Acquisition of particle processing capability in juvenile oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis>: ontogeny of gill mucocytes

Acquisition of particle processing capability in postlarval oysters depends upon the structural development of the pallial organs, as well as the development of cilia and mucocytes used (either directly or indirectly) in particle capture and transport. Mucocyte mapping was therefore used to identify mucocyte types and distributions throughout gill development in juvenile oyster Crassostrea gigas (Thunberg 1793) specimens from 2.9 mm to 2.4 cm in shell length. Three categories of gill filaments were identified: apical, lateral and principal filaments, corresponding to filament location or future location in gill plicae. Mucocyte densities were recorded per linear μm (l μm) of frontal surface, and converted to potential total volumes, using the mean volumes of each of the two major mucocyte types: acid mucopolysaccharide (AMPS)-mucocytes and mixed mucopolysaccharide (MMPS)-mucocytes. While AMPS secretions were dominant up to 1.0 cm (flat homorhabdic gill, to semi-heterorhabdic differentiation and plication), MMPS secretions increased progressively, dominating in 2.4 cm and adult specimens (fully heterorhabdic and plicated). Mucus composition, and hence mucus viscosity, thus appears to evolve in relation to the degree of enclosure of the gill frontal surfaces. Total (AMPS + MMPS) potential mucus secretion increased allometrically with juvenile growth, characterized by a sharp increase between 10 and 24 mm shell length, suggesting a marked improvement in particle processing capability. Mucocyte distributions on the gill were heterogeneous from the onset of heterorhabdic differentiation (7.5 mm): the apical filaments of the plicae contained much greater mucocyte total volumes, compared to the lateral and principal filaments. In addition to mucus composition, total potential mucus volume thus also evolved in relation to the degree of enclosure of the gill frontal surfaces. These results show that functional specialization in mucocyte distribution precedes the complete anatomical heterorhabdic differentiation. The completely functional adult gill system is thus attained in 2.4 cm juveniles. This information should be of use in understanding the dynamics of juvenile feeding, growth, and mortality, both in natural systems and in rearing operations.     

Development and biology of <Emphasis Type="Italic">Periphylla</Emphasis><Emphasis Type="Italic">periphylla</Emphasis> (Scyphozoa: Coronatae) in a Norwegian fjord

The globally distributed coronate scyphomedusa Periphylla periphylla (Peron and Lesueur, 1809) occurs in permanent and extremely high abundance in some Norwegian fjords. Studies on the species in Lurefjorden, 40 km north of Bergen, have revealed a holopelagic life cycle with direct development. We distinguished 14 successive developmental stages, characterised by size and morphology. Eggs and early stages are non-mobile, neutrally buoyant, and found mainly at intermediate depths. Rearing studies indicated a development time of 2-3 months from fertilisation to stage 9, when the medusa starts feeding and becomes motile. The fjord population usually shows a strong diel vertical migration, with aggregations at shallower depths during the night. The development includes a gradual increase in pigmentation (porphyrins), starting with the stomach, and thereafter extending to tentacles and the whole exumbrella. This pigment is photodegraded by natural light. Rearing experiments in the laboratory have shown a lethal effect of light. Here, development stops at stage 5. Pigments become visible in stage 7. Older medusae are severely damaged and killed within a few days by exposure to daylight. All stages of P. periphylla are bioluminescent, with this capacity increasing with growth, probably in connection with the development of the nervous net. The light reaction starts at the point of stimulation and spreads at two different speeds corresponding to the two nerve nets of the medusae. The coronal furrow has a central function in transmission of the reaction. It is hypothesised that the bioluminescence is used mainly as a warning to some potential predators, signalling that the porphyrin-containing medusa is unpalatable. P. periphylla has the same two categories of nematocysts as other coronates, with a total of six different types. Nematocyst abundance, distribution and morphology indicate their function. The giant euryteles (capsule up to 100 µm and tubule up to 1160 µm) are the largest ones known among Scyphozoa, and are unique in that this size increases during medusa development.     

Dynamics of pigment degradation by the copepodite stage of <Emphasis Type="Italic">Pseudodiaptomus</Emphasis><Emphasis Type="Italic">euryhalinus</Emphasis> feeding on <Emphasis Type="Italic">Tetraselmis</Emphasis><Emphasis Type="Italic">suecica</Emphasis>

Short-term (3 h) changes in concentration of chlorophylls and their derivatives in stage V Pseudodiaptomus euryhalinus and their fecal material were followed by HPLC during a 24 h experiment. Copepodites were fed with the prasinophyte Tetraselmis suecica. Intact chlorophyll a and b were found in animals and fecal material and had similar dynamics of accumulation over time. The extent of transformation of chlorophyll a and b to colorless compounds was different with chlorophyll a being more extensively degraded. Additionally, several chlorophyll derivatives (pheophytin and pyropheophytin-like pigments) were found. Pyropheophytin a was the most abundant followed by pheophytin b, pheophytin a, and pheophorbide a. Relative amounts of pheopigments were different in copepodites and fecal material, and pheophytin a, pheophorbide a, and pheophytin b concentrations were low and variable. The amount of ingested chlorophyll recovered as chlorophyll a and its derivatives in fecal and copepodite pools was generally low (<5%), with one exception occurring after 9 h, when it accounted for >70%. These data suggest individual pheopigments are produced at different rates and that chemical or enzymatic mechanisms in the gut of copepodites act on the two chlorophylls in different ways.     

Effects of the toxic dinoflagellate <Emphasis Type="Italic">Gymnodinium catenatum</Emphasis> on uptake and fate of paralytic shellfish poisons in the Pacific giant lions-paw scallop <Emphasis Type="Italic">Nodipecten subnodosus</Emphasis>

Juvenile Pacific giant lions-paw scallops Nodipecten subnodosus were fed the toxic dinoflagellate Gymnodinium catenatum, a producer of paralytic shellfish poison (PSP), supplied with Isochrysis galbana (a nontoxic microalgae). Short-term (<24 h) experiments were performed to determine clearance and ingestion rates of G. catenatum. Kinetics of PSP was examined in longer-term experiments (>2 days). At high food concentrations, juvenile scallops showed production of pseudofeces, partial shell valve closure, and reduction in feeding. According to HPLC analysis, the only toxins present in the dinoflagellate G. catenatum and in the scallops were the gonyautoxins (GTXs), except in the labial palps and digestive gland, where trace amounts of saxitoxin (STX) were present in scallops. These tissues could play an important role in toxin biotransformation. The ranking of toxin concentration in tissues was: digestive gland > labial palps > intestine > gills > mantle > adductor muscle, where the total contribution of viscera was more than 80% of the total toxin body burden. Juvenile scallops exhibited no apparent detrimental physiological responses during the long-term feeding experiment. The dinoflagellate may contribute nutrients to the scallop, in addition to the microalgae I. galbana. The dinoflagellate may enhance cell uptake and byssus production. Once PSP accumulated during the first 12 days, it was slowly eliminated. The limited capacity for accumulating toxins in the adductor muscle favors domestic marketing of scallops.     

Distribution of the alien ctenophore <Emphasis Type="Italic">Mnemiopsis</Emphasis><Emphasis Type="Italic">leidyi</Emphasis> in the Caspian Sea in August 2001

In this study, spatial and vertical distributions of the invasive ctenophore Mnemiopsis leidyi in the Caspian Sea were evaluated by using data collected at 41 stations during the August 2001 cruise. A comparison of data from different depths revealed that M. leidyi were generally confined to surface waters. The maximum size of the ctenophore was only 41-45 mm, and the bulk of individuals (85.5%) were <10 mm in length. The average and maximum biomasses of M. leidyi were calculated as 120 and 351 g wet weight m^-2, respectively. Whilst highest biomasses were observed in the western and central Middle Caspian Sea, hot spot areas of reproduction were present along the coasts of the western Caspian Sea, with abundance values of up to 2285 ind. m^-2. The impact of such high densities of M. leidyi is expected to be significant for the pelagic ecosystem of the Caspian Sea.     

Multiple-front iron-mineralisation in chiton teeth (<Emphasis Type="Italic">Acanthopleura</Emphasis><Emphasis Type="Italic">echinata</Emphasis>: Mollusca: Polyplacophora)

The mineralisation of iron in the major lateral teeth of the chiton Acanthopleura echinata has been examined using light and electron microscopy, together with energy dispersive and Raman spectroscopy. The deposition of magnetite in the posterior region of the tooth cusp has been shown to occur simultaneously on two fronts, from both the anterior and posterior sides. In contrast, in the lepidocrocite region, the mineral is formed by aggregation over the whole region. In addition, evidence is presented for the existence of a thin veneer of ferrihydrite over the surface of the magnetite region. The veneer preserves this region from oxidation prior to use in the mature tooth. Data also suggest that the junction zone plays a vital role in the overall biomineralisation process, contributing large amounts of iron at critical stages of development.     

Cardiac output during exposure to <Emphasis Type="Italic">Chattonella</Emphasis><Emphasis Type="Italic">marina</Emphasis> and environmental hypoxia in yellowtail (<Emphasis Type="Italic">Seriola</Emphasis><Emphasis Type="Italic">quinqueradiata</Emphasis>)

The experiments were undertaken to measure, for the first time, cardiac output in yellowtail (Seriola quinqueradiata) during exposure to a harmful red tide flagellate (Chattonella marina). The responses were compared with those during exposure to environmental hypoxia to evaluate the significance of the drop of arterial oxygen partial pressure (PaO₂) in the fish-kill mechanisms by C. marina. PaO₂ immediately decreased, whereas heart rate (HR) was maintained until shortly before death during exposure to C. marina. Suffocation developing during the exposure resulted from a decrease in blood oxygen content, but not from lowered blood flow to the tissue. Although exposure to both C. marina and hypoxia immediately decreased PaO₂, arterial oxygen content (CaO₂) and pH (pHa) were significantly lower, whereas HR and cardiac output (&Qdot;) remained significantly higher, for the C. marina-exposed fish than for hypoxia-exposed fish. Although the drop in PaO₂ appears to play a pivotal role in the mechanisms of fish death by C. marina, other physiological response(s) should also be considered.     

Effect of oxygen concentration on metabolism and locomotory activity of <Emphasis Type="Italic">Moina</Emphasis><Emphasis Type="Italic">micrura</Emphasis> (Cladocera) cultured under hypo- and normoxia

The influence of oxygen concentration on total and basal metabolism, scope of activity, drag force and duration of jerks, time spent swimming and energy cost of locomotion in Moina micrura Hellich females cultured under hypo- and normoxia was investigated. Scope of activity (Q_l) of hemoglobin-rich red individuals (M_a) acclimated to hypoxia depended less upon oxygen concentration than that of non-acclimated, pale individuals (M_na). Within the range 10-0.3 mg O₂ l^-1 Q_l decreased 4.4-fold in M_a and 62.5-fold in M_na. In both M_a and M_na the integral drag force of antenna fell from 0.22ǂ.07 to 0.12ǂ.04 dyn (1 dyn=1·10^-5 N), the duration of jerks increased from 0.06ǂ.01 to 0.1ǂ.02 s in the range from ~2.0 mg O₂ l^-1 to sublethal oxygen concentrations. At 0.7-0.8 mg O₂ l^-1 M_na stopped filtration and increased time spent swimming. In contrast, even under more severe hypoxia (~0.2 mg O₂ l^-1), M_a maintained their filtering activity using energy (up to 80% of total metabolism) achieved due to increased oxygen capacity of the blood. Separating locomotion and feeding functions, M. micrura can spend less energy for swimming and use its energy budget more plastically under changing environmental conditions.     

Evidence of a spawning area of <Emphasis Type="Italic">Anguilla</Emphasis><Emphasis Type="Italic">marmorata</Emphasis> in the western North Pacific

Leptocephali of the tropical eel Anguilla marmorata have been consistently collected in the same area of the North Equatorial Current (NEC) in the western North Pacific during three consecutive cruises in June and July of 1991 (N=28) and 1994 (N=20), and July and September of 1995 (N=27), indicating that this is a spawning area of this species. These leptocephali were collected from 130°E to 142°E and 12°N to 20°N, to the west of the Mariana Islands, in 20 tows in 1991, in 13 tows in 1994 and in 17 tows in 1995, indicating a widespread presence, but a relatively low abundance. Six of these specimens (16.3-36.0 mm total length) from the 1995 cruise, which were of the typical size range of these leptocephali, were genetically confirmed to be A. marmorata in a previous study. The consistent presence of recently spawned A. marmorata leptocephali (9-20 mm) in all 3 years, suggests that the western region of the NEC is the spawning area of the northern population of A. marmorata that was identified in a recent population genetics study. These leptocephali would thus be transported westward by the NEC and then transported north into the Kuroshio Current and toward Taiwan and Japan, or south toward the southern Philippines and into the Celebes Sea by the Mindanao Current. Available evidence indicates that A. marmorata may have potentially year-round spawning, and the presence of a spawning area of this species in the same region as that of Anguilla japonica suggests that the northern population of A. marmorata has evolved a spawning migration from East Asia, the Philippines and the Celebes Sea region to the NEC area, but differs from A. japonica by having some individuals that recruit to more southern areas.     

Mechanisms of range expansion in the intertidal brown alga <Emphasis Type="Italic">Fucus</Emphasis><Emphasis Type="Italic">serratus</Emphasis> in northern Spain

The intertidal brown alga Fucus serratus is naturally expanding its distributional range along the north shore of Spain. Shores being colonised by F. serratus harbour saturated algal communities, with almost no free space. Processes responsible for the successful expansion were investigated in a locality where the alga was experimentally introduced. After a short period, the number of F. serratus plants increased exponentially at a monthly intrinsic rate of 0.111. At early and intermediate stages of invasion, colonisation occurred by the growth of patches of the invader around the transplant points. F. serratus is a bad disperser, with most of propagules settling in near proximity to parent plants. Colonisation of manipulated and control areas was influenced by the proximity of propagule sources. Experimental removals of canopies, irrespective of whether the turf was removed or left intact, accelerated the expansion, as this component of the community had a negative influence on recruitment of both F. serratus and F. vesiculosus. The unmanipulated algal beds were, however, invasible, as some control quadrats close (<10 m) to F. serratus patches had a 100% cover of F. serratus at the end of the study. Prior or concurrent to increases of F. serratus, a decrease in cover of F. vesiculosus and other resident macroalgae was evident. No formal competition experiment was done but results suggest that F. serratus outcompetes F. vesiculosus and Bifurcaria bifurcata at mid-tidal levels. In scraped quadrats, in which both Fucus species recruited at large densities, F. serratus grew at a higher rate than F. vesiculosus and eventually monopolised the experimental surfaces.     

Effects of substrate type on growth and mortality of blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis><Emphasis Type="Italic">edulis</Emphasis>) exposed to the predator <Emphasis Type="Italic">Carcinus</Emphasis><Emphasis Type="Italic">maenas</Emphasis>

Structure and complexity of the substrate are important habitat characteristics for benthic epifauna. The specific growth and mortality rates and inducible defence characters on medium-sized blue mussels (Mytilus edulis L.) exposed to shore crabs (Carcinus maenas L.) were examined on three different substrate types in combined field and laboratory experiments. The experiments showed that complexity of the substrate increased blue mussel survival significantly, through a decrease in predation pressure. However, increased intraspecific competition for food on the complex substrate resulted in significantly lower growth rates of the mussels. Inducible defence characters were also influenced by substrate type. Blue mussels were more affected by predators on the structurally simple substrate, where they developed thicker shells and a larger posterior adductor muscle.     

Settlement of the wreckfish (<Emphasis Type="Italic">Polyprion</Emphasis><Emphasis Type="Italic">americanus</Emphasis>)

The wreckfish (Polyprion americanus) is a panoceanic, long-lived, deep-water demersal species, characterized by an extended pelagic juvenile phase associated with floating objects. In the present study morphometric data from 435 specimens collected from Greece (Crete), Italy (Ionian Sea) and the Atlantic coast of France from September 1999 to March 2001 were analyzed to estimate the settlement size of the species and associated changes in morphology. The fishery sample included specimens from both the pelagic and the demersal stage. Length-at-settlement (TL₅₀) and 95% confidence intervals were estimated by fitting a logistic function. Eleven morphometric characters were analyzed, and the existing inflection points, reflecting alterations in body shape, were defined: (1) iteratively, as the transition point, splitting the data set into two groups, for which the reduced major axis functions, between a character and total length, of the successive groups best fit (minimum sum of squares of residuals) the combined data set and (2) where the second derivative of the fitted third-order polynomial functions to morphometric ratios equaled zero. The main size range within which wreckfish settled was 56-65 cm, and the means of the inflection points defined by the two methods were 61 and 64 cm. These results indicate changes in functional morphology associated with settlement. The monitoring of wreckfish juveniles caught in the wild and kept individually in captivity showed changes in growth and food intake at approximately the estimated settlement length and suggested that temperature was the most likely factor triggering settlement.     

Behavioral adaptations of the cubozoan medusa <Emphasis Type="Italic">Tripedalia cystophora</Emphasis> for feeding on copepod (<Emphasis Type="Italic">Dioithona oculata</Emphasis>) swarms

The cubozoan medusa Tripedalia cystophora preys on dense swarms of the copepod Dioithona oculata in the mangrove prop-root habitat of Puerto Rico. The copepod swarms form in shafts of sunlight that penetrate the mangrove canopy during the day. T. cystophora are found primarily within the same illuminated areas, feeding heavily on the dense swarms of copepods. Laboratory studies were conducted to examine the behavioral adaptations of T. cystophora that allow them to remain within these dense copepod swarms. In the presence of vertical light shafts, T. cystophora showed both a significant change in swimming speed and a significant increase in turning rate. T. cystophora turn abruptly when they leave a vertical shaft of light and quickly re-enter. The presence or absence of copepod prey, either in swarms or dispersed, had no significant effect on T. cystophora swimming speed or turning behavior. Feeding rates of T. cystophora on the copepod D. oculata increase rapidly with copepod density, and maximum rates are not achieved until the high densities associated with swarms are reached. It seems doubtful that these small medusae could survive without their ability to encounter and exploit these dense swarms of copepods.     

Nematode assemblages from <Emphasis Type="Italic">Avicennia</Emphasis><Emphasis Type="Italic">marina</Emphasis> leaf litter in a temperate mangrove forest in south-eastern Australia

Meiofauna from Avicennia marina leaf litter in a temperate mangrove forest was enumerated, and the nematode assemblages compared on the bases of leaf colour (used as a guide to leaf age) and shore horizon where samples were collected. Twenty-one putative nematode species were collected from 48 leaf litter samples. Univariate analyses indicated that neither the colour of the leaf nor the shore horizon significantly affected abundance of nematodes. However, of the four (2Ƕ) treatment groups, rarefaction curves revealed highest diversity on brown leaves from under the shade of the tree canopy (H'=0.751ǂ.126 SE, n=17). Species diversity of leaf litter nematodes was lower in this temperate mangrove system than reported from tropical mangrove studies. ANOSIM tests confirmed a significant effect of shore horizon on nematode assemblages. The dominant feeding group among nematodes was non-selective deposit feeders (7/21 species, but 77% of all nematodes). Epigrowth grazers were represented by 8/21 species of nematodes, but only 19% of the total number. Excised leaves became skeletonised by about 15 weeks. Shorter temporal scales of life cycles of nematodes compared with leaf degradation, and the dynamic nature of epibiontic assemblages, probably explain the similar assemblage structure on yellow and brown leaves.     

Sexual conflict,sexual selection and sperm competition in the spawning decisions of bitterling, <Emphasis Type="Italic">Rhodeus</Emphasis><Emphasis Type="Italic">sericeus</Emphasis>

Sexual selection may operate through either direct selection on preference genes or indirect genetic benefits. However, in many species both direct and indirect selection may operate and can influence female mate and oviposition choice. Adaptations by males for sperm competition can also affect female mate and oviposition choice and can lead to sexual conflict. We investigated the role of direct and indirect benefits and male adaptation to sperm competition for female oviposition decisions in the bitterling, Rhodeus sericeus, a freshwater fish that spawns on the gills of living unionid mussels. Field experiments showed that females chose mussels for spawning on the basis of the number of embryos already in mussels and the body size and/or extent of red coloration of the eyes of territorial males. There was also a significant interaction between these direct and indirect benefits of oviposition choice. Territorial male bitterling avoided leading females to mussels into which rivals had already released sperm. Males also increased their rate of sperm release into mussels in response to sperm release by sneaking males. We discuss the possibility that sexual conflict in oviposition choice occurs in this species.     

Effects of symbiotic status,flow speed,and prey type on prey capture by the facultatively symbiotic temperate coral <Emphasis Type="Italic">Oculina</Emphasis><Emphasis Type="Italic">arbuscula</Emphasis>

Symbiotic temperate corals can supplement prey capture by the coelenterate host with autotrophic carbon production by endosymbiotic zooxanthellae. To test the relationship between heterotrophic consumption and photosynthetic energy, prey capture by symbiotic and aposymbiotic specimens of the temperate scleractinian coral Oculina arbuscula (Verrill) was measured in January-April 2001. Corals were tested in a laboratory flume at five flow speeds, using Artemia franciscana cysts and nauplii as prey. Per-polyp capture rate and feeding efficiency were independent of symbiotic condition. Capture rate increased with flow speed, while capture efficiency declined. The location of capture shifted from the upstream to downstream side of the coral as flow speed increased. Differences in capture rate, location, and feeding efficiency for cysts and live brine shrimp nauplii were likely due to prey size rather than swimming ability.     

<Emphasis Type="Italic">Semibalanus</Emphasis><Emphasis Type="Italic">balanoides</Emphasis> in winter and spring: larval concentration,settlement, and substrate occupancy

This study measured the progression from pelagic larvae to juvenile barnacles, and examined whether recruitment of barnacles, Semibalanus balanoides Linnaeus, at two intertidal sites in contrasting hydrodynamic regimes was determined by pre-settlement or post-settlement processes. The two sites were 1.5 km apart in the vicinity of Woods Hole, Mass., USA. Quantitative plankton samples were taken twice weekly from December 1997 to May 1998 at a nearby site as an estimate of nearshore larval abundance. The presence of S. balanoides nauplii was noted, and cyprids were enumerated and measured. Larval settlement at the two sites [Gansett Point, Buzzards Bay (GP) and Little Harbor, Vineyard Sound (LH)] was estimated from examination of replicate settlement plates exposed for 2 or 3 days throughout the settlement season, and from replicate plots on marked rock quadrats at each site. On both plates and rocks settled cyprids and metamorphs were enumerated. Space occupancy on unmanipulated rock quadrats by all stages from cyprids to adult barnacles was also examined. Settlement occurred from 2 January to 20 May, and major settlement peaks coincided with peaks in pelagic cyprid concentration at LH, but not at GP. Space occupied by juvenile barnacles was close to zero up until late February despite substantial settlement prior to that. At LH, juvenile barnacle cover was zero at the end of the observations; all settlement failed. Almost 100% of settled cyprids failed to metamorphose within 2 days from late January to late March. Then the proportion metamorphosing increased sharply coinciding with a sudden increase of 3°C in water temperature. Observed site differences in space occupancy by juvenile barnacles suggest that while cyprid supply is a necessary condition for barnacle settlement, other factors affecting metamorphosis of settled cyprids and early juvenile mortality determine recruitment.     

Interspecific infection of aposymbiotic juveniles of <Emphasis Type="Italic">Codakia</Emphasis><Emphasis Type="Italic">orbicularis</Emphasis> by various tropical lucinid gill-endosymbionts

Previous molecular phylogenetic analyses have shown that five tropical lucinid species living in or near Thalassia testudinum seagrass beds are colonized by the same bacterial symbiont species. In addition, a new lucinid species belonging to the genus Anodontia, which inhabits reducing sediment found near seagrass beds and in mangrove swamps, has been included in the present study. Endosymbiosis in Anodontia alba was examined according to symbiont phylogenetic and gill ultrastructural analysis. Phylogenetic analysis showed that partial 16S rDNA sequences of A. alba- and Codakia orbicularis-symbionts were 100% identical at all nucleotide positions determined, suggesting that A. alba also harbors the same symbiont species as C. orbicularis (and, consequently, as C. orbiculata, C. pectinella, Linga pensylvanica and Divaricella quadrisulcata). Based on light and electron microscopy, the cellular organization of the gill filament appeared similar to those already described in other lucinids. The most distinctive feature is the lack of "granule cells" in the lateral zone of A. alba gill filaments. In order to confirm the single-species hypothesis, purified fractions of gill bacterial symbionts obtained from the gills of each of the six tropical lucinids cited above were used to infect aposymbiotic juveniles of C. orbicularis. In each case, aposymbiotic juvenile batches were successfully infected by the gill-endosymbiont fractions, whereas, during the experiments, juveniles from the negative control were still uninfected. These experimental data confirm the phylogenetic data and also demonstrate that chemoautotrophic bacterial endosymbionts from their host cells can colonize aposymbiotic juveniles. The conclusion also follows that intracellular gill-endosymbionts still have the capacity to recognize and colonize new host generations. Lucinids provide a unique model for the study of sulfide-oxidizing symbiosis, even if symbionts remain unculturable.     

Seasonal variation in the degree of heterospecific association of two syntopic hyraxes (<Emphasis Type="Italic">Heterohyrax</Emphasis><Emphasis Type="Italic">brucei </Emphasis>and <Emphasis Type="Italic">Procavia capensis</Emphasis>) exhibiting synchronous parturition

Adult rock hyrax (Procavia capensis) and bush hyrax (Heterohyrax brucei) allocate much of their surface activity to being vigilant for vertebrate predators. Individuals of the two species associate for thermoregulatory basking on rock outcrops (koppies) and, in the Matobo National Park, Zimbabwe, frequently constitute heterospecific groups that produce offspring synchronously. Exposure to predators during basking is thought to put a high premium on vigilance, particularly during the presence of dependent offspring in nurseries. We predicted, therefore, an increase in the association of the two species of hyraxes when offspring were present because this would lead to larger groups, thus enhancing the ability to detect predators. Koppie aggregations of hyraxes were classified as homospecific or heterospecific according to basking group composition. Heterospecific association increased when offspring were present, and heterospecific groups were larger than homospecific ones. Fifty-one percent of nurseries were heterospecific for adults/subadults. During the pre-weaning phase (<2 months after birth), 85% of P. capensis and 65% of H. brucei juveniles were in nurseries of heterospecific basking groups. Heterospecific groups had more offspring than homospecific groups. Juvenile mortality is most evident after young are weaned and heterospecific associations break down. Heterospecific aggregation, through increased group size, may result in a higher probability of avoiding predators, thereby reducing the vulnerability of young.