acids were substituted, on diets containing 20:63 fatty acid. The radioactive tracer experiments indicate that 16:17, 18:0 and 18:19 fatty acids may be synthesised from palmitic acid and that P. japonicus larvae may possess the ability to elongate 18:33 to 20:53 and 22:63, and 18:26 to 20:46. However, the rates of these reactions appear to be too slow to meet the larval requirements for essential fatty acids and the 3 series of polyunsaturated fatty acids must be provided in the diet.     

Elemental composition (C,N,H) in larval and crab-1 stages of Pagurus bernhardus (Decapoda,Paguridae) and Carcinus maenas (Decapoda,Portunidae)

Analyses of individual content of carbon (C), nitrogen (N), and hydrogen (H) were carried out for all larval stages of Pagurus bernhardus and Carcinus maenas, and for newly metamorphosed crabs. Maximum range in total larval development is 12.8 to 165.8 g C, 3.2 to 35.1 g N, and 1.9 to 24.9 g H in P. bernhardus and 3.1 to 43.2 g C, 0.7 to 10.1 g N, and 0.4 to 6.3 g H in C. maenas. From these data energy equivalents were calculated. Maximum range in total larval life is 0.43 to 6.38 J ind. ^-1 in P. bernhardus and 0.1 to 1.49 J ind. ^-1 in C. maenas. There is a 32.4% mean loss of energy in P. bernhardus megalopa development; this seems to describe the normal developmental pattern in this stage. Biomass was determined as fresh and dry weight respectively. Individual dry weight is about 3.6 to 5.6 times higher in P. bernhardus (44 to 340 g) than in C. maenas (12 to 93 g) larvae.Contribution to research project Experimentelle marine Ökosystemanalyse sponsored by Bundesministerium für Forschung und Technologie, Bonn (Grant No. MFU-0328/1)     

The burrows and physiological adaptations to a burrowing lifestyle of Natatolana borealis (Isopoda: Cirolanidae)

The isopod Natatolana borealis Lilljeborg constructs U-shaped burrows in soft mud, the bore of which closely approximates the width of the occupant. Within artificial burrows, the isopods are largely quiescent and often adopt a position close to one of the burrow openings. Conditions within burrows constructed in the laboratory are moderately hypoxic [11.7 to 14.9 kPa (88 to 112 torr)], with isopods showing discontinuous irrigation behaviour (pleopod beating). Rates of oxygen consumption (measured at 10°C) are maintained approximately constant over a wide range of oxygen partial pressure (P_{O 2}) due, in part, to a pronounced increase in pleopod beat rate. Values for the critical partial pressure of oxygen (P_c), the P_{O 2} at which can no longer be maintained independent of P_{O 2}, were 2.0 to 3.3 kPa (15 to 25 torr). N. borealis can survive lengthy periods (65 h at 5°C) of anoxia, during which there is a significant reduction in the carbohydrate concentration and an increase in the l-lactate concentration of the tissues. The oxygencarrying capacity of the haemolymph of N. borealis was low. The haemocyanin showed a relatively high oxygen affinity [P₅₀=0.39 kPa (2.99 torr) at 10°C at the in vivo pH of 7.80] and a pronounced Bohr effect (-1.22). These characteristics may be advantageous to a burrowing mode of life and also for the conditions likely to be encountered in fish carcasses into which they burrow en masse to feed.     

The influence of landmarks on the systematic search behaviour of the desert isopod Hemilepistus reaumuri

Summary The desert isopod Hemilepistus reaumuri uses embankments of faeces, which each family builds around the entrance of its burrow, as an aid to homing after a foraging excursion. Though an isopod must touch its family's embankment (outer radius 8–15 cm) with its antennae before it can detect it, this landmark eases the return to the burrow appreciably. However, this advantage is imperiled by problems with similar landmarks. If during foraging the isopod goes astray and has to search for its landmark it also explores most of the alien families' embankments it detects, at least until it has located the burrow entrance within it. But it is not trapped by such similar landmarks. Whereas an isopod explores its own embankment until it comes to its burrow, digging for hours if the burrow entrance happens to be covered by sand, it leaves an alien embankment after most a few minutes and resumes its search. This shows that the isopod is able to distinguish this landmark from its own, probably by the same chemical badge it uses for the identification of family members. A desert isopod must explore alien embankments predominantly because it is not able to distinguish an alien embankment which is near its own burrow and may intersect its own embankment from others. Even when the animal explores and embankment in vain for a long time it could simply have overlooked its own burrow's entrance. In addition the isopod does not recognize an alien embankment which it has already explored. Therefore during a longer search, it has to explore an alien embankment again and again. H. reaumuri solves these identification problems, which correspond to the problems of other arthropods using landmarks for orientation, in a very successful manner. By repeatedly returning to a particular area it searches there more intensively, the greater the probability that its burrow is in this area according to the information, independent of landmarks, available to the animal. In most cases when it detects a particular alien embankment it explores it for a constant short time (on average 20.4 s). It follows that the isopod explores an alien embankment more intensively, the greater the probability that its burrow is within it. In this simple manner it approximately fulfills the rules of the best mathematical procedures that have recently been developed for solving search problems in which an object detected must be explored for some time before it can be distinguished from the real target. Theoretically these procedures are more successful than the search behaviour of H. reaumuri, but they require that either a particular landmark can be identified with certainty by exploration or that at least it can be recognized on a later contact. Although H. reaumuri does not meet these requirements, the success of its search behaviour is almost identical to that of the mathematical procedures.     

Impact of irrigation on oxygen flux into the sediment: intermittent pumping by Callianassa subterranea and “piston-pumping” by Lanice conchilega

O₂-flux into sediments attributed to the pumping behaviour of two macrofauna species, Callianassa subterranea (Decapoda) and Lanice conchilega (Polychaeta) was investigated. Samples were obtained from the North Sea near Helgoland in 1989 and 1990. The two species were found to transport roughly similar amounts (3 mmolm^-2d^-1) of oxygen into the sediment although they displayed markedly different pumping behaviours. Irrigation by C. subterranea was intermittent and characterized by regularly recurring breathing currents which lasted 2.6 min and were separated by 40-min pauses. In addition to this regular intermittent irrigation, an irregular mode was observed. C. subterranea constructed a complex burrow system. At least half of the burrow wall was not in contact with oxygenated water, however, and thus not effective as additional interface for O₂-exchange. Sediment expelled from the burrow increased the total oxygen uptake (TOU) relative to the surrounding sediment surface. L. conchilega moved water much more frequently (every 4 min) than C. subterranea. We suggest that L. conchilega acted as a piston when moving in its tube, exchanging burrow water with the overlying water. This mechanism, termed piston-pumping, is also potentially important in other smaller tube dwelling organisms. At a shallow water station in the southern North Sea 21 ind of C. subterranea constructed 1.6 m² burrow surface per m². L. conchilega (300 ind m^-2) created only 0.37 m²m^-2 tube surface. On the basis of the abundance and oxygen transport associated with pumping activity, it is calculated that the two species increase TOU by 85% compared to O₂-flux across the sediment-water interface.     

Grazing and ingestion rates of nauplii,copepodids and adults of the marine planktonic copepod Calanus helgolandicus

The average grazing and ingestion rates of all stages of the marine planktonic copepod Calanus helgolandicus (Calanoida) from nauplius stage IV to adults were measured experimentally at 15°C in agitated cultures. The chain-forming diatom Lauderia borealis and the unarmoured dinoflagellate Gymnodinium splendens were offered as food. The food concentrations were close to natural conditions and ranged from 36 to 101 g of organic carbon per liter. The medium body weights expressed in g of organic carbon of almost all larval stages raised at 49 g C/1 were identical with the weight of the same stages caught in the Pacific Ocean off La Jolla, California, USA. In a log-log system, grazing and ingestion rates increased almost linearly with increasing body weight. Grazing rates ranged from 4 to 21 ml/day/nauplius stage IV to 286 ml to 773 ml/day/female. Ingestion rates increased from 0.2 g to 0.8 g C/day/nauplius stage IV to 18 g to 69 g C/day/female. Grazing and ingestion rates per unit body weight decreased gradually with increasing body weight. The daily ingested amount of food decreased from 292 to 481% of the body weight (g C) of nauplius stage V to 28–85% of the body weight of adult females. Grazing and ingestion performances of all stages increased with increasing particle size. Grazing rates decreased and ingestion rates increased with increasing food concentrations. The published data on food intake of the different age groups of C. helgolandicus show that the young stages of herbivorous planktonic copepods can play a major part in the consumption of phytoplankton in the sea due to their high grazing and ingestion rates.     

Electrophoretic identification and genetic analysis of bivalve larvae

Taxonomic identification and genetic analysis of larval marine invertebrates have been vexing problems. We describe a polyacrylamide mini-gel electrophoresis technique for resolving proteins from individual larval bivalves (shell length 250 to 350 m) and apply this technique to three species of laboratory-cultured larval oysters [Ostrea edulis L., 1758, Crassostrea gigas (Thunberg, 1793) and c. virginica (Gmelin, 1791)] reared during summer 1989. Electrophoretic patterns of proteins clearly discriminate among the three species and allow genetic analysis of a polymorphic allozyme locus (Pgi) in field-collected larvae and juveniles of C. virginica. This technique provides an economical tool for largescale taxonomic, ecologic, and genetic studies of meroplanktonic stages of various species.     

Brood defense and age of young: a test of the vulnerability hypothesis

Summary In many altricial species including the great tit (Parus major) the intensity of brood defense against predators has been shown to increase with the age of the offspring. This effect has been ascribed amongst others to the young becoming more vulnerable as they age (vulnerability hypothesis). In a great tit population suffering heavy losses from brood depredation by the great spotted woodpecker (Dendrocopus major), we rendered first and second broods more vulnerable by artificially enlarging the entrance of the nest hole. Contrary to the vulnerability hypothesis, 16 experimental pairs defended their brood against a dummy great spotted woodpecker less vigorously than did 16 control pairs. Nest concealment behavior potentially compromising active defense was minimized by simultaneous playback of nestling distress calls, thus simulating the act of nest predation. This leaves the brood value hypothesis as an alternative functional explanation of the defense level — age effect. It predicts that parents should defend their brood in proportion to the reproductive value (or some more suitable cohortal equivalent measure) of their offspring. At present, this explanation pertains to one predator species. In first broods, but not in second broods, males defended them more vigorously than did their females. While this parallels previous experiments on brood defense against predators posing a much greater risk to the parents, two functional explanations previously put forward can hardly apply.     

Larval development of the euphausiid Euphausia pacifica in the Yellow Sea

The complete larval development of Euphausia pacifica in the Yellow Sea is described and the stages are compared with larvae of E. nana. Euphausiid larvae examined in the present study were collected at 30 stations in the Yellow Sea in 1989. During the stages of Calyptopis III to Furcilia II, E. pacifica larvae are smaller than E. nana larvae but they are larger from Furcilia III onward. There are geographical variations in body size of Calyptopis III among the southern California waters, the eastern Japan waters, the Japan Sea, and the Yellow Sea; body size is smallest in the Yellow Sea and Japan Sea, while it is largest in southern California waters and intermediate in eastern Japan waters. Segmentation of antennal endopods was observed in the 56 to 51 furcilia forms in the Yellow Sea population, suggesting that this characteristic is not as reliable for identification of furcilia stages as the pleopods and terminal telson spines. Thus, we propose here that there are six furcilia stages of E. pacifica, instead of the seven previously reported by Boden (1950).     

Seasonal activity and thermoregulation in Meriones unguiculatus: A gerbil's choice

Summary Mongolian gerbils were observed over seasonal changes in day length and temperature in an outdoor enclosure containing an extensive burrow system. Animals could choose between the stable environment of their burrow and the daily and seasonal flunctuations aboveground. Surgically implanted radio transmitters provided body temperature and location of animals.Gerbils avoided temperature extremes by emerging from their burrow when ambient temperature fell below burrow temperature in hot weather and rose above burrow temperature in cold weather. They delayed emergence in the winter until several hours after sunrise and emerged before or after sunset in summer (Fig. 1).Circadian changes in core-body temperature corresponded to increases and decreases in behavioral activity. Emergence from the burrow was predictable by an increase in body temperature, as a warm-up period preceded emergence (Fig. 3).We conclude that gerbils, when subjected to extreme temperatures in the wild, probably time their activity to correspond to preferred ambient temperatures and thus shift from diurnal activity in the winter to nocrurnal activity in the summer (Fig. 2). This desert rodent's exceptional thermoregulatory capacity adapts it for synchronizing its daily activity to temperature.     

The effect of salinity and cyclic temperature on larval development of the mud-crab Rhithropanopeus harrisii (Brachyura: Xanthidae) reared in the laboratory

Larvae of Rhithropanopeus harrisii (Gould) were reared from hatching to the first or second crab stages in 11 combinations of salinities and cyclic temperatures (5, 20, and 35 S at 20° to 25°C, 25° to 30°C, and 30° to 35°C; 25 S at 20° to 25°C and 30° to 35°C). The larvae survived to the megalops and first crab stages in all salinities and cycles of temperature other than 5 S at 30° to 35°C. The best survival to the megalops (94%) and first crab (90%) stages occurred in 20 S, 20° to 25°C. In all other combinations of salinities and temperatures there was a reduction in survival to the first crab stage. The duration of the larval stages was affected significantly by temperature, whereas the effect of salinity on the mean days from hatching to the first crab stage was not consistent at the different temperature cycles. Development to the first crab stage required the shortest time in 20 S, 30° to 35°C (mean 12.3 days), and the longest time in 5 and 35 S, 20° to 25°C (mean 22.6 days and 21.6 days, respectively). Megalops larvae reared in 35 S at all cycles of temperature, as well as larvae in 20 and 25 S, 30° to 35°C, showed a high percentage of abnormality, with the highest percentage occurring in 35 S, 30° to 35°C. It appears that larval development of R. harrisii is strongly influenced by environmental factors and not solely related to genetic differences.This research was supported by grants from the Nordic Council for Marine Biology and the U.S. Atomic Energy Commission [Grant No. At-(40-1)-4377].Contribution No. 116, Zoological Museum, University of Oslo, Norway.