A comparative study of feeding and growth in two coexisting species of carnivorous gastropods

The effects of variable food supply on growth rate and feeding behaviour in two coexisting intertidal gastropods were examined through field and laboratory experiments involving mark and recapture procedures, growth experiments under controlled food supply and video observations of feeding. All cohorts of both Japeuthria species grew most in summer, with the highest growth rate being achieved by the youngest cohort of both sexes. In contrast, almost all cohorts (except the youngest one) showed zero to negative growth in winter. The growth rates of Japeuthria species clearly varied depending on feeding frequencies and the initial size of individuals. Females of both species had a significantly higher growth rate than males, though the difference became less marked with decreasing food supply. The total time spent on feeding and the length of the first feeding bout generally increased as the duration of the interval between meals was increased across treatments. From both the field growth experiment and the observations of feeding behaviour it is clear that J. ferrea had a higher frequency of feeding than J. cingulata in the field; 1- to 3-d intervals of feeding frequency in the former and 3- to 6-d intervals in the latter. Most notably, the two species showed a reversal in growth rates with changing conditions of food supply. When food was relatively abundant J. ferrea achieved better growth than J. cingulata, but J. cingulata grew better than J. ferrea under low food supply. In view of the fact that J. ferrea seems to be competitively superior to J. cingulata under normal levels, the present observation that J. cingulata can perform better than J. ferrea when food availability is low gives an important advantage to the latter in terms of its coexistence with the former. Received: 9 March 1999 / Accepted: 8 October 1999     

Population structure,food relations and ecological role of marine oligochaetes,with special reference to meiobenthic species

Data on abundance, biomass and biovolume demonstrate the significant ecological role of oligochaetes in the littoral marine benthos. Their numerical and productive importance is comparable to that of many other common meio-and macrofauna groups from various littoral areas. Oligochaetes often exhibit nutritional specialization (e.g. bacteria or diatoms attached to detritus or sand grains). Consequently, food supply can control their population structure and distribution. Few oligochaetes are, apparently, consumed by predators. Hence, only a small portion of their biomass is transferred to higher trophic levels, while the main part is decomposed directly. Most oligochaetes seem to represent final links of rather short food chains. Ecologically, marine oligochaetes attain major importance only in littoral areas.This investigation was supported by the Sonderforschungsbereich 94 (Meeresforschung) at the University of Hamburg.     

Size and structure of echinoderm populations associated with different coexisting coral species at Aldabra Atoll,Seychelles

The associated echinoderm fauna of 125 living heads of massive microatolls of Porites spp., branched heads of Porites nigrescens Dana and small, densely packed heads of the hydrocoral Millepora exacea Forskål of a shallow sublittoral lagoon flat at Aldabra Atoll, Seychelles was examined. The weight and number of echinoderms increased with coral size in all cases. The microatolls were the largest coral heads and sheltered the most diverse, underlying echinoderm cryptofauna comprising the largest bodied species, regardless of class or family. The infauna in heads of P. nigrescens was less diverse and smaller bodied than the echinoderms under microatolls. Only with P. nigrescens did species number increase with increasing coral size. Heads of M. exacea were the smallest and provided a habitat of compact interstitial spaces for the most depauperate, smallest bodied infauna which included juveniles of larger species. Ophiuroids dominated the coral-associated echinoderm faunas whereas larger bodied classes, such as echinoids and holothuroids, were less well represented. Large bodied ophiuroids such as certain ophiodermatids occurred only under microatolls, whereas the usually small bodied ophiotrichids dominated in the heads of M. exacea. The generally ubiquitous ophiocomids were well represented in, or under all corals. Heads of M. exacea could be nursery areas for recently metamorphosed juvenile ophiuroids and echinoids which, as they grow, seek shelter in corals with larger interstitial spaces such as P. nigrescens. Significant interspecific ophiuroid associations were related to space availability for larger species under microatolls and in heads of P. nigrescens. The possibility of microhabitat partitioning in corals among some ophiuroids is suggested.     

Growth,stomach contents and parasites in a tidepool population of the hippolytid shrimp Heptacarpus futilirostris,with reference to reproduction

The biology of the hippolytid shrimp Heptacarpus futilirostris (Bate) was studied in a tidepool at Kominato, central Japan from January 1983 to June 1984. In males, the relative growth ratio between the third maxilliped length and body length revealed a turning point at approximately 15 mm in body length. Population recruitment was presumed to continue from April to December. The range of body length did not increase from July to October, and thereafter increased rapidly in both sexes up to a maximum range, i.e. from 6 to 29 mm in January. The wide range was presumably due to the simultaneous occurrence of rapid growth with population recruitment. The growth rates of size groups above and below 15 mm were different in the males observed in January. Large males disappeared, probably due to death, and small males participated in reproduction after July. The growth rate increased from late autumn to spring and decreased from summer to early autumn, and was influenced by relative food abundance, especially seaweeds. Two parasitic isopods were identified: Bopyrinella antilensis nipponica in the branchial cavity, and Epiphryxus sp. on the abdomen. It is suggested that parasites have more influence on maturity than on growth. Ovigerous females were found from January to October, with the breeding season peak between April and June. The relationship between female body length (L) and clutch size (N) was expressed by the regression equation: N=5.11 L^1.6347 (r=0.57). This species is identified as a multiple breeder. The relatively large number of larvae presumably compensates for the low larval survival rate.     

Prey defense,predator preference,and nonrandom diet: The interactions between Pycnopodia helianthoides and two species of sea urchins

Interactions between the predatory sea star Pycnopodia helianthoides (Brandt, 1835) and two of its natural prey, the sea urchins Strongylocentrotus purpuratus (Stimpson, 1857) and S. franciscanus (Agassiz, 1863), are examined with regard to predator preference, predator diet, and prey defenses. The sea star is able to detect both species of sea urchin upstream in a Y-trough, but does not consistently choose one over the other (i.e., no preference). However, when the sea star is presented with equal numbers of similar-sized specimens of the two species of sea urchin, its diet is markedly nonrandom, since S. purpuratus is eaten almost 98% of the time. The defensive responses of the two species of sea urchin differ in form and effectiveness. S. franciscanus employs its long spines as defensive weapons, pinching the rays of an attacking sea star. This defensive response is more effective than the pedicellarial response used by S. purpuratus. The nonrandom diet of the predator seems to result primarily from prey defensive responses that differ in effectiveness, rather than from an intrinsic, behavioral preference of the predator at an earlier stage in the predator/prey interaction.     

Distribution pattern, reproductive traits, and molecular analysis of two coexisting vermetid gastropods of the genus Petaloconchus: a Caribbean endemic and a potential invasive species

Vermetid gastropods are characterized by complex taxonomy and unusual ecology. A survey of the fouling community in the intake channel at the Planta Centro Power Plan in Puerto Cabello, Venezuela, found massive colonies of vermetid gastropods of the genus Petaloconchus. We arbitrarily named two prevalent varieties as black and brown-orange morphs, distinguishing based on the color of their soft bodies. Spatial distribution was different for the morphs. The black morph was present along the jetty, with higher average densities in the shallower intertidal area (410 vs. 143 ind m⁻²), while the brown-orange morph was only present at the initial part of the channel (μ = 83 ind m⁻²). Both produced small eggs (142 vs. 180 μm diameter on average), with the orange-brown eggs being slightly larger, but the morphs differed in other reproductive aspects. The brown-orange morph produced significantly fewer capsules (up to nine simultaneously per female) with fewer embryos (average of 27 viable embryos/capsule) that hatched at a larger size (577 μm on average), allocating about 17.64% of the initial egg production to nurse eggs. In contrast, the black morph produced up to 14 capsules simultaneously, hatched an average of 178 veligers of 212 μm, and produced no nurse eggs. The intra-capsular development reached a more advanced stage in the brown-orange morph than in the black one. Several cytochrome subunit I (COI) and 16S rRNA (16S) haplotypes were found for the black morph, when compared to only one haplotype for both genes present for the brown-orange morph. Preliminary phylogenetic analyses separated the morphs into different clades, supported by robust bootstrap values and posterior probabilities (>98). Our results indicate that the morphs are two different species: the black morph was identified as Petaloconchus cf. varians and the brown-orange morph as a non-described species, Petaloconchus sp. (orange). The first is endemic to the Caribbean, while the second is potentially an introduced species.     

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.     

Observations on the embryonic development of some deep-living decapod crustaceans,with particular reference to species of Acanthephyra

Eggs of meso- and bathypelagic decapod crustaceans were removed from gravid females and allowed to develop in vitro. The time course of embryonic development, as indicated by the appearance of certain morphological and physiological features, was determined for most species at a constant temperature of 12°C. The embryonic period, and the time between the appearance of certain key features such as naupliar and compound eyes and heartbeats, showed little variation in any one species. Significant differences were observed in the development times of different species. In general, species with large eggs have a longer embryonic period than those with small eggs, but there also variations between species with eggs of similar size. During the development of Acanthephyra eggs there is an approximate doubling of the egg volume, correlated with a decrease in egg density and an increase in the water content. The increase in the water content and egg volume is similar to that of other decapod eggs. It is concluded that those species with small eggs could have several broods in 1 year, whereas those species with large eggs are likely to be restricted to one, or in some cases possibly two, broods.     

Vertical distributions and photosynthetic action spectre of two oceanic picophytopianlcters,Prochlorococcus marinus andSynechococcus sp.

Two picophytoplankters,Prochlorococcus marinus andSynechococcus sp., were isolated from the bottom of the euphotic zone (150 m depth) in the western Pacifie Ocean. The concentration ofP. marinus at this depth was more than 10⁴ cells ml^–1 while that ofSynechococcus sp. was less than 10² cells ml^–1. TheP. marinus isolate has a high divinyl-chlorophylla:b ratio similar to that of the Mediterranean strain, while theSynechococcus sp. isolate is of the phycourobilinrich type. The growth rate ofP. marinus was higher thanSynechococcus sp. when both were cultured under weak blue-green to blue-violet light (ca. 2 E m^–2 s^–1). While the chlorophyll-specific absorption spectra showed higher values inSynechococcus sp., the photosynthetic action spectre revealed thatP. marinus was able to use blue-violet light, whereasSynechococcus sp. was able to use blue-green light, more efficiently for photosynthesis. The photosynthetic quantum yield ofP. marinus was higher than that ofSynechococcus sp. at any wavelength between 400 and 700 nm. The calculated in situ photosynthesis rates per Gell volume forP. marinus were estimated to be higher than forSynechococcus sp. at 50 and 150 m depth. These results indicate thatP. marinus photosynthetically surpassesSynechococcus sp. in the blue-light-rieh environment of the oceanic euphotic zone. This may be why the former predominates at depths in temperate to tropical open ocean waters.     

An examination of photosynthetic production,excretion of photosynthetic products,and heterotrophic utilization of dissolved organic compounds with reference to results from a coastal subtropical sea

The rate of primary production, excretion of photosynthetic products and turnover of glucose and amino acids was measured at a station in a coastal region in the Bahamas. Over the depths 0 to 50 m, total photosynthetic rates varied from 1.7 to 12.7 gC fixed 1^-1day^-1, averaging 4.3. The extent of extracellular photosynthetic products ranged from undetectable to 23%, averaging 6.9%. Neither the field data nor studies with axenic cultures of Dunaliella tertiolecta, Skeletonema costatum, and Monochrysis lutheri showed any evidence of an increase in the percentage excretion at low population densities or low photosynthetic rates. Rates of amino acid turnover varied from 21 to 168% day^-1, and that of glucose from 8.3 to 41% day^-1. Light seems to have little effect on the uptake and respiration of these substrates by the planktonic population. There was a significant relationship between the fraction of the substrate used for respiration and that retained by the cell. On average, 42% of the glucose taken up was respired and 21% of the amino acid mixture. Tentative calculations suggest that the production of dissolved organic material as extracellular photosynthetic products would be insufficient to supply the heterotrophic population, and it was concluded that some other route(s) must be of major importance.     

Reproductive,growth and life-history contrasts between two species of grapsid crabs,Sesarma cinereum and S. reticulatum

Two grapsid crabs, Sesarma cinereum (Bosc) and S. reticulatum (Say), were studied in the region of Beaufort, North Carolina, USA, during the summers of 1977, 1983, and 1984. The two grapsid species showed differences in social organization, habitat preferences, and number and size of first-stage larvae. We analyzed several other characters (growth rates, size at sexual maturity, reproductive effort per clutch) to determine if these differences could be related to a coherent pattern of life-history trends. While some relationships between characters were consistent with those found among other crab species, few seemed peculiarly matched to the ecological contrasts represented by our species. Our results suggest that other factors (phyletic predisposition; decoupling of selection between larval and adult stages; equally adaptive alternatives) may obscure the rules governing relationships between life-history characters and evolutionary trends.     

Morphological and histochemical observations on the nervous system of two sipunculid species,Siphonosoma australe and S. cumanense

The nervous systems of Siphonosoma australe Keferstein and S. cumanense Keferstein, 1867 are histologically similar. However, the two species differ with regard to the structure of the nuchal organ and ventral sensory organ. The nuchal organ in S. cumanense is multilobulated, innervated by 4 pairs of nerves and secretes acidophilic granules, whereas that in S. australe is bilobed, innervated by 2 pairs of nerves and is nonsecretory. Whilst the ventral sensory organ in S. cumanense appears empty and collapsed, that in S. australe contains concretions rich in glycoprotein. The central nervous system consists of the cerebral ganglion, circumesophageal commissures, and the ventral medullary nerve cord. The ovoid brain consists of a centrally placed neuropile and a cellular cortex. Three types of neurons are found in the brain: the giant (cyanophil), median (acidophil), and the globular neurons. The cell bodies of the giant and median neurons are located in the posterodorsal part of the brain, whereas the globular neurons predominate in the rest of the cortex. Most of the protein tests stained the giant and median but not globular neurons. The giant neurons also showed a weak PAS and sudanophilic reaction, suggesting the presence of small amounts of neutral mucopolysaccharides and lipids. Cyanophils and acidophils containing fuchsinophilic granules show possible neurosecretory activity.     

A simulation model of estuarine subsystem coupling and carbon exchange with the sea. II. North Inlet model structure,output and validation

A first-stage verified model of carbon/energy flux through the North Inlet (South Carolina) marsh—estuarine ecosystem is presented. The time series output for model compartments and overall septem carbon flow are compared with observed data collected over the past five to ten years. Results indicate that the model is stable and can broadly reproduce some of the major trends of a salt marsh—estuarine system. Further avenues of research are suggested.     

Not all ctenophores are hermaphrodites. Studies on the systematics,distribution, sexuality and development of two species of Ocyropsis

The lobate ctenophores Ocyropsis maculata and O. crystallina are not simultaneous hermaphrodites, based on morphological, histological and experimental evidence. Sex ratios in populations, sex ratios of size classes within populations, and average sizes of males and females support the hypothesis that both species are dioecious, rather than sequential hermaphrodites. We have divided each species into two subspecies, based on morphology and geographic distribution. Preliminary evidence suggests that the subspecies also differ in reproductive behavior. One subspecies, O. crystallina guttata, spawns on a daily cycle in the laboratory, and spawning becomes more synchronous when males and females are placed together. Species of Ocyropsis, all of which are oceanic in distribution, are probably descended from a common ancestor that was a simultaneous hermaphrodite. That this group of oceanic ctenophores evolved dioecy directly contradicts the assertion that there is a selective advantage to hermaphroditism in environments where the probability of finding a mate is reduced.     

Defense mechanisms and antipredator behavior in two sympatric species of spiny lobsters, Panulirus argus and P. guttatus

The congeneric spiny lobsters Panulirus argus and P. guttatus co-occur throughout the Caribbean Sea, where they may share the coral reef habitat. Despite their phylogenetic closeness, both species have many different life-history traits that may partially explain their coexistence. However, even though both species may face the same predators, their defense mechanisms and antipredator strategies had not been compared. We compared the performance between species in 18 morphological and behavioral defense mechanisms commonly expressed by most spiny lobsters, including predator-avoidance mechanisms (activity schedule, sheltering behavior, delay to disturbance, and effect of conspecific damage-released scents on shelter choice) as well as antipredator mechanisms (body size, several parameters of the escape response and limb autospasy, clinging strength, antennal strength, and cooperative defense). As hypothesized, both species expressed all these defense mechanisms (except cooperative defense, shown only by P. argus), reflecting their phylogenetic closeness, but performed significantly differently in most, in accordance with their particular ontogenetic traits. Their comparative performance in individual defense mechanisms as well as the antipredator strategies displayed by groups of lobsters of each species in the presence of a common predator (the triggerfish Balistes vetula) showed that, in general, the defensive behavioral type of P. argus is more bold and that of P. guttatus more shy. Therefore, their distinct defensive behaviors contribute to their niche differentiation.     

The breeding biology of two sympatric species of tropical intertidal hermit crabs,Clibanarius chapini and C. senegalensis

A one-year study of breeding in the two intertidal hermit crabs Clibanarius chapini Schmitt and C. senegalensis Chevreux and Bouvier showed that both species maintain a high reproductive activity throughout the year. Brief low reproductive activities, found in both species, occurred in different months: C. chapini in November, C. senegalensis in May. While these drops cannot be attributed to any recognisable environmental factor, it is suggested that such pattern may result from competition between the two species. Sex-ratio studies indicated slightly more females than males in both species. The maximal size attained by males of both species is considerably greater than that of the females; in C. senegalensis this is very marked. It is suggested that it may be selectively advantageous for a female to attain a smaller maximal size since her brood must also be accomodated within the shell. As expected for continuously breeding species, recruitment of young into the population is also continuous.     

Sperm traits of the quacking frog, Crinia georgiana: intra- and interpopulation variation in a species with a high risk of sperm competition

Sperm traits often show extreme variation both between and within species. Between-species variation may often be interpreted in the context of a sperm competition theory, but within-species variation has remained unexplained. Previous studies on intraspecific variation in sperm traits have focused on a limited set of variables and may have failed to explain observed variation because of potential trade-offs between different sperm traits. We report on variation in number, size, motility and longevity of sperm in the frog Crinia georgiana, a species where sperm competition is common. We found intrapopulation variation in sperm size and motility and interpopulation variation in relative sperm number and size. When we combined relative sperm number and size into one variable, and motility and longevity into another, we found significant interpopulation variation in these variables as well. We also detected considerable intra- and significant interpopulation variation in cumulative sperm quality, a combination of all four sperm traits. Furthermore, a significant effect of the interaction between population origin and male size on sperm characteristics indicated interpopulation variation in the strength of selection acting on sperm traits of males adopting different mating strategies. We discuss heterogeneity in the reproductive environment, a complex genetic background in the determination of sperm characteristics and varying levels of developmental noise as potential contributors to the observed variation in sperm traits.     

Spatial segregation amongst goby species within an Australian estuary,with a comparison of the diets and salinity tolerance of the two most abundant species

The present study was undertaken to determine whether the various species of gobies that are found within the large Swan Estuary in south-western Australia are segregated within that system, and to attempt to determine the basis for any differences in their spatial distributions. The Swan Estuary comprises a long entrance channel (lower estuary), two wide basins (middle estuary) and the saline reaches of the tributary rivers (upper estuary). A total of 26232 gobies, representing seven species, was collected using a 3 mm-mesh seine net at 15 sites throughout this estuary on at least one occasion monthly over seven consecutive seasons between September 1983 and March 1985. Favonigobius lateralis and Pseudogobius olorum contributed 47.0 and 47.8%, respectively, to the total catch of gobies at all sites. The densities of each species at each site were used to determine the relative contribution of each species to the gobiid fauna at each of the sites in the lower, middle and upper estuary. Comparisons of these data with those published on the distribution and abundance of gobiid larvae confirmed that F. lateralis, which was found predominantly in the lower estuary, is a marine species that spawns in high salinities near the estuary mouth or in inshore coastal waters. In contrast, the life cycle of P. olorum and Papillogobius punctatus are typically completed within the saline reaches of the upper estuary, and that of Arenigobius bifrenatus within both this region and parts of the middle estuary where the substrate is particularly soft. Afurcagobius suppositus also spawns in this area, as well as in fresh water. Tridentiger trigonocephalus, represented by only eight individuals, is an introduced, marine species that was found mainly in the lower estuary. A single representative of the marine species Callogobius depressus was caught. The relatively low numbers of gobies caught in the middle estuary, where they contributed only about 3.5% to the total number of all gobies at all sites, may represent an aversion to the presence of rougher waters in the large basins. Circumstantial evidence suggests that the sandy substrate and consistently high salinities found in the lower estuary are preferred by F. lateralis, whereas the silty surface to the substrate and lower salinities of the upper estuary are preferred by Pseudogobius olorum. Densities of three of the four most abundant species were higher in either spring or summer than in winter, reflecting the influx of 0 + recruits, and possibly also the tendency for species in estuaries to congregate in the shallows during the warmer periods of the year. F. lateralis fed mainly on polychaetes and crustaceans, whereas P. olorum ingested predominantly algae, reflecting differences in mouth morphology and feeding behaviour, rather than the type of food available.     

Light quality in relation to growth,photosynthetic rates and carbon metabolism in two species of marine plankton algae

The effect of light quality on growth, photosynthesis and carbon metabolism in two species of marine algae,Cyclotella nana (Hustedt) andDunaliella tertiolecta (Butcher), was examined. Relative growth constants forC. nana were 0.37, 0.29 and 0.25 in blue, white and green light, respectively. Corresponding constants were 0.41, 0.31 and 0.29 forD. tertiolecta. Photosynthetic rates in both species were higher in blue light and lower in green light compared with white light of the same intensity. More than 60% of¹⁴C assimilated byC. nana orD. tertiolecta grown in blue or green light was incorporated into the ethanol-insoluble fraction, compared with 10 to 30% in this fraction in white light. The relative importance of the various components within this fraction was independent of light quality. Although less¹⁴C was assimilated into the ethanol-soluble fraction in blue or green light, there was a relative increase in some amino acids and organic acids in this fraction and a decrease in sugars and sugar phosphates relative to white light of the same intensity. These differences were independent of light intensity, photosynthetic rate and cell density in the cultures.