Covering behaviour in <Emphasis Type="Italic">Paracentrotus</Emphasis><Emphasis Type="Italic">lividus</Emphasis>: is light important?

"Covering" or "heaping" behaviour is common to a number of regular echinoids living in a variety of different habitats. Many theories have been proposed to explain this behaviour, among which are several that link covering to light, including ultraviolet (UV) light. However, previous investigations of this light theory have been largely qualitative. In the present study, we used a systematic laboratory protocol to examine quantitatively the covering behaviour of the shallow water echinoid, Paracentrotus lividus, under four different light regimes: white light (400-700 nm), UV-A+B (315-400 nm), UV-A (320-400 nm), and darkness. These experiments demonstrated that light, in particular UV light, influences the covering behaviour of P. lividus. Under the UV regimes, significantly more individuals were found to display covering behaviour, and individuals spent more time at the base of aquaria, farthest from the light source. Moreover, covering items were retained for the longest period of time under the UV-A+B regime. We propose that protection against the harmful effects of UV radiation may be one of the functions of covering behaviour in P. lividus.     

(<Emphasis Type="Italic">Z</Emphasis>)-9-Pentacosene − contact sex pheromone of the locust borer, <Emphasis Type="Italic">Megacyllene robiniae</Emphasis>

Summary. Male locust borers, Megacyllene robiniae (Förster), responded to females only after contacting them with their antennae, indicating that mate recognition was mediated by a contact sex pheromone. GC-MS analyses of whole-body extracts of males and females determined that the profiles of compounds in the extracts were qualitatively similar, but differed considerably in the ratios of compounds between sexes. Biological activities of reconstructed blends of the most abundant straight-chain (nC₂₃, nC₂₄, nC₂₅, nC₂₆), methyl-branched (3me-C₂₃, 3me-C₂₅), and unsaturated (Z9:C₂₃, Z9:C₂₅, Z9:C₂₇ compounds in extracts from females were assessed in arena bioassays, assessing four distinct steps in the mating behavior sequence of males (orientation, arrestment, body alignment, mounting and attempting to couple the genitalia). Males were unresponsive to freeze-killed, solventwashed females treated with blends of straight-chain and methyl-branched alkanes, but responded strongly to females treated with the blend of alkenes. Further trials determined that the complete sequence of mating behaviors, up to and including coupling the genitalia, was elicited by Z9:C₂₅ alone. Z9:C₂₅ comprised 16.4 ± 1.3% of the total hydrocarbons in whole-body hexane extracts of females and was co-dominant with two other hydrocarbons that were not active. In contrast, in solid phase microextraction (SPME) wipe samples from several areas of the cuticle, Z9:C₂₅ appeared as the single dominant peak, comprising 34.6 – 37.8% of the sampled hydrocarbons. Our data indicate that Z9:C₂₅ is a contact sex pheromone of M. robiniae, being the most abundant hydrocarbon on the surface of the cuticular wax layer of females where it is readily accessible to the antennae of males.     

Sibling competition in guinea pigs (<Emphasis Type="Italic">Cavia aperea</Emphasis> f. <Emphasis Type="Italic">porcellus</Emphasis>): scrambling for mother’s teats is stressful

Lactation is the most energy-intense period in the life of a female mammal. This can cause severe conflict between mother and offspring over the duration of lactation but also between siblings over the amount of milk each pup gets from its mother. Thus, competitive interactions between siblings are expected, and competition is likely to increase with litter size, particularly in species where the number of offspring exceeds the number of teats. We studied sibling competition in the domestic guinea pig (Cavia aperea f. porcellus), which has two teats, but frequently bears litters of up to five pups. By cross-fostering we created non-competition (control) litters with two pups and competition litters with four pups and observed nursing behaviour on days 5, 10, 15 and 20 postpartum. Pups of larger litters had lower growth rates, indicating increased competition among siblings in these litters. Pups of larger litters had to wait longer for access to a teat and spent less time suckling than pups of smaller litters but ate more solid food instead. Additionally, we manipulated the individual short-term need of pups by separating half of the pups of each litter for 2 h from their mothers before observation. Within a litter, hungry pups achieved access to milk faster and spent more time suckling than non-hungry pups. Pups competed mostly by scramble competition. Aggressive interactions occurred only in large litters. Pups of large litters had higher cortisol levels than pups in small litters. These effects decreased with age as pups became increasingly independent of maternal milk. Pup behaviour appears to fit better with models of scramble competition than with those of honest signalling. This contribution is part of the special issue “Sibling competition and cooperation in mammals” (guest editors: Robyn Hudson and Fritz Trillmich).     

High-frequency observations of early-stage larval abundance: do storms trigger synchronous larval release in <Emphasis Type="Italic">Semibalanus balanoides</Emphasis>?

The acorn barnacle, Semibalanus balanoides, is thought to release larvae in response to phytoplankton blooms, but there is evidence that another, unidentified cue for release may exist. We conducted high-frequency sampling in Little Harbor, Massachusetts, USA, to determine whether early-stage larval abundance was related to several environmental variables, and to characterize vertical distributions of the larvae. Larval concentrations peaked at 2.52 and 1.02 individuals l⁻¹ during two storms. Larvae were more abundant near the surface than near the bottom. We suggest the hypothesis that turbid conditions and upward-swimming behavior may protect newly-released larvae from predation and cannibalism. Future studies should test this hypothesis with barnacles and other invertebrates.     

Is <Emphasis Type="Italic">Hippolyte williamsi</Emphasis> gonochoric or hermaphroditic? A multi-approach study and a review of sexual systems in <Emphasis Type="Italic">Hippolyte</Emphasis> shrimps

Sexual systems vary considerably among caridean shrimps and while most species are gonochoric, others are described as sequential protandric hermaphrodites or simultaneous hermaphrodites with an early male phase. At present, there is confusion about the sexual system exhibited by several species mostly because those studies attempting to reveal their sexual system draw inferences solely from the distribution of the sexes across size classes. Here we investigated the sexual system of the shrimp Hippolyte williamsi from Chile to determine if the species is protandric or gonochoric with sexual dimorphism (males smaller than females). Morphological identification and size frequency distributions indicated that the population comprised small males, small immature females, and large mature females, which was confirmed by dissections. No transitional individuals were found. Males maintained in the laboratory molted 1–8 times, and many grew up to reach sizes observed in only a small fraction of males in the field. No indication of sex change was recorded. Our results indicate that H. williamsi is a sexually dimorphic gonochoric species and emphasizes the importance of using several kinds of evidence (size measurements, growth experiments, morphological dissections, and histological studies) to reveal the sexual system of Hippolyte species. Whether the observed size dimorphism between males and females in many species of Hippolyte is expression of contrasting sexual and natural selection, and whether divergent sexual fitness functions can contribute to the evolution of hermaphroditism remains to be revealed in future studies.     

Laboratory investigation of <Emphasis Type="Italic">Fallopia</Emphasis> × <Emphasis Type="Italic">bohemica</Emphasis> fruits dispersal by watercourses

Seed and fruit dispersal along watercourses favours the long-distance migration of invasive species, not only for aquatic or wetland species, but also for terrestrial wind-dispersed plants, like the Japanese knotweed. The present paper aims at investigating the role of watercourses in the dispersal of the knotweed due to its frequent occurrence on riverbanks and production of fertile achenes (type of fruit of the Japanese knotweed). This dispersal occurs along two steps after the fruits deposit on the water surface: floatation first and then sinking towards the bottom of the watercourse. Regarding the first step, the effects of agitation of the water, temperature, surface tension and luminosity on the achenes floatability are experimentally studied. While no influence of luminosity is observed, an increase of temperature greatly decreases the floating time. Floating time also decreases as the contact between water and the fruit is enhanced (through submersion of achenes, agitation of the water or lower surface tension). Regarding the second step, the fall velocity of the fruits in water at rest is measured and appears to be independent of the seed history (floating time). 3D helical motions are systematically observed with constant tangential velocity with respect to the falling velocity. The trajectory of the fruits in a shear flow is then measured and the evolution of their velocity components along the sinking process is discussed. Finally, the contribution of both steps to the long-distance migration of the seeds is estimated.     

Life cycle of <Emphasis Type="Italic">Oithona</Emphasis> <Emphasis Type="Italic">similis</Emphasis> (Copepoda: Cyclopoida) in Kola Bay (Barents Sea)

The annual population dynamics (nauplii, old copepodites CIV–CV and adults) and seasonal variations in reproductive parameters of the cyclopoid copepod Oithona similis were investigated on the basis of the data 1999–2006 in Kola Bay, a large subarctic fjord in the Barents Sea. Population density of O. similis ranged from 110 to 9,630 ind m⁻³ and averaged 1,020 ± 336 ind m⁻³. The relative abundance of adults was high during winter (~60%). At the end of winter (mid-March), the population included a large percentage of later-stage copepodites (stage CIV 23% and stage CV 57%). There were two periods of mass spawning, in late June and September. Autumn and summer generations strongly differed in abundance, average prosome length (PL), clutch size (CS), egg diameter (D), egg production rates (EPR and SEPR) and female secondary production. Average PL decreased with increasing water temperature, while D and CS were strongly correlated with PL but unaffected by temperature. Annual average EPR and SEPR were 0.55 ± 0.18 eggs female⁻¹ day⁻¹ and 0.0011 ± 0.003 day⁻¹, respectively. Female secondary production averaged 0.8 ± 0.3 μg C m⁻³ day⁻¹ (range 0.001–3.58). There were positive relationships between abundance, EPR, SEPR, production and water temperatures. Reproductive parameters appeared to be controlled by hydrological factors and food conditions.     

<Emphasis Type="Italic">Hippophae rhamnoides</Emphasis> on a coastal dune system: a thorny issue?

The study mapped the spread of the invasive non-native shrub, Hippophae rhamnoides, on a coastal dune system in South Wales. H. rhamnoides colonies spread across the system, covering around 60.9 ha in 1996 compared to 2.4 ha in 1957. Clearance activities have since decreased the total to around 23 ha. The effects of this expansion on ground flora were assessed through comparison of species assemblages in colonised, never colonised and cleared areas. Dichotomisation presented a number of markedly different communities which correlated to their status of H. rhamnoides invasion. Colonised quadrats were overwhelmingly composed of H. rhamnoides and a few shade-tolerant species such as Galium aperine and Urtica dioica. Cleared quadrats did not support desirable dune grassland species present in never colonised sites, although those which had been cleared for the longest period may be regaining some characteristic species. Clearance work in future needs to concentrate on the complete removal of smaller H. rhamnoides patches whilst also preventing the establishment of Chamerion angustifolium and other ruderal species which in turn inhibit the development of typical dune grassland communities.     

Function of the major cheliped in male–male competition in the hermit crab <Emphasis Type="Italic">Pagurus nigrofascia</Emphasis>

Functions of the major cheliped in pagurid hermit crabs have been studied in fights for shells. The major cheliped often shows sexual size dimorphism, suggesting that sexual selection favors the development of the male major cheliped. The function of the major cheliped in male–male competition was examined in Pagurus nigrofascia collected from April to June 2009 on the intertidal rocky shore in southern Hokkaido, Japan (41°N, 140°E). Sexual size dimorphism of the major cheliped was observed, and precopulatory guarding males had larger major chelipeds than solitary ones. Guarding males used the major cheliped to deter intruders during competitive interactions. Males without a major cheliped were disadvantaged even if they were larger than opponents and had ownership. Cheliped size affected the outcomes of contests between similar sized males. This suggests that the male major cheliped in P. nigrofascia protects mates from competitors and, consequently, enhances male mating success. Sexual selection may favor the development of the major cheliped in male pagurids.     

Variation in colony geometry modulates internal light levels in branching corals, <Emphasis Type="Italic">Acropora humilis</Emphasis> and <Emphasis Type="Italic">Stylophora pistillata</Emphasis>

Colonial photosynthetic marine organisms often exhibit morphological phenotypic plasticity. Where such plasticity leads to an improved balance between rates of photosynthesis and maintenance costs, it is likely to have adaptive significance. To explore whether such phenotypic plasticity leads to more favourable within-colony irradiance for reef-building branching corals, this relationship was investigated for two coral species Acropora humilis and Stylophora pistillata, along a depth gradient representing light habitats ranging from 500 to 25 μmol photons m⁻² s⁻¹, during 2006 at Heron Island, Great Barrier Reef (23.44°S, 151.91°E). In the present study changes in flow-modulated mass transfer co-varied with light as a function of depth. In low-light (deep) habitats, branch spacing (colony openness) in A. humilis and S. pistillata was 40–50% greater than for conspecifics in high-light environments. Also, branches of A. humilis in deep water were 40–60% shorter than in shallow water. Phenotypic changes in these two variables lead to steeper within-colony light attenuation resulting in 38% higher mean internal irradiance (at the tissue surface) in deep colonies compared to shallow colonies. The pattern of branch spacing was similar for S. pistillata, but this species displayed an alternate strategy with respect to branch length: shade adapted deep and cave colonies developed longer and thinner branches, allowing access to higher mass transfer and irradiance. Corals in cave habitats allowed 20% more irradiance compared to colonies found in the deep, and had a 47% greater proportion of irradiance compared to colonies in the shallow high-light environment. Such phenotypic regulation of internal light levels on branch surfaces partly explains the broad light niches of many branching coral species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Strategies of a parasite of the ant–<Emphasis Type="Italic">Acacia</Emphasis> mutualism

Mutualisms can be exploited by parasites—species that obtain resources from a partner but provide no services. Though the stability of mutualisms in the presence of such parasites is under intensive investigation, we have little information on life history traits that allow a species to be a successful mutualist or rather a parasite, particularly in cases where both are closely related. We studied the exploitation of Acacia myrmecophytes by the ant, Pseudomyrmex gracilis, contrasting with the mutualistic ant Pseudomyrmex ferrugineus. P. gracilis showed no host-defending behavior and had a negative effect on plant growth. By preventing the mutualist from colonization, P. gracilis imposes opportunity costs on the host plant. P. gracilis produced smaller colonies with a higher proportion of alates than did the mutualist and thus showed an “r-like” strategy. This appears to be possible because P. gracilis relies less on host-derived food resources than does the mutualist, as shown by behavioral and stable isotope studies. We discuss how this system allows the identification of strategies that characterize parasites of mutualisms.     

How does burrowing by the isopod <Emphasis Type="Italic">Limnoria agrostisa</Emphasis> (Crustacea: Limnoriidae) affect the leaf canopy of the southern Australian seagrass <Emphasis Type="Italic">Amphibolis griffithii</Emphasis>?

In south-western Australia, the isopod Limnoria agrostisa commonly burrows into leaf clusters and immature shoots of Amphibolis griffithii. The isopod also burrows into the sheath and rhizomes of Posidonia species. In A. griffithii, the isopod consumes new tissue within the sheath, damaging or destroying the meristem. This results in malformation of new leaves or destruction of whole leaf clusters with the potential to reduce the photosynthetic area of a shoot. The isopod has been found in all but one meadow of A. griffithii examined over 1,000 km of the Western Australian coastline. It was present throughout the year and showed little variation in abundance. Young were produced year round, but were more abundant in the summer months. Females, 3.5 mm in length or larger, produced 2–5 young that were brooded within the leaf cluster or base of an immature shoot. Within a meadow, 40–70% of shoots and 10–20% of leaf clusters were damaged by isopods. Seasonal trends were not consistent, but damage appeared to be higher in summer when isopod abundance was higher. Approximately 40% of clusters were destroyed by isopod damage. Isopods attack shoots of all ages, but damage was often located on apical clusters. There was no evidence that isopod damage initiated branching or leaf cluster formation. Estimations of clusters damaged or destroyed may be conservative, as only those clusters remaining on a shoot could be counted, and cluster loss could not be quantified. Examination of defoliated stems on upright shoots and horizontal rhizomes indicated that many were destroyed by isopods. The effect of L. agrostisa although substantial appears to be a feature of healthy seagrasses throughout southern Australia.     

Why are larvae of the social parasite wasp <Emphasis Type="Italic">Polistes sulcifer</Emphasis> not removed from the host nest?

A challenge for parasites is how to evade the sophisticated detection and rejection abilities of potential hosts. Many studies have shown how insect social parasites overcome host recognition systems and successfully enter host colonies. However, once a social parasite has successfully usurped an alien nest, its brood still face the challenge of avoiding host recognition. How immature stages of parasites fool the hosts has been little studied in social insects, though this has been deeply investigated in birds. We look at how larvae of the paper wasp obligate social parasite Polistes sulcifer fool their hosts. We focus on cuticular hydrocarbons (CHCs), which are keys for adult recognition, and use behavioral recognition assays. Parasite larvae might camouflage themselves either by underproducing CHCs (odorless hypothesis) or by acquiring a chemical profile that matches that of their hosts. GC/MS analyses show that parasite larvae do not have lower levels of CHCs and that their CHCs profile is similar to the host larval profile but shows a reduced colony specificity. Behavioral tests show that the hosts discriminate against alien conspecific larvae from different colonies but are more tolerant towards parasite larvae. Our results demonstrate that parasite larvae have evolved a host larval profile, which overcomes the host colony recognition system probably because of the lower proportion of branched compounds compared to host larvae. In some ways, this is a similar hypothesis to the odorless hypothesis, but it assumes that the parasite larvae are covered by a chemical blend that is not meaningful to the host.     

Simulation of exchange flow between open water and floating vegetation using a modified RNG <Emphasis Type="Italic">k</Emphasis>-<Emphasis Type="Italic">ε</Emphasis> turbulence model

The paper focuses on the numerical simulation of the exchange flow between open water and floating vegetation, which plays an important role in maintaining the ecological balance by transporting nutrient matter. The simulation was conducted using a new solver developed upon OpenFOAM. A modified RNG k-ε turbulence model, which is expected to model both the high- and low-Reynolds number flows correctly, was used to determine the eddy viscosity. Several particular terms were added into the momentum equations and turbulence model equations to model the effects of vegetation and buoyancy. Among these terms, the term for the effect of vegetation in the ε-equation was re-modelled. The model was validated by properly predicting the profiles of mean velocity and turbulent kinetic energy for flows through suspended canopies. The density flow between open and vegetated water was simulated with the same conditions as those of the experiment conducted by Zhang and Nepf. The predicted results agreed well with the experimental data and provided more detailed information of such exchange flow. The convection between the root layer and the layer beneath the roots, which was not observed in the experiment, was observed in the numerical simulation.     

Sibling species or poecilogony in the polychaete<Emphasis Type="Italic"> Scoloplos armiger</Emphasis>?

In marine invertebrates multiple modes of development, or poecilogony, may occur in a single species. However, after close examination, many of such putative cases turned out to be sibling species. A case in point may be the cosmopolitan orbiniid polychaete Scoloplos armiger, which inhabits marine shallow sediments. In addition to the well-known direct, holobenthic development from egg cocoons, pelagic larvae have also been described. Our culture experiments revealed a spatially segregated source of the two developmental modes. All females of an intertidal population produced egg cocoons and no pelagic larvae. All but 2 out of 15 females of an adjacent subtidal population produced pelagic larvae and no egg cocoons. Based on these results we performed a molecular genetic analysis (RAPD-PCR) on three intertidal and four subtidal populations in the North Sea. Selected samples from all sites were analysed also by the AFLP method. We found significantly higher genetic diversity within subtidal than within intertidal populations. This is consistent with a wider dispersal by pelagic larvae and a smaller effective population size when development is holobenthic. Total genetic divergence is not related to distance but to the intertidal/subtidal division. We suggest that S. armiger actually represents two sibling species.     

Where are larvae of the vermetid gastropod <Emphasis Type="Italic">Dendropoma maximum</Emphasis> on the continuum of larval nutritional strategies?

This study evaluated whether larvae of the Indo-Pacific vermetid gastropod Dendropoma maximum are obligate planktotrophs, or whether they exhibit an intermediate feeding strategy. Experiments were conducted in Moorea, French Polynesia (149°50′W, 17°30′S), Sep–Oct 2009, to examine D. maximum larval growth and metamorphic responses to different diets and amounts of food. Dendropoma maximum larvae required particulate food to undergo metamorphosis, but were able to survive and grow in the absence of food for up to 20 days. Larvae in Low and Unfed food treatments exhibited phenotypic plasticity by growing a larger velum (the larval feeding structure) compared with those in high food. Unfed D. maximum larvae had a slower initial growth rate; however, by 11-day post-hatch fed and unfed larvae had converged on the same mean shell height (553 μm), which was only 10% larger than the initial size at hatching. Therefore, although the nutritional strategy of D. maximum larvae is best described as obligate planktotrophy, it appears to approach an intermediate feeding strategy.     

Optimal maternal investment in the dung beetle<Emphasis Type="Italic"> Onthophagus taurus</Emphasis>?

Optimal investment theory is based on the assumption that the proximate constraint acting on parental investment is resource based. A trade-off between per offspring investment and total investment seems intuitive. Consequently, a parents investment strategy is expected to represent a trade-off between the benefits of investment for current offspring and the costs to future reproduction for parents. In this study, we provide clear evidence that the costs and benefits of maternal provisioning in the dung beetle Onthophagus taurus influence the amount of provisions provided by the mother. Horse dung is typically of a higher nutritional value than cow dung and females were shown to provide 20% less dung to offspring when provisioning with horse dung. By reducing their investment per offspring and exhibiting a clear preference to provision offspring with horse dung, females were able to produce significantly more offspring. Females provisioning with horse dung received greater fitness returns per unit of investment and experienced lower provisioning costs, in terms of the minimum amount of dung required to produce a surviving offspring, than females provisioning with cow dung. Females provisioning in soil of low moisture content were found to have higher tunneling costs than those provisioning in soil of high moisture content, while the fitness returns per unit of investment did not differ. We adopted a marginal value theorem (MVT) approach to calculate the theoretical optimal level of investment for each dung type and for each soil moisture. Predicted levels of provisioning were lower for horse dung than for cow dung and for moist soil than for dry soil. Therefore, the results of this study are in qualitative agreement with MVT predictions and provide empirical support for the proposal that females can adaptively adjust their level of investment in response to resource and/or habitat quality. However, the theoretically predicted optimal investment yielded a poor quantitative fit with our observed levels of investment, with females providing over twice the investment predicted by the MVT approach. We suggest that this difference may reflect either our inability in directly quantifying all the necessary costs and benefits of investment in O. taurus and/or the applicability of the underlying assumptions of MVT.Communicated by D. Gwynne     

Does the antidiabetic drug metformin affect embryo development and the health of brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis> f. <Emphasis Type="Italic">fario</Emphasis>)?

Background

Due to the rising number of type 2 diabetes patients, the antidiabetic drug, metformin is currently among those pharmaceuticals with the highest consumption rates worldwide. Via sewage-treatment plants, metformin enters surface waters where it is frequently detected in low concentrations (µg/L). Since possible adverse effects of this substance in aquatic organisms have been insufficiently explored to date, the aim of this study was to investigate the impact of metformin on health and development in brown trout (Salmo trutta f. fario) and its microbiome.

Results

Brown trout embryos were exposed to 0, 1, 10, 100 and 1000 µg/L metformin over a period from 48 days post fertilisation (dpf) until 8 weeks post-yolk sac consumption at 7 °C (156 dpf) and 11 °C (143 dpf). Chemical analyses in tissues of exposed fish showed the concentration-dependent presence of metformin in the larvae. Mortality, embryonic development, body length, liver tissue integrity, stress protein levels and swimming behaviour were not influenced. However, compared to the controls, the amount of hepatic glycogen was higher in larvae exposed to metformin, especially in fish exposed to the lowest metformin concentration of 1 µg/L, which is environmentally relevant. At higher metformin concentrations, the glycogen content in the liver showed a high variability, especially for larvae exposed to 1000 µg/L metformin. Furthermore, the body weight of fish exposed to 10 and 100 µg/L metformin at 7 °C and to 1 µg/L metformin at 11 °C was decreased compared with the respective controls. The results of the microbiome analyses indicated a shift in the bacteria distribution in fish exposed to 1 and 10 µg/L metformin at 7 °C and to 100 µg/L metformin at 11 °C, leading to an increase of Proteobacteria and a reduction of Firmicutes and Actinobacteria.

Conclusions

Overall, weight reduction and the increased glycogen content belong to the described pharmaceutical effects of the drug in humans, but this study showed that they also occur in brown trout larvae. The impact of a shift in the intestinal microbiome caused by metformin on the immune system and vitality of the host organism should be the subject of further research before assessing the environmental relevance of the pharmaceutical.

     

Spike dives of juvenile southern bluefin tuna (<Emphasis Type="Italic">Thunnus maccoyii</Emphasis>): a navigational role?

Tunas make sharp descents and ascents around dawn and dusk called spike dives. We examine spike dives of 21 southern bluefin tuna (Thunnus maccoyii) implanted with archival tags in the Great Australian Bight. Using a new way to categorize this behavior, we show that spike dives are similar among all the fish in the study. The dive profiles are mirror images at dawn and dusk and are precisely timed with respect to sunrise and sunset. We analyze the possible reasons for spike dives, considering the timing of spike dives, the characteristic dive profile, and the tuna's magnetic habitat. In addition, we present anatomical evidence for elaboration of the pineal organ, which is light mediated and has been implicated in navigation in other vertebrates. The new evidence presented here leads us to suspect that spike dives represent a survey related to navigation.     

Genetic variation among life-history stages of mussels in a<Emphasis Type="Italic"> Mytilus edulis–M</Emphasis>.<Emphasis Type="Italic"> trossulus</Emphasis> hybrid zone

The mussel species Mytilus edulis L. and M. trossulus Gould coexist and hybridize throughout a large area that includes the north coast of Maine and Atlantic Canada. Previous studies provided genetic evidence for limited hybridization between the two species for mussels >15 mm. The present study used two genetic markers (ITS, Glu-5) to examine the genetic composition of early life-history stages by sampling veliger and pediveliger larvae, juveniles (<2.0 to 15.0 mm shell length) and adults (>15 mm shell length) in Trinity Bay, Newfoundland, during three consecutive years (1995–1997) to determine if differential mortality limits the relative abundance of hybrids. The relative frequency of the two species and the different hybrid genotypes was similar among the larvae, juveniles and small adult mussels. The double hybrid genotype (F₁-like) was the rarest genotype observed. There was no evidence for differential mortality during the early life-history stages, and factors limiting production of hybrids appear to operate before the late larval stage. The observed frequency of hybrids is probably due to a combination of pre- and postzygotic reproductive isolating mechanisms operating early in the mussels life history. M. trossulus dominated the early life-history stages, possibly due to a higher population density and a greater reproductive output than M. edulis. Differential mortality may explain the observed decrease in frequency of M. trossulus and increase in frequency of M. edulis with increasing shell length. A similar frequency of hybrid mussels from larvae to the size class of 55 mm shell length may indicate a rate of mortality intermediate between the two parental species. The M. edulis–M. trossulus hybrid zone appears to be maintained by reproductive isolating mechanisms limiting the production of hybrids and life-history differences that allow the two species to coexist.Communicated by J.P. Grassle, New Brunswick