Niche overlap among two species of coral dwelling fishes of the genus Dascyllus (Pomacentridae)

Synopsis The two damselfish, Dascyllus aruanus and D. marginatus are coral-dwelling species, occurring in separate groups or together in the corals Stylophora pistillata and Acropora spp. The two species, however, occur together more frequently than would be expected by chance. The niches of these species overlap in space, time and food. Looking for mechanisms that allow such coexistence, it was found that in such a bispecific group, no difference exists in agonistic acts between intra — or interspecific encounters. This phenomenon seems to be produced by two independent mechanisms — random settling of juveniles from the plankton and predation pressure that strongly limits the re-settling of the fish.     

Practical coexistence of two species in the chemostat - A slow-fast characterization

We show that the chemostat model with two species having different but close break-even concentrations exhibits a slow-fast dynamics. Considering small perturbations about the dilution rate for which break-even concentrations are identical, we use the Fenichel theory to show the coexistence of species for large times. Then we determine the reduced dynamics, which is non-trivial and characterized by the slopes of the growth functions about their break-even concentrations.     

Protandric hermaphrodite peculiarities in Amphiprion frenatus Brevoort (Teleostei, Pomacentridae)

Electron microscopy of the male phase of the ovotestis of Amphiprion frenatus , a protandric hermaphrodite, showed no connective tissue between male and female areas and, as the basal lamina was lacking both along the seminiferous tubules and round the previtellogenic oocytes, the male and female germ cells were only separated by their respective surrounding somatic cells (Sertoli and follicle cells). Besides previtellogenic oocytes, oocytes in meiotic prophase and very small (young) previtellogenic oocytes, were detected in the ovarian part, as spermatogenesis proceeded, revealing oogenetic activity. Degeneration of some previtellogenic oocytes and their follicle cells was discernible.     

Interspecific variation of body shape and sexual dimorphism in three coexisting species of the genus <Emphasis Type="Italic">Petrotilapia</Emphasis> (Teleostei: Cichlidae) from Lake Malawi

Differences in color patterns have been the most used feature in describing cichlid species belonging to genus Petrotilapia from Lake Malawi. In this study, we quantified morphological variation in body shape within and among three coexisting Petrotilapia species using landmark-based geometric morphometric methods. Statistic analyses revealed significant body shape differences among species but not between sexes. Post hoc multiple comparisons based on Mahalanobis distances revealed that P. nigra was significantly different from P. genalutea and Petrotilapia sp., whereas the latter two were not significantly different. The splines generated showed that the most pronounced variation was in the head region, in which P. nigra had a relatively longer and deeper head than the other two. The most clear-cut distinction was in gape length; P. genalutea had the longest gape, followed by Petrotilapia sp., whereas P. nigra had the shortest gape. Body depth was shallower in P. nigra than the others. When comparing sexes by their centroid size, ANOVA revealed that males were bigger than females. Therefore, we conclude that color is not the only feature that can distinguish these congeners. We discuss the observed sexual dimorphism in terms of sexual selection and relate morphological variation among species to feeding behavior, which may help explain their coexistence in nature.     

Isolation and characterization of 16 microsatellite loci in the humbug damselfish, Dascyllus aruanus (family Pomacentridae)

Here we report on 16 microsatellite loci designed for the damselfish Dascyllus aruanus. All loci were tested on 98 individuals and were polymorphic (seven to 35 alleles). Expected heterozygosity ranged from 0.705 to 0.942. Six loci showed Hardy-Weinberg disequilibrium due to the occurrence of null alleles. Cross-species amplifications conducted within the genus Dascyllus (D. carneus, D. strasburgi, D. trimaculatus) lead to polymorphic fragments in 32 out of 48 tests. These 16 loci will enable future research into the behavioural ecology and population ecology of Dascyllus aruanus throughout the Indo-Pacific.     

Comparative feeding ecology of two herbivorous damselfishes (Pomacentridae: Teleostei) from the Gulf of California,Mexico

Two species of benthic damselfishes from the Gulf of California, Mexico, use contrasting behaviors when feeding on benthic algal communities. The small (±70 g) Cortez damselfish, Eupomacentrus rectifraenum (Gill, 1862), feeds selectively from a multi-species algal mat, eats fleshy red and green algae and ignores brown and calcareous algae. The giant blue damselfish, Microspathodon dorsalis (Gill, 1862), is a large (±450 g), lethargic, nonselective feeder which grazes on a near monoculture of a fleshy red alga, Polysiphonia sp. Feeding activity for both species is low in the morning peaks during late afternoon, and drops sharply as night approaches. Based on feeding rates, gut-filling times, and weights of gut contents, Cortez damselfish process six to eight full guts of food and giant blue damselfish three full guts of food per day. The algal mat exhibits high standing crops (291–618 g dry wt · m^?2) and low productivity, but the preferred food of the Cortez damselfish (Ulva) appears to colonize the mat frequently and grow rapidly. The Polysiphonia dominated community on giant blue damselfish territories exhibits low standing crops (23 g · m^?2) and high productivity (34–47 times that of the mat per gram algae). Even though the feeding behaviors and resources used by the two damselfishes differ, both species eat similar food (delicate red and green fleshy algae, and depend on rapid colonization and/or high productivity to maintain their primary foods in the grazed algal community.     

Direct mate choice for simultaneous acoustic and visual courtship displays in the damselfish, Dascyllus albisella (Pomacentridae)

Acoustic signals are well established as key components of mate selection in terrestrial species, but not in aquatic species. It has long been known that damselfish (Pomacentridae) use a combined visual and acoustic display in their courtship.. This study examined several male qualities including individual size, courtship vigor, territory size and complexity, as well as components of the acoustic call including dominant frequency, pulse characteristics and repetition rate. The objective was to determine which male traits were correlated with mating success. Observations made over ten reproductive cycles revealed that female mate choices were not random and that male mating success was correlated with courtship rate (a simultaneous visual and acoustic cue) and the number of neighboring females, but not with male morphological traits, territory quality, or acoustic call structure. These results suggest that females choose mates based on a condition-dependent trait (courtship rate) that advertises quality of paternal care, which supports good parent models of sexual selection, thereby demonstrating the importance of the combined acoustic/visual display for sexual selection in fishes     

Mating rendezvous in monogenean gill parasites of the humbug Dascyllus aruanus (Pisces: Pomacentridae)

The gills of the humbug, Dascyllus aruanus (Pomacentridae), were infected by a monogenean genus Haliotrema at a high prevalence (83%) but with a low mean intensity (5.6 worms/fish). All the gill arches of 365 fish, caught on the fringing reef of Moorea Island (French Polynesia), were examined for parasites. Each hemibranch was divided into 12 subequal sections. Monogeneans showing microhabitat overlap were defined as couples. Hosts with low intensity of infection (fewer than 5 monogeneans per gill) were selected and couples were recorded. Among the 37 hosts harboring 2 worms on their gills, 18 fish were infected with these 2 monogeneans on the same gill side of the body; 50% (n = 9) of these harbored monogeneans within the same gill arch and 55% (n = 5) of these last fish showed individual parasites within the same section of the gill. In the case of hosts with few monogeneans (3 and 4 individuals; n = 37) on the same arch, more than 40% (n = 16) harbored worms in couples. There may be some chemical communication that allowed these monogeneans to migrate toward each other and thus enhance mating success. Mating rendezvous appears to be a more important factor than site location for these gill monogeneans.     

Historical aspects of protandrous sex change in the anemonefish Amphiprion melanopus (Pomacentridae, Teleostei)

Gonadal structure and cellular composition were examined in juveniles, males and females of the protandric hermaphrodite, Amphiprion melanopus. Functional sex change was experimentally induced in the field and gonad structure was histologically examined both qualitatively and quantitatively at 10, 20, 30 and 45 days after its initiation. Juvenile gonads consist primarily of immature ovarian tissue. Functional male gonads are ovotestes with co-existing mature spermatogenic tissue and immature ovarian tissue, while females possess only ovarian tissue. The initiation of sex change is marked by a rapid maturation of spermatogenic tissue and proliferation of putative oogonia. Gonads were essentially female by 20 days into sex change, but evidence of mature female function (marked by the initiation of vitellogenesis) was not observed until 45 days. Considerable variation between individuals was seen in quantitative measures of gonadal change in the early stages of sex change, but not in later stages. Progress in sex change as indicated by histological indicators was, however, consistent within stages. Duet systems for gamete transport changed from the male to the female form after all male tissue had been replaced.     

On the coexistence of three predatory stonefly species in a central Japanese stream

Three large-bodied stonefly species (Paragnetina tinctipennis, Oyamia lugubris, and Kamimuria tibialis) coexist in a central Japanese stream. These species have been classified as predators. Here we study their microhabitat use while focusing on the physical environments, physiological activity, and food resources. We show that Paragnetina uses a niche with faster currents than other species throughout the year. Oyamia has seasonal flexibility in microhabitat preference, physiological activity, and food resources. Kamimuria is a rather stable species, independent of seasonal patterns.     

Unusual cell organelles during spermiogenesis in two species of gobies (Gobiidae,Teleostei)

Summary Testes of Lesuerigobius friesii and Gobius bucchichi were studied in adult reproductive fish. During the onset of spermatid development, a peculiar system of alternating rough (RER) and smooth (SER) endoplasmic reticular tubules form rings distally to the cell nucleus. The RER tubules are seen to possess up to 12 ribosomes in cross-section, whereas the SER are strongly electron-dense. Nanotubules connect these stacks of tubules to the developing head and tail of the sperm. With ripening of the sperm these tubules disintegrate within the excessive cytoplasm. It seems likely that these are special forms of Macro-Golgi System that possibly provide protamines for the developing sperm.     

Attack intensity by two species of territorial damselfish (Pomacentridae) as estimates of competitive overlap with two species of wrasse (Labridae)

The intensity of interspecific territorial defense should be based upon the degree of competitive overlap. We tested this relationship in two territorial Caribbean damselfish (dusky, Stegastes adustus, and longfin, S. diencaeous) with intruders being the bluehead wrasse (Thalassoma bifascatus) and the slippery dick wrasse (Halichoeres bivittatus). Based on food habits, the slippery dick and the bluehead wrasse should have the same degree of competitive overlap to the two damselfish species. We also predicted that the larger slippery dick wrasse intruder would receive more aggression than the smaller bluehead wrasse intruder. Neither damselfish species distinguished between the two wrasses suggesting that they were ecologically equivalent. We also tested size differences within both species of wrasse and found that size had no influence on the aggression in the dusky damselfish. The longfin also did not show a size preference for the bluehead but did prefer to attack the larger slippery dick wrasse. In spite of the similarities between the dusky and the longfin damselfish and the similar food habits of the bluehead and the slippery dick wrasse, our results suggest that using intensity of interspecific territorial defense alone may not be an adequate measure of competitive overlap.     

Bioaugmentation and coexistence of two functionally similar bacterial strains in aerobic granules

The survival of the inoculated microbial culture is critical for successful bioaugmentation but impossible to predict precisely. As an alternative strategy, bioaugmentation of a group of microorganisms may improve reliability of bioaugmentation. This study evaluated simultaneous bioaugmentation of two functionally similar bacterial strains in aerobic granules. The two strains, Pandoraea sp. PG-01 and Rhodococcus erythropolis PG-03, showed high phenol degradation and growth rates in phenol medium, but they were characterized as having a poor aggregation activity and weak bioflocculant-producing and biofilm-forming abilities. In the spatially homogeneous batch conditions, strain PG-01 with higher growth rates outcompeted strain PG-03. However, the two strains could stably coexist in the spatially heterogeneous conditions. Then the two strains were mixed and bioaugmented into activated sludge in two sequencing batch reactors, which were operated with the different settling times of 5 and 30 min, respectively. Aerobic granules were developed only in the reactor with a settling time of 5 min. Fluorescence in situ hybridization and denaturing gradient gel electrophoresis showed that the two strains could coexist in aerobic granules but not in activated sludge. These findings suggested that the compact structure of aerobic granules provided spatial isolation for coexistence of competitively superior and inferior strains with similar functions.     

Germinal potentialities during sexual state changes in a protandric hermaphrodite, Amphipnon frenatus (Teleostei, Pomacentridae)

In Amphiprion frenatus , a protandric hermaphrodite, male sex inversion is characterized by a decrease of spermatogenic activity in the ovotestis followed by a degeneration of male gevm cells and an increase of oogenic activity. Among female germ cells, undifferentiated primordial germ cells (PGCs) were identified; their participation in building up the ovary is suggested. In addition, the unusual association of juveniles with a single adull member or juvenile groupings lacking the presence of a monogamous pair, induced in juveniles the anticipated sex differentiations (in either male or female orientation) in which not only spermatogonia and oogonia but also PGCs are involved.     

Strategies for coexistence in two species of New Zealand Hydropsychidae (Trichoptera)

Strategies for the coexistence of two caddisflies,Aoteapsyche raruraru and A. colonica(Trichoptera: Hydropsychidae) were considered in aSouth Island lake outlet, New Zealand. Three mainstrategies were assessed, firstly that competitionbetween species may be reduced by the presence ofselective predators, secondly, that coexistence ismaintained by periodic disturbance which reducescompetition effects, and thirdly that interspecificcompetition is negated by segregation of either lifehistories or feeding strategies, and diet ormicrohabitat preferences. The first strategy wasrejected, as gut analyses of common fish andmacroinvertebrate predators showed that both speciesof Aoteapsyche were taken in approximately equalproportions to their benthic densities (i.e. 10:1 A. raruraru to A. colonica), indicating thatpredation was unlikely to influence coexistence.Similarly the second strategy was not supported byobservations of flow conditions during the study whichwere insufficient to move the substrata extensivelycolonised by both species of Aoteapsyche.Finally, temporal segregation of life histories wasnot observed, but analyses of larval guts indicatedthat diet was affected by shelter location on thesubstrate. Sampling of substrate microhabitats showedthat A. raruraru larvae occurred on the uppersurfaces, sides and under surfaces of large cobbles,although significantly higher densities were collectedfrom the upper surfaces and sides. In contrast, A. colonica aggregated on the sides and undersurfacesof cobbles. In other streams A. colonica showsa similar microdistribution but in a silted streamlacking seston, and in the absence of A. colonica, A. raruraru larvae occurred mainly onthe under surfaces of stones. Co-existence of thesetwo congeners would seem to be possible bymicrohabitat segregation where food availability ishigh, however in the absence of a plentiful foodsupply and A. colonica the microdistribution ofA. raruraru may differ.