Reproduction and annual gametogenic cycle in Nephtys hombergii and N. caeca (Polychaeta: Nephtyidae)

The gametogenic cycles of Nephtys hombergii Audouin et M. Edw. and N. caeca Fabricius are described. Both species are polytelic (=discrete, iteroparous) and breed in their second and subsequent years. They spawn in May in the Tyne estuary. After spawning, N. hombergii retains a large number of gametocytes, which are ultimately resorbed. A new generation of gametocytes is proliferated by September and mature gametes accumulate during the winter months. Gametic development is not well synchronised.     

Reproductive energetics,endocrinology and population dynamics of Nephtys caeca and N. hombergi

An investigation of gametogenetic cycles and spawning success in populations of Nephtys caeca and N. hombergi from the River Tyne estuary, North Shields, England, in relation to production of gonadotrophic (GH) and spawning hormones (SH) has been made, based on data collected from 1978–1984. The data show that a consistent pattern of gametogenesis and efficient spawning occurs in populations of N. caeca in north-east England, but that the reproductive cycle of N. hombergi is erratic. Premature oosorption and gametogenic failure in this latter species is associated with a low level of GH production. Following high levels of GH production, gravid females sometimes fail to spawn owing to the non-release of SH. The reproductive energetics and reproductive success of N. caeca and N. hombergi are described in relation to the observed patterns of endocrine activity and the consequent changes in population structure.     

Population dynamics and secondary production in an estuarine population of Nephtys hombergii (Polychaeta: Nephtyidae)

From July 1978 to March 1980, a study was made on the distribution, population dynamics and secondary production of Nephtys hombergii Audouin et Edw. occurring in the sublittoral industrialised region of Southampton Water in south England. The distribution of the worm was related to the silt content and copper level of the sediment, the greatest densities of N. hombergii being found in sediment containing 60 to 100% silt. Breeding occurred at a low level throughout the year, with a maximum in July to September and November to January in the second year of growth. Spawning occurred when the oocytes measured 200m in diameter, and unshed gametes were resorbed. Annual production varied between 0.092 and 4.32 g C m^-2 yr^-1 (ash-free dry weight) and amounted to 1.9–39.4% of the total macrofaunal production at the sampling stations. The production:biomass (P:B) ratio of the species varied between 1.6 and 2.9.     

Local variation in mortality rates and life-expectancy estimates of the coral-reef fish Centropyge bicolor (Pisces: Pomacanthidae)

Mortality rates of the coral reef fish Centropyge bicolor were based on the disappearance of known individual fish. Data were collected over three years from 1978 at four sites around Lizard Island, Queensland, Australia. To measure disappearance, membership of 36 harems (comprising about 150 fish at any one time) was determined at bimonthly intervals. Mortality rates varied significantly among the four study areas. A tenfold difference in mean mortality occurred between two of the areas. The ranking of areas according to mortality rate was similar for 1978–1979 and 1979–1980. The ranking of areas was again similar for 1980–1984 when mortality rates were determined from an additional census in June 1984. Average further life-expectancy of mature fish based on mortality rates varied from 1 yr in one study area to 5 to 13 yr in a different area.     

Population dynamics and reproductive cycle of Pholoë minuta (Polychaeta: Sigalionidae) in Galway Bay

Pholoë minuta (Fabricius) is a prominent member of the Amphiura filiformis (O.F. Müller) subcommunity in Galway Bay, Ireland. The aim of the present investigation was to study the population dynamics and reproductive cycle of P. minuta in this area. Monthly sampling over a 2 yr period (November 1978–October 1980), at a permanent station (18 m) with a fine sand substrate, revealed a mean density of 96 individuals per 0.1 m². The width of the fifth elytron was correlated with total body length and was used as a basis for analysing population structure. Four distinct year classes were evident and estimates of recruitment levels and growth rates were calculated. P. minuta is polytelic and reproduces for the first time when approaching 3 yr of age. Spawning occurred in March—April and appeared to be associated with a seasonal low in bottom-water temperature.     

Influence of mussel rafts on spatial and seasonal abundance of crabs in the Ría de Arousa,North-West Spain

The crab community in the Ría de Arousa, North-West Spain, was studied for 2 yr (1974–1975; 1975–1976), firstly to detect any differences due to mussel (Mytilus edulis) raft aquaculture; secondly to study temporal changes in spatial distributions in beach, raft and non-raft areas throughout the ría. Portunid crabs dominated the fauna, the dominant species being Macropipus puber and M. depurator in the raft areas, M. depurator in non-raft subtidal areas, and M. arcuatus in beach stations. No substantial seasonal changes were observed in species composition throughout the ría. In general, the highest densities occurred in fall and winter. Raft stations showed the highest values of abundance, up to 0.47 individual per m², and 8.20 g wet wt m^-2. The food resource provided by the rich epifauna associated with the mussel rafts, and changes in the sediment due to shell deposits could account for the high density values in raft areas. Raft areas (polygons) cover about 10% of the surface of the Ría de Arousa and, thus, the raft aquaculture of mussels significantly increases crab production.     

Long-term changes in populations of subtidal bivalves (Abra alba and A. prismatica) from the Bay of Morlaix (Western English Channel)

Two populations of Abra alba (Wood) and one of A. prismatica (Montagu) (Mollusca: Bivalvia) were studied over a 10 yr period (1977–1987) in two muddy fine-sand subtidal communities of the Bay of Morlaix, France. The survey provided an example of long-term changes in the three Abra spp. populations, which displayed synchronized changes, with a regular annual cycle and increasing densities during 1979–1980 related to the higher concentration of organic matter resulting from the Amoco Cadiz oil spill in March 1978. A. alba rapidly adapts its demographic strategy to eutrophic conditions by increasing its reproductive potential, growth, and abundance. During times of eutrophication, A. alba has three spawning periods and three recruitments per year as opposed to two spawning periods and two recruitments per year during oligotrophic conditions. Growth of the juveniles of this species is insignificant until spring for individuals recruited in the autumn, whereas individuals which settle during spring or summer display immediate rapid growth. A. prismatica has a low capacity to adapt to eutrophic conditions. It has one annual period of sexual maturation at the end of the summer, with spawning in September–October and settlement beginning in mid-November. Growth of the juveniles after settlement is also insignificant until April. These results enable comparison of the demographic strategies of these two sympatric species.     

An investigation on the anaerobic metabolism of Nephtys hombergii (Annelida: Polychaeta)

In Nephtys hombergii Savigny (Annelida: Polychaeta: Errantia), succinate and less pronounced L-alanine are accumulated as endproducts during an initial phase of anaerobiosis. In this phase aspartate is utilized as substrate for anaerobic energy production in addition to glycogen. Prolonged anaerobiosis results in the formation of propionate and acetate which, to a large extent, are excreted into the water. The concentrations of aspartate and succinate were found to remain unchanged down to a Pw_{O 2}(oxygen partial pressure in incubation water) of approximately 20 torr, indicating a fully aerobic metabolism. At a Pw_{O 2}of 15 torr characteristic changes can be observed: energy production becomes partially anaerobic. At a Pw_{O 2}of 7 torr the level of aspartate is largely reduced, but the accumulation of succinate is still significantly less than at anoxia. [In the habitat of N. hombergii, the intertidal mud flats, a Pw_{O 2}in the range from 0 torr (Pamatmat, 1968) to 12 torr (Wells and Warren, 1975) was measured.] The activities of some important enzymes involved in anaerobic energy production (among them pyruvate kinase, phosphoenolpyruvate carboxykinase and lactate dehydrogenase) were measured. Although N. hombergii has a high glycolytic capacity, no lactate dehydrogenase was detected. Instead strombine and alanopine dehydrogenases were present in comparatively high activities.     

Population dynamics and secondary production in Melinna palmata (Polychaeta: Ampharetidae)

A Southampton Water (England) population of Melinna palmata Malmgren was studied from July 1978 to March 1980. M. palmata is a gregarious sedentary polychaete living in muddy substratum. The distribution of the polychaete in Southampton Water was related to the silt content of the sediment. The species occurs in sediments of <60% silt, it is dioecious, and the gametes are shed during a prolonged breeding season throughout the year, with peaks in March and July. Mature oocytes measure between 180 and 240 m in diameter at spawning, and fertilization is external. Unshed oocytes are resorbed. Annual secondary ary production was 0.42 gC m^–2yr^–1 and the P:B ratio was 2.19.     

Nephtys hombergii, a free-living predator in marine sediments: energy production under environmental stress

Nephtys hombergii is a free-living, burrowing predator in marine sediments. The worm is, therefore, exposed to various environmental conditions which tube-dwelling polychaetes of the same habitat most likely do not encounter. The worms have to survive periods of severe hypoxia and sulphide exposure, while at the same time, they have to maintain agility in order to feed on other invertebrates. N. hombergii is adapted to these conditions by utilising several strategies. The species has a remarkably high content of phosphagen (phosphoglycocyamine), which is the primary energy source during periods of environmental stress. With increasing hypoxia, energy is also provided via anaerobic glycolysis (pO₂<7 kPa), with strombine as the main end-product. Energy production via the succinate pathway becomes important only under severe hypoxia (<2 kPa), suggesting a biphasic response to low oxygen conditions which probably is related to the worm's mode of life. The presence of sulphide resulted in a higher anaerobic energy flux and a more pronounced energy production via glycolysis than in anoxia alone. Nevertheless, after sulphide exposure under anaerobic conditions of <24 h, N. hombergii is able to recover completely. Although N. hombergii appears to be well adapted to a habitat with short-term fluctuations in oxygen and appearance of hydrogen sulphide, its high energy demand as a predator renders it likely to limit its survival in an environment with longer lasting anoxia and concomitant sulphide exposure. Received: 28 May 1997 / Accepted: 21 June 1997     

Seasonal changes of benthic megafauna in the Ría de Muros e Noia (Galicia,North-West Spain)

A community of decapod crustaceans (Brachyura) was sampled seasonally (October 1978–July 1979) from three habitats (raft, middle and beach) in the Ría de Muros e Noia (North-West Spain), with the purpose of studying spatial and temporal changes in the community and comparing with communities in the neighbouring Ría de Arousa, which supports an intense mussel (Mytilus edulis)-raft culture. The Portunidae family dominated the decapod community. Polybius henslowi, a species with pelagic stages, which enters the rías periodically in large numbers, was the dominant species at all the stations throughout the sampling period, attaining densities of 1.6 individuals m^-2 (18.7 g wet wt m^-2) in summer. Macropipus depurator and M. puber were the next most important species at the raft station, M. depurator at middle stations, and M. vernalis and Carcinus maenas at beach stations. The highest population densities were recorded in summer, due to the great abundance of Polybius henslowi at this time. When the data were reconsidered omitting p. henslowi, highest densities were in autumn and winter in the inner ría. In general, the nature of the substratum, the presence of mussel rafts, depth and salinity were the main factors determining the structure of the community. Density and biomass in the Ría de Muros e Noia (P. henslowi omitted) were lower than in the Ría de Arousa (up to six times lower in some areas). The Ría de Muros lacks the extensive number of mussel rafts present in the Ría de Arousa which constitute an important food resource for decapods.     

Population and species diversity fluctuations in a rocky intertidal community relative to severe aerial exposure and sediment burial

From a series of 10 quarterly assessment between October 1975 and May 1978 (inclusive), fluctuations in abundance were determined for macroinvertebrates and macrophytes of a rocky intertidal community on Santa Cruz Island, California, USA. During afternoon low tides in late fall and winter of the first 2 yr of study, organisms of the upper and middle intertidal zones were subjected to prolonged aerial exposure. Many species there tolerated this exposure, but die-backs occurred for a barnacle (Chthamalus fissus/C. dalli) and several algae (Endocladia muricata, Pelvetia fastigiata f. gracilis, Corallina officinalis var. chilensis, Corallina vancouveriensis, Cylindrocarpus rugosus and Codium fragile). These die-backs were succeeded by blooms of Ulva californica and Porphyra perforata. In the upper and middle intertidal zones, the major cover organisms that could tolerate prolonged aerial exposure were disproportionately prevalent and appeared to be maintained by the periodic repression of species intolerant to such exposure. In February 1978, heavy rainfall caused sediment inundation of the middle and lower intertidal zones. This event was followed by declines in abundance of a barnacle (Tetraclita rubescens) and several algae (Pelvetia fastigiata f. gracilis and corallina spp.). Shannon-Wiener H species diversity fluctuated from a high in October to a low the following May during both 1975–1976 and 1976–1977 in conjunction with the period of increased daytime aerial exposure in late fall and winter. A further decline in diversity following sediment inundation in February 1978 contributed to a long-term trend of declining H species diversity (3.06 in October 1975 to 1.87 in May 1978). We hypothesize that predictable late fall to winter aerial exposure stresses, in combination with a random physical disturbance (sediment burial), exceeded an optimal intermediate level of disturbance predicted for maximal species diversity.     

Population dynamics of the acorn barnacles, Tetraclita squamosa and Tetraclita japonica (Cirripedia: Balanomorpha), in Hong Kong

Tetraclita squamosa and Tetraclita japonica are common, intertidal barnacles. In Hong Kong, the population dynamics of the two species exhibited spatial and temporal variation on two semi-exposed shores. T. squamosa produced egg masses from May–June and annual settlement and recruitment occurred from June–July. In contrast, settlement and recruitment of T. japonica was sparse from March–May, but intensified in July–October. Mature gonads and egg masses were, however, only present from September–November, suggesting the larvae from the two settlement pulses originated from other locations as well as Hong Kong populations. Settlement intensity and post-recruitment mortality of the two species varied between sites, possibly due to spatial variation in free rock space, physical transport of larvae and abundance of food in the water column.Communicated by T. Ikeda, Hakodate     

The importance of sandflat morphology to recruitment of the intertidal snail Nassarius pauperatus during ten consecutive years at three sites in South Australia

This project was desinged to confirm the temporal consistency of inter-site differences in recruitment of an intertidal gastropod, and to test the hypothesis that these differences are accounted for by differences in shore topography. Annual recruitment of the intertidal sanil Nassarius pauperatus (Lamarck) was estimated at two sandflats in Gulf St Vincent, South Australia, over a period of 12 yr (1979 to 1990). At one sandflat the gradient was slight, the zone containing N. pauperatus was wide, and there were extensive pools of water at low tide, whilst at the other sandflat the gradient was steeper, the zone containing N. pauperatus was narrower, and there were fewer or no pools. Although recruitment at both sites varied temporally, the density of recruits was always higher at the first site. These data, together with data covering 10 yr (1981–1990) from a sandflat where the gradient of the substratum was decreasing due to accretion of sediment, suggest that shore topography is an important determinant of relative recruitment for N. pauperatus in South Australia.     

Habitat harshness and morphodynamics: life history traits of the mole crab Emerita brasiliensis in Uruguayan sandy beaches

The habitat harshness hypothesis (HHH) postulates that in reflective beaches the harsh environment forces organisms to divert more energy towards maintenance and they therefore have lower abundance, fecundity, growth and survival rates than in dissipative beaches. Recent investigations have tested this hypothesis through single comparisons of only two beaches, and thus the observed trends in population level variables cannot be attributed incontestably to the beach state, but only to location. Here, abundance, reproduction, recruitment, population structure and body size of the intertidal mole crab Emerita brasiliensis were compared between populations from eight microtidal exposed sandy beaches with contrasting morphodynamics, sampled bimonthly during 22 months throughout the 180 km Uruguayan Atlantic coast. Physical variables and compound indices of the beach state were used to categorize sandy beaches. The results of this bi-annual large-scale analysis were fully consistent with the predictions of the HHH: abundance (total and population components), duration of the reproduction and recruitment seasons and the individual size of megalops and females of the mole crab E. brasiliensis decreased from dissipative to reflective beaches. This was reflected by linear or, mostly, nonlinear relationships between biological and both physical variables and compound indices of beach state. In conclusion, this multi-beach sampling provides compelling evidence of a consistent response of demographic and life history traits of an intertidal beach species to morphodynamic characteristics.     

Tisular and population level responses to habitat harshness in sandy beaches: the reproductive strategy of <Emphasis Type="Italic">Donax hanleyanus</Emphasis>

Microscopic analysis and field sampling procedures were used to compare demographic and reproductive strategies of the intertidal wedge clam Donax hanleyanus (Bivalvia: Donacidae) in two exposed sandy beaches with contrasting morphodynamics (reflective vs. dissipative) during 13 consecutive months. Histological analysis showed that: (1) the reproductive cycle of D. hanleyanus was more extended in the dissipative beach, and this was true for all the three pre-active (beginning of gonadal activity), active (maturation) and spawning stages; and (2) males and females showed significantly smaller sizes at sexual maturity at the reflective beach. Even though successive increments in proportion and mature at size were observed, the sigmoid function was significantly steeper at the reflective Arachania for both sexes, suggesting an abrupt transition to maturity. Field sampling revealed a more extended recruitment period at the dissipative beach, where recruits were also significantly more abundant than at the reflective beach. These results give support for the habitat harshness hypothesis, which predicts that in intertidal species capable of sustaining populations across a wide spectrum of physical conditions, such as D. hanleyanus, abundance, recruitment, size at maturity and extent of reproductive and recruitment seasons increase from reflective to dissipative beaches. However, a recent hypothesis suggests that reflective beaches acting as sink populations were not sustained, because mature and spawning individuals of both sexes were found in the reflective beach throughout the study period. Thus, we suggest that post-settlement processes are critical in modulating population patterns for this bivalve.     

Abundances of surgeonfishes on patch-reefs in Caribbean Panamá: due to settlement,or post-settlement events?

I examined whether patterns of settlement of planktonic juveniles determined the size and composition of adult populations of three Caribbean surgeonfishes (A canthurus bahianus, A. chirurgus and A. coeruleus) on six isolated patch-reefs in Panamá. I monitored settlement monthly on six reefs (0.14 to 0.67 ha) for 8 yr (1978–1986), and censused adults on those reefs annually. Year-to-year fluctuations in the absolute and relative abundances of settlers arriving on each reef were not extreme. Densities of adults in 1986 were not correlated with the densities of settlers that arrived between 1979 and 1984. Survival of settlers, although apparently unrelated to the density of adults, was negatively related to the density of settlers per unit of adult habitat (but not per unit of settlement habitat) for each species. Changes in adult populations of each species between 1978 and 1986 were not correlated with the abundance of settlers relative to the initial number of adults. The relative abundances of adults of each species in 1986 did not reflect the relative abundances of settlers (1979–1984); these abundances were due to both spatial variation and interspecific variation in survival. In some cases, relocation of adults between isolated patch-reefs (as much as 100 m from other reefs) substantially altered the size and composition of surgeonfish populations. Thus, either post-settlement mortality or relocation overrode effects of settlement patterns in determining adult abundances on the six reefs. Relocation is important in organizing reef-fish communities and must be further examined, particularly in large habitat mosaics.     

Comparison of spatial overlap between the polychaetes Nereis virens and Nephtys caeca in two intertidal estuarine environments

An intensive study of spatial overlap between the polychaetes Nereis virens (Sars) and Nephtys caeca (Fabricius) was conducted in 1992 on two tidal sand flats, which differ by their exposure to dominant winds and residual currents in the Lower St. Lawrence Estuary (Québec, Canada). Results showed that spatial overlap (Lloyd's index and spatial distribution) was higher among adults of both species at the lower tidal elevation of l'Anse-à-l'Orignal (north-east oriented). Results also suggest weaker interspecific interactions among juveniles of both species because of limited spatial overlap. In Baie-du-Ha!Ha! (south-west oriented), spatial overlap was greater than that observed in l'Anse-à-l'Orignal and appeared important in adults as well as in juveniles. Levins' directional measure of competition indicated an asymmetric spatial overlap between N. virens and N. caeca in Baie-du-Ha!Ha! and a symmetric overlap in l'Anse-à-l'Orignal. The intraspecific encounter values, estimated from Lloyd's mathematical expression, were significantly higher than values of interspecific interactions only in l'Anse-à-l'Orignal. Moreover, no vertical stratification was found inside the sediment, with no effect of the densities and individual body weights of the polychaetes. Juveniles of both species mainly inhabited the organic-rich upper portion of the sediments (0 to 12 cm), while adults colonised greater depths (>25 cm) where organic matter content was lower. A complementary field experiment was conducted in 1993 to investigate interspecific interactions (predation and competition) existing between N. virens and N. caeca. Results from this experiment depend on which species was first-introduced and showed an important variation in mortality rates between allopatric and sympatric conditions. The influence of competition and predation on the structure of these populations is also discussed in relation to their spatial overlap.     

Distributions and population structures of two intertidal estuarine polychaetes in the lower St. Lawrence Estuary,with special reference to environmental factors

An intensive study of the spatial distribution ofNereis virens (Sars) andNephtys caeca (Fabricius) was conducted during the spring and autumn of 1986 in the lower St. Lawrence estuary. Statistical analysis showed that spatial variations in density, individual body weight and sexual maturity, particularly in the case ofNereis virens, are correlated with the intertidal level, with certain sediment characteristics, and with the thickness of the colonizable sediment layer. The density ofN. virens increases in an offshore-onshore direction, whereas that ofNephtys caeca decreases in the same direction. For both species, mean body weight increases downshore from the upper intertidal level. Other specific relationships exist in relation to sediment characteristics. Sexually matureNereis virens are found only at the lower intertidal level; sexual maturity inNephtys caeca was not studied. There were no changes in spatial distribution patterns between spring and autumn. The spatial distribution ofNereis virens parallels depth contours and may reflect its ability to inhabit environments which become more physically unstable in an offshore-onshore direction. This spatial distribution is consistent with a model whereby larvae are recruited in the upper intertidal zone and juveniles migrate downshore.     

Color patterns of the shrimps Heptacarpus pictus and H. paludicola (Caridea: Hippolytidae)

Color patterns of the shallow-water shrimps Heptacarpus pictus and H. paludicola are formed by chromatosomes (usually termed chromatophores) located beneath the translucent exoskeleton. Development of color patterns is related to size (age) and sex. The color expressed is determined by the chromatosome pigment dispersion, arrangement, and density. In populations with well-developed coloration (H. pictus from Cayucos, California, 1976–1978, H. paludicola from Argyle Channel, San Juan Island, Washington, June–July, 1978), prominent coloration was a characteristic of maturing females, breeding females, and some of the larger males. In the Morro Bay, California, population of H. paludicola (sampled 1976–1978), color patterns were poorly developed except in a few large females. In both species, most shrimp lose color at night because of pigment retraction in certain chromatosomes. In both species, there are 5 basic morphs: 1 transparent and 4 colored morphs. In the colored morphs, the color patterns are composed of bands, stripes, and spots which appear to disrupt the body outline. Each color morph also has a common environmental color in its color pattern, e.g the green of green algae, the whites and pinks of dead and living coralline algae, and various shades of tidepool litter. These shrimps are apparently under heavy predation pressure by fish, and it is suggested that the color patterns are camouflage against such visually-hunting predators.