Bioluminescence in the Monterey Submarine Canyon: image analysis of video recordings from a midwater submersible

Video images of bioluminescence were recorded in situ during a 1985 study of the midwater environment of the Monterey Canyon, using a single-person, untethered submersible. Gelatinous organisms were responsible for the most brilliant bioluminescent displays, often exhibiting elaborate kinetics in response to mechanical stimulation. Images of bioluminescent displays recorded from identified organisms are shown and display patterns are described. All bioluminescence emission spectra from captured specimens were blue, with peak emissions between 460 and 494 nm. Image-analysis of recordings of mechanically stimulated bioluminescence revealed source densities between 43 and 175 m^-3 and intensities between 2.5 and 37.3 W sr^-1 m^-3. The predominant display type at all depths studied (between 100 and 560 m) was luminous secretions. Despite high intensities of mechanically stimulated bioluminescence, no spontaneous light production was recorded in the absence of mechanical stimulation.     

Sites of bioluminescence in the appendicularians Oikopleura dioica and O. labradoriensis (Urochordata: Larvacea)

Although bioluminescence is known in larvacean tunicates, its origin has not been reported. Two species, Oikopleura dioica and O. labradoriensis, from the northeastern Pacific Ocean, were examined during 1979–1982 to determine the sites of luminescence. An appendicularian lives within a secreted, mucous house, and its body is tightly invested by a house rudiment. Mechanical stimulation elicited multiple, summated blue-green flashes from free individuals invested by house rudiments and from empty houses that were virtually free of contamination by exogenous luminescent bacteria or dinoflagellates. Direct microscopic observations showed that the light is produced by clusters of 1 to 2 m fluorescent granules that form intricate, species-specific patterns of inclusions in the house rudiment. These granular inclusions are probably present in the expanded house, where they account for the multiple, point-sources of light observed in flashing houses. Neither fluorescence nor luminescence were observed in any other parts of the house rudiments, expanded houses, or free appendicularians (including the oral glands, which were previously suspected of producing bioluminescent secretions).     

Vertical and horizontal movements of striped marlin (Tetrapturus audax) near the Hawaiian Islands,determined by ultrasonic telemetry,with simultaneous measurement of oceanic currents

We measured the vertical and horizontal movements of striped marlin (Tetrapturus audax) off the leeward coast of the Island of Hawaii between 20 November and 18 December 1992 while simultaneously gathering data on water temperature and oceanic currents. Fish movements were monitored by ultrasonic depth-sensitive transmitters, depth-temperature profiles by an expendable bathythermograph system, and oceanic current patterns by an acoustic Doppler current profiler. Like Indo-Pacific blue marlin (Makaira mazara), striped marlin near Hawaii spend >85% of their time in the mixed layer (i.e., above 90 m depth). The maximum depth for striped marlin appears to be limited by water temperatures 8 C° colder than the mixed layer, rather than by an absolute lower temperature. We also found that the horizontal displacements of some striped marlin can be strongly influenced by currents.     

Population genetics of the blue crabCallinectes sapidus: modest population structuring in a background of high gene flow

To determine the genetic population structure of blue crabs (Callinectes sapidus Rathbun), electrophoretic allozyme analysis was performed on 750 individuals collected from 16 nearshore locations ranging from New York to Texas, USA. Twenty enzymes and non-enzymatic proteins coded by 31 presumptive loci were examined. Twenty-two loci were either monomorphic or polymorphic at less than theP ₉₅ level; alleles for these polymorphic loci were geographically dispersed. Allele frequencies for three of the remaining polymorphic loci were homogeneous over all populations, as were levels of polymorphism and heterozygosity. Phenograms generated by the UPGMA (unweighted pair-group method using arithmetic averages) and distance Wagner methods exhibited no geographic pattern in the clustering of populations. Estimates ofN _em (effective number of migrants per generation between populations) indicated substantial gene flow, with aalues sufficiently high to infer panmixia between all blue crab populations from New York to Texas. However, despite this high level of gene flow, two striking patterns of geographic differentiation occurred: genetic patchiness and clinal variation. Allele frequencies atEST-2, GP-1, IDHP-2, DPEP-1, DPEP-2, andTPEP exhibited genetic patchiness on local and range-wide geographic scales, and allele frequencies atEST-2 varied temporally. Genetic patchiness in blue crabs is likely to be the result of the pre-settlement formation and subsequent settlement of genetically heterogeneous patches of larvae; allele frequencies of those larval patches may then be further modified through ontogeny by localized selection. In the Atlantic Ocean, a regional latitudinal cline ofEST-2 allele frequencies was superimposed on the range-wide genetic patchiness exhibited by that locus. This pattern against a background of high gene flow is highly likely to be maintained by selection. In estuaries along the Atlantic Ocean coast, a combination of low adult long-distance migration and a high retention rate of locally spawned larvae could serve to segregate populations and allow for the development of the geographic cline inEST-2. The Gulf of Mexico showed no apparent cline, perhaps due to long-distance migration of females in some regions of the Gulf, or to masking by genetic patchiness. These results emphasize the importance of both ecological and evolutionary time scales and structuring mechanisms in determining genetic population structure.     

Diel patterns of planktonic bioluminescence in the northern Sargasso Sea

Day-night changes in the vertical distribution, intensity, and size of bioluminescence flashes were investigated during a series of cruises to the northern Sargasso Sea in 1987 and 1988. Overall, depth integrated bioluminescence potential and flash density estimated from in situ measurements with a pumping bathyphotometer were 2 to 5 x higher at midnight than at midday. Depths from 50 to 100 m exhibited the most substantial day to night increases in bioluminescence potential and flash density. When classified by flash size (photon output per flash event), the increase from day to night was significant for all flash sizes, but was most dramatic for small flashes producing <7 x 10⁸ photons flash^-1. Bioluminescence potential and flash density increased 2 to 3 x during bathyphotometer measurements made at dusk. Bioluminescent light budgets estimated from day and night net collections in May and August 1987 also predicted 2.5 x higher nighttime than daytime mesoplankton bioluminescence. However, large bioluminescent taxa (mesoplankton) capable of significant vertical migrations only contributed on the order of 15% of the total bioluminescence in surface waters. Our results do not support the idea that most of the nightly increase in bioluminescence potential and flash density are due to vertical migration of bioluminescent organisms; rather they are consistent with an alternate view that photoinhibition of bioluminescent flashing by dinoflagellates may be primarily responsible for the diel patterns.     

Chaotic genetic patchiness in an intertidal limpet,Siphonaria sp.

Variation of 4 polymorphic enzymes was studied for 2 yr (1978 and 1979) in an undescribed species of Siphonaria, a pulmonate limpet, from a rocky shore at Rottnest Island, Western Australia. Depending on the locus, significant genetic differences were found among sites along 50 m of shore, between high and low portions of the shore within sites, between adults and recruits, and between recruits in the 2 yr. This genetic heterogeneity does not follow a simple, consistent pattern, but forms a shifting, ephemeral genetic patchiness best described as chaotic. This patchiness may result from temporal variation of numbers and genotypes of recruits, which leads to the proposal that planktonic dispersal, although causing uniformity on a large scale, can give rise to fine-scale genetic patchiness.     

Combining fishing and acoustic monitoring data to evaluate the distribution and movements of spotted ratfish <Emphasis Type="Italic">Hydrolagus colliei</Emphasis>

Direct and indirect methods have been used to describe patterns of movement of fishes, but few studies have compared these methods simultaneously. We used 20 years of trawl survey data and 1 year of acoustic telemetry data to evaluate the vertical and horizontal movement patterns of spotted ratfish Hydrolagus colliei in Puget Sound, WA, USA. Densities of large ratfish (≥30 cm) were higher at the deepest depths trawled (70 m) during daylight hours, whereas densities were similar across depth zones (to 10 m) at night. Acoustic tracking of ratfish showed distinct diel patterns of movement and activity level; ratfish moved into shallow, nearshore habitats at night from deeper, offshore habitats during the day and made ~3 times more moves at night than day in shallow habitats. Broader spatial patterns depended on where ratfish were tagged: one tag group remained in one general location with few excursions, whereas a second tag group moved within a 20-km band with some individuals moving >90 km. These data will help inform food web models’ abilities to quantify interspecific interactions between ratfish and other components of their community.     

Spatial and temporal variations in the population size structure of three lanternfishes (Myctophidae) off Oregon,USA

Size-frequency distributions were determined for 3 common lantern-fishes (Stenobrachius leucopsarus, Diaphus theta, and Tarletonbeania crenularis) off Oregon in the summer. The fishes were caught mainly in sound-scattering layers by a large pelagic trawl with 5 opening-closing nets. Changes in depth distribution and diel vertical migration with growth were evident for all 3 species. The size of S. leucopsarus increased markedly with depth both at 0 to 90 m at night and 250 to 500 m during the day. Larger D. theta were also found deeper during the day (between 250 and 450 m), but neither D. theta nor T. crenularis demonstrated size segregation in the upper 90 m at night. Large D. theta and small T. crenularis did not appear to migrate into surface waters at night. Age-Group O (15 to 20 mm) S. leucopsarus were most abundant in deep water (400 to 480 m) in the daytime and did not migrate into near-surface waters at night. Age-Group I (30 to 40 mm) S. leucopsarus were common at about 300 m by day and within the upper 30 m at night. Age-Group II–III (50 to 60 mm) apparently followed the evening ascent of Age-Group I fish and most resided at 75 to 90 m at night, beneath Age-Group I fish. Age-Group III+fish (70 to 80 mm) were associated with Age-Group O at 400 to 480 m by day and usually did not migrate above 200 m at night. The size structure of S. leucopsarus differed among the nets of a single tow at one depth, or between two tows that fished the same depths on successive nights, indicating horizontal patchiness in age structure. D. theta demonstrated low within-tow variability in size composition which indicated a spatially more uniform age structure on a scale of kilometers. The size structures of these 3 lanternfishes were different in the same area and the same season during two different years, suggesting variable survival of year classes or horizontal patchiness of age composition in the area sampled.     

Effects of environmental manipulation on the locomotor activity and agonistic behavior of cultured juvenile American lobsters,Homarus americanus

The locomotor activity patterns and agonistic encounters of cultured juvenile (IX–XIII stage) American lobsters, Homarus americanus (Milne-Edwards) held in 8 experimental environments were investigated to examine their relative contribution to aggressive level. Three variables: (1) lobsters individually or communally maintained, (2) shelters present or absent, and (3) open or closed seawater system were tested in a set of factorial experiments. Locomotor activity of lobsters held in a 12 h: 12 h light: dark regime was recorded for individually separated lobsters for the first 5 days and for communally held lobsters for the next 5 days of a 10-day experimental period. All lobsters maintained individually had similar levels of activity. When the same individuals were communally maintained, there appeared to be social inhibition of activity. In addition, communally held lobsters without shelter were twice as active as those provided with shelter. The activity levels of groups of communally held lobsters with shelter progressively decreased over the experimental period. All lobsters became somewhat entrained to the light: dark cycle and were nocturnally active. However, the degree of entrainment was strongly influenced (P<0.001) by the presence or absence of shelter and other lobsters. Individually maintained lobsters all showed good entrainment, although those provided with shelter were slightly better entrained. The presence of conspecifics desynchronized activity patterns when shelter was not provided. Aggressive level, as measured by the frequency of agonistic encounters per 15-min observation period and the number and types of displays per encounter, decreased over an 8-day period for groups of 3 lobsters in all experimental environments. Lobsters maintained in a closed system initially showed higher frequencies of agonistic encounters than those held in an open system. The observed decrease in aggressive level with time primarily resulted from decreased activity levels. When two lobsters met, the probability of an agonistic encounter was about 0.8 throughout the experimental period, but decreasing activity levels resulted in progressively fewer encounters. The results suggest several approaches for limiting aggression among communally maintained lobsters in culture systems by environmental manipulation.     

Genetic structure at different spatial scales in the pearl oyster (Pinctada margaritifera cumingii) in French Polynesian lagoons: beware of sampling strategy and genetic patchiness

In order to study further the genetic structure of the pearl oyster Pinctada margaritifera in French Polynesia with a special consideration for the sampling scale, we analyzed or re-analyzed sets of data based on nuclear DNA markers obtained at different spatial scales. At a large scale (several 1,000 km), the remote Marquesas Islands were confirmed to be significantly differentiated from Tuamotu–Gambier and Society archipelagos, with a marked difference however for the two main islands that are different from each other. At a medium scale (several 10 to several 100 km), overall homogeneity was observed within and between these two archipelagos, with some exceptions. This could be attributed both to large-scale larval dispersal and to human-driven spat translocations due to pearl oyster cultivation. These results contrast with those observed (1) at a small scale (less than 10 km) in a lagoon heavily impacted by translocation and cultural practices, where significant genetic differentiation was detected among three laying beds, and (2) at a micro scale where we detected an important variability of the genetic composition of young spat recruited on artificial collectors. Such patterns could result from a high variance in the number of genitors at the origin of each cohort, or from pre- or post-settlement selection on linked loci. Altogether, our data support the hypothesis that under certain conditions populations of bivalves may exhibit patterns of chaotic genetic patchiness at local scale, in line with the increasing report of such patchiness in marine benthic organisms. This underlines the importance of sampling scale that should be rigorously defined depending on the questions to be answered. Nevertheless, a survey of about 80 articles dealing with population genetics of marine invertebrates showed that only 35% of those studies disclosed details about the sampling strategy (particularly the area explored). These results emphasize the need for cautious interpretation of patterns of genetic structure at medium scale when rigorous sampling strategies are not deployed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Movements,environmental associations,and presumed spawning locations of Atlantic halibut (Hippoglossus hippoglossus) in the northwest Atlantic determined using archival satellite pop-up tags

Large Atlantic halibut (Hippoglossus hippoglossus) off the eastern coast of Canada were tagged with pop-up satellite archival transmission tags (N = 17) to track movements, determine ambient depth and temperature, and infer spawning activity. Many halibut showed seasonal movements from deepwater slope areas in fall and winter to shallower feeding grounds on the Scotian Shelf and Grand Banks in summer. Halibut depths ranged between 0 and 1,640 m. Mean temperature of occupation was 4.7 °C. Multiple short-term vertical ascents from a consistent baseline depth, characterized as spawning rises, were identified in seven of the tagged halibut south of the Grand Banks. All presumed spawning rises occurred in multiples of 2–6 events at 2- to 9-day intervals between October and January, spanning an average vertical extent of 50–100 m at depths of about 800–1,000 m. Given the direction and velocity of the slope water currents and the duration of the pelagic stage, the calculated 300–500 km drift of the eggs and larvae would take them onto the Scotian Shelf, as well as into the Gulf of St. Lawrence. Therefore, the location of the presumed spawning grounds is consistent with expectations based on migration compensation theory, the northeasterly migratory patterns of the juveniles, the relatively static distribution of the adults off southern Newfoundland, and the prevailing currents at depth.     

Landscape bioacoustics allow detection of the effects of habitat patchiness on population structure

Landscape structure may affect individual dispersal abilities, thus influencing the genotypic and phenotypic composition of populations. We analyzed the interplay among landscape, behavior, and evolutionary processes by correlating habitat patchiness to the variability in vocalizations of Dupont's Lark Chersophilus duponti, one of the most habitat-selective and rare European songbirds. We tape-recorded males throughout the species distribution in Spain, analyzed the spatial patterns of territorial call variation at different scales (individuals, populations, and broad geographic areas), and related acoustic variability to patterns of isolation by geographic distance and by landscape unsuitability (calculated by building a predictive model of habitat suitability). The differentiation of spectro-temporal call features resulted from both isolation by distance and isolation by landscape unsuitability mechanisms. Landscape connectivity was often a better determinant of call differentiation than simple straight-line distance between individuals, providing the first evidence that call transmission can be limited by the presence and distribution of patches of adequate habitat, which likely mediates bird dispersal. Landscape patchiness resulted in a reduction of acoustic diversity (repertoire size) within populations, and a parallel increase in differentiation among populations. Landscape bioacoustics can represent a promising tool for estimating population structure, although the study of animal communication cannot be viewed as an alternative, but a source of complementary information to genetics, given that it provides evidence of male-male transmission and social and cultural phenomena that are currently undetectable from molecular data.     

Diel vertical movements of adult male dolphinfish (Coryphaena hippurus) in the western central Atlantic as determined by use of pop-up satellite archival transmitters

The vertical movements of six adult male dolphinfish (Coryphaena hippurus) (95–120 cm estimated fork length), caught using standard sportfishing methods, were investigated using high-rate single-point pop-up satellite archival transmitters from 2005 to 2011 in the western central Atlantic. Data revealed a diel activity pattern within the mixed surface layer with dives below the thermocline suggesting temperature is not a barrier to vertical movements for short periods of time. Dolphinfish were tracked for periods of 4.96–30.24 day (Σ = 83.37 day), reaching depths >200 m, and in temperatures ranging from 16.20 to 30.87 °C. The six tags allowed comprehensive vertical movement analyses by time of day, duration at depth, and based on vertical movement patterns. The longest (>60 min), deepest (>30 m), and most extensive vertical movement patterns occurred during night rather than day, with the most time spent near the surface during the day. Dolphinfish spent 66 % of their time in the surface layer (0–9.9 m) and only one individual spent 8 % of the monitoring period diving >8 °C from the maximum surface temperatures recorded while tracked. Two tags were analyzed based on lunar phase and revealed contrasting relationships between vertical movements during new and full phases. Our results suggest dolphinfish vertically shift between surface and at-depth feeding strategies to exploit aggregating epipelagic and mesopelagic prey items leading to predictable diel vertical movements.     

Mesoscale, finescale and microscale distribution of micro- and nanoplankton in the Irish Sea, with emphasis on ciliates and their prey

We examined the degree of mesoscale (km), finescale (m), and microscale (cm) patchiness of ciliates and their prey in waters of varying hydrographic conditions. Samples were taken throughout the water column, along a transect across the Irish Sea (54°N), at scales ranging from 0.15 to 10⁵ m. We examined physical, chemical, and biological characteristics. The eastern and western Irish Sea were stratified, with a pycnocline at ∼20 to 30 m. The central waters were mixed and had adjacent frontal regions. Euphotic depth was ∼20 to 35 m. Generally, the upper waters were nitrogen-limited, with elevated levels associated with frontal regions and deeper waters. Microphytoplankton exhibited fine-mesoscale patchiness: diatom numbers were low in stratified waters, with higher levels in mixed and frontal regions; dinoflagellates were abundant in subsurface waters near the fronts. Nanoflagellate numbers and biomass decreased with depth below the pycnocline, and exhibited microscale distribution in upper waters; these micropatches may provide increased food levels for ciliates. Microscale distribution of ciliates was rare and only occurred at mixed/frontal sites; finescale ciliate patches were a more prominent feature of the water column. These finescale patches can be composed of a variety of taxa but can also be virtually monospecific. Finescale patches may produce localised regions of high productivity that is available to fishes and copepods, but may also be a sink, as patches can be short-lived and thus unavailable to predators. Received: 28 September 1998 / Accepted: 26 January 1999     

Temporal and spatial variability in settlement of the sea urchin<Emphasis Type="Italic"> Paracentrotus lividus</Emphasis> in the NW Mediterranean

Settlement into the benthic habitat may be an important process in regulating sea urchin abundance, which potentially modifies the structure of benthic communities. Strong settlement events may increase sea urchin abundance beyond a certain threshold, leading to the formation of coralline barrens (overgrazed communities with a dominance of encrusting coralline algae). To understand the role of settlement in regulating sea urchin populations we first need to determine settlement variability. Temporal variation in settlement of the sea urchin Paracentrotus lividus was monitored at three sites in the Medes Islands, NW Mediterranean, during three settlement seasons (March 1998 through October 2000). Spatial variation in settlement was studied in 1999 at 50 sites along a gradient of exposures to waves and currents, inside and outside the archipelago, and separated by distances from tens to thousands of meters. Bathymetric distribution of settlement was also studied in 2000 at six sites at 5, 10, 15 and 20 m depths. Settlement of P. lividus occurred in a single annual peak within 3 weeks in May–June. Differences in settlement between years were more than two orders of magnitude. Spatial variability was found at all scales investigated, showing strong patchiness at the smallest spatial scales (tens of meters). Sea urchins settled preferentially at depths between 5 and 10 m. Substratum type, level of protection, and adult population densities were not significant in determining settlement. However, settlement was found to be related to the degree of exposure to waves and currents, indicating that physical processes are very important at the spatial scales investigated. This greatly variable settlement is a necessary, although not sufficient, condition to create gradients of adult P. lividus abundance. Further studies should be designed to investigate the interaction between settlement strength and post-settlement mortality.Communicated by O. Kinne, Oldendorf/Luhe     

Social facilitation of selective mortality

Territorial defense by breeders influences access to resources near defended nest sites by intruder species and may have indirect effects on other species within the territory, leading to local patchiness in distribution patterns. The present study demonstrates that adult males of a damselfish, Pomacentrus amboinensis, indirectly facilitate the increased survival of conspecific juveniles through the territorial defense of their nesting site from potential egg predators. Moreover, male territoriality results in a shift in the selectivity of predation on newly settled juveniles. We monitored the fate of pairs of predator-naive, newly settled P. amboinensis placed inside and outside nesting territories. Individuals within a pair differed in size by approximately 1 mm and were tagged for individual identification. Away from male territories larger juveniles had greater survival, while within territories, larger juveniles suffered higher mortality. Behavioral observations indicated that the moonwrasse Thalassoma lunare, a predator of benthic eggs and small fishes, had reduced access to juveniles within male territories, while another predator on small fishes, the dottyback Pseudochromis fuscus, had unobstructed access to male territories. Experimental removal of P. fuscus indicated that the shift in the direction of phenotypic selection on newly settled juveniles was the indirect effect of aggression by nest-guarding male damselfish, which resulted in differential access to male territories by these two predators of small fishes. Evidence suggests that behavioral interactions between the resident community and intruders will influence patchiness in selective pressures imposed on benthic prey by influencing both the composition of predator types that can access the prey resource and their relative abundance. How this spatial and temporal patchiness in predator pressure interacts with spatial patchiness of recruiting prey will have a major influence on the resulting distribution of juveniles and their phenotypic traits.     

Spatial pattern of soft-bottom Polychaeta off Nova Scotia,Canada

Patchiness of polychaetes at the micro-scale (i.e. proportional to the sphere of influence of an organism) and macro-scale was investigated in the Northwest Atlantic in 1977 and 1981. Seven box-core samples (each 2 500 cm²) from three sites off Nova Scotia, Canada were completely subsampled as 25 contiguous 100cm² cores and analysed for their polychaete faunas. Dispersion chi-square and autocorrelation analyses were performed on the complete data set. The inshore fauna (St. Margaret's Bay) was strongly clumped with patchiness at several scales: <10cm in diameter, equal to 10 to 50cm (37% of the species), and>50cm. Polychaetes in samples from off-shore stations were much less aggregated, and discordantly dispersed to varying degrees. The results conform with the present understanding of infaunal spatial patterns and reject the hypothesis that a depth gradient in patchiness exists in addition to the well documented density and biomass gradients.     

Some observations of sea turtle nesting activity in Costa Rica

Low level aerial observations were used to obtain synoptic records of the distribution of sea turtle nesting activity along both coasts of Costa Rica. Pertinent environmental information was simultaneously recorded including beach characteristics, river effluents, and evidence of coastal currents. Other correlative information was obtained from detailed maps, current charts, and climatological data. On the Caribbean coast, as expected, green turtle (Chelonia mydas) nesting was concentrated on the beaches between the Tortuguero and Parismina Rivers. On the Pacific coast, two major nesting beaches for the Pacific ridley Lepidochelys olivacea were found, each having over 100 thousand turtles aggregated offshore during the peak period between September and November. Aggregations were present at least from July through December. Massed nesting occurs each year on these same beaches and the event is known as the salida de flota by the natives of Guanacaste Province. Numerous less important nesting beaches were also found. Nesting density did not correlate well with beach quality but, instead, appeared to be related to the proximity of the beach to offshore currents. Oceanic current systems apparently facilitate the transport of sea turtles to the general vicinity of the important nesting beaches on both coasts of Coata Rica.Contribution No. 1557 from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA.     

Spatial distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum in coastal bottoms (Cap de Creus, northwestern Mediterranean Sea)

Current knowledge on the abundance and distribution patterns of different soft coral species is relatively limited when compared to other benthic suspension feeders such as gorgonians and hard coral species. To overcome this scarcity of information, the distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum were investigated in northwestern Mediterranean benthic communities over a wide geographical (60 km of coastline) and bathymetrical (0–70 m depth) extent using a remotely operated vehicle. A. acaule was the most abundant species in the study area with highest recorded density of 18 col m^?2 found at depths of 35–45 m in areas that are directly exposed to strong near-bottom currents. Conversely, A. palmatum was only found as scattered solitary colonies at greater depths in soft bottoms, with maximum density of 2.4 col m^?2. Medium and large colonies of A. acaule were preferentially found on sloping and vertical rocky bottoms where they form dense patches. High-density patches of A. acaule were preferentially found on vertical rocky bottom, while isolated colonies were preferentially observed on coralligenous substrata as well as on flat soft and maërl substrates. A. acaule biomass distribution showed highest values between 40 and 45 m depth, and between 60 and 65 m depth. This suggests that deeper populations are formed by colonies that are bigger than the equivalent shallower ones. Although both species are almost genetically identical, ecologically they are very different. For this reason, conservation plans should consider the differential ecological traits shown by these two soft coral species.     

The growth response of a model Gymnodinium splendens in stationary and wavy water columns

A computer model is used to investigate the simulated growth of a theoretical dinoflagellate resembling Gymnodinium splendens in response to a variety of field conditions. Literature data on G. splendens are combined with probable estimates of organism response (where direct data are lacking) to yield light-and temperature-dependent production curves. These production curves are superimposed on a physical model characterized by a diurnally variable light cycle, by a two-layered water column (16°C water overlaying 12°C water) of variable layer thicknesses, and by variable extinction coefficients in the upper layer. The water column is either stationary or perturbed by a semidiurnal (12.4 h) internal wave. Organism behavior ranges from the continuous occupation of selected strata (stationary or wavy) to diurnal vertical migrations within the upper layer or across the thermocline. In stationary water columns, species patchiness depends on spatial differences in the depth preferences of nonmigrating organisms or in the details of the behavior of migrating organisms. In water columns perturbed by a semidiurnal internal wave, spatial differences in the phase relationship between the wave form and daylight supplement organism behavior as a source of patchiness. The models result in their most complex spatial patterns when a population migrates through a thermocline perturbed by a semidiurnal internal wave.University of Texas Marine Science Institute Contribution No. 280.