Depth Segregation Research Articles

Sex-specific differences in diving behaviour of two sympatric Alcini species: thick-billed murres and razorbills

At the Gannet Islands, Labrador, sympatric thick-billed murres ( Uria lomvia (L., 1758)) and razorbills ( Alca torda L., 1758) are slightly sexually dimorphic and have similar intersexual differences in parental roles; females are the main meal providers and males are mostly involved in brooding and chick defence at the breeding site and at sea. The question is whether differences in parental roles influence the foraging behaviour patterns of males and females. Murre females foraged during twilight periods and dived shallower than males. In razorbills, although sex differences were not as clear, females also tended to dive shallower (<10 m) and more often at twilight. Males of both species foraged during daylight hours and tended to dive deeper than females. Females of both species had shorter dive bouts (i.e., duration of a series of dives) even though the number of bouts and dives per day were equal between sexes. In both species, female dives were mostly shallower W-shaped dives, likely for capturing crustaceans at twilight. In contrast, males performed mostly deeper U-shaped dives for capturing mid-water species (e.g., capelin, Mallotus villosus (Müller, 1776)). Altogether, our results show that the two sympatric auks had relatively similar intersexual segregation in feeding time, depth, and prey. Sex differences in nest attendance, driven by differences in parental roles, seem to explain these findings.

Cryptic speciation in the vermilion rockfish (Sebastes miniatus) and the role of bathymetry in the speciation process

A recent phylogenetic review of the genus Sebastes suggested the existence of a cryptic species of vermilion rockfish (Sebastes miniatus). To evaluate the geographical and bathymetric range of the Type 1 and Type 2 forms reported in that study, cytochrome b sequences were examined from 548 fish. Type 1 fish were found primarily south of Point Conception on reefs deeper than 100 m. Type 2 fish were common range-wide at sites shallower than 100 m. Reproductive isolation between the two types was tested using nine microsatellite loci. Estimates of genetic divergence were made using the fixation index (F(ST)) and correspondence between haplotype and genotype was tested by Bayesian population assignment and multivariate plotting of individual genotypes. Microsatellite analyses gave strong support for the presence of two distinct groups of genotypes. All fish with Type 1 haplotypes and fish with Type 2 haplotypes from < 100 m depth had genotypes unique to their haplotype group. However, most (68%) fish with Type 2 haplotypes from > 100 m depth assigned strongly to the Type 1 genotype group. Morphometric comparisons between the two genotypic groups revealed significant differences at three of the six examined measurements. Differences in both genetics, depth of occurrence, and morphology suggest these are separate species. This observation along with evidence of depth segregation in many recent species pairs led us to hypothesize a speciation model for Sebastes spp. by which the loss or truncation of a depth-related ontogenetic migration can lead to the creation of reproductively isolated populations.

Daphnia galeata in the deep hypolimnion: spatial differentiation of a “typical epilimnetic” species

Daphnia galeata is traditionally regarded to be a non-migratory species, which lives in warm epilimnetic waters. Depth segregation or vertical migration is usually attributed to other Daphnia species such as D. hyalina or D. longispina. In a two-year study, we found that in a deep, dammed-valley reservoir (Řimov Reservoir, Czechia) the majority of the population of D. galeata lives in the warm epilimnetic waters during the summer months, but some specimens of this species could be always found in the deep strata as well. This hypolimnetic subpopulation stays in the cold hypolimnetic water and does not migrate. The abundance of hypolimnetic D.galeata does not exceed one specimen per litre and usually shows seasonal variation (minimal densities in early spring, maximal in late summer). Using allozyme electrophoresis, we found that the subpopulation from the deep hypolimnion was clearly genetically differentiated from the population in the epilimnion. We found significant differences in both allele and multilocus genotype frequencies; the FST values at most sampling dates exceeded 0.05. However, the spatial segregation between the epilimnetic and hypolimnetic subpopulations is not permanent. The reservoir is dimictic and hence, at least twice per year, all vertically segregated parts of the population are mixed together. Our results suggest that the deep hypolimnetic subpopulation is repeatedly re-established in spring by deepwater “colonists”, at least some of which seem to be ecologically specialised for the hypolimnetic conditions, and dominate the hypolimnion by the end of the season. The genetic differentiation is likely the result of both the different depth preferences of various D. galeata clones and different selective pressures in the epilimnion and hypolimnion.

Spatial and Temporal Variability of Zooplankton in a Great Plains Reservoir

Seasonal and daily patterns of zooplankton populations are often predictable in natural lakes. Distinct zonation and geomorphic differences in reservoirs, however, make ecological extrapolations from lakes to reservoirs uncertain. We describe the spatial and temporal distribution of zooplankton, algae, and water clarity across reservoir zones and along a depth gradient in Glen Elder Reservoir, Kansas. Daphnia species were most abundant in the lacustrine zone, with D. pulicaria numerically dominant in early spring and D. mendotae dominant later in 1999 and 2000. Rotifers (Keratella quadrata, Asplanchna spp.) were dominant in the riverine zone in 1999, 2000, and 2001. Algal biomass was not significantly different between zones through most of the sampling periods, except late April in 1999 and mid-April in 2000. Chlorophyll a exceeded 81 l gl )1 in the lacustrine zone in mid- to late-April in 1999 and 2000, and exceeded 90 l gl )1 in the riverine zone in mid-April. Water clarity was significantly lower in the riverine zone in 1999 and 2000. Most zooplankton taxa had similar depth distributions at night and day, indicating a lack of diel vertical migration behavior on a large scale. However, in small scale (30 cm vertical enclosures) laboratory experiments, both D. pulicaria and D. mendotae occupied significantly deeper depths (>25 cm) under lighted conditions in the presence of fish chemical cues compared to shallow water (7–17 cm) under dark conditions and in the absence of fish cues. These contrasting results suggest that, as in other studies, Daphnia sense cues from predators and alter their depth in the water column on small scales without natural constraints on movement and choices. However, other factors in the reservoir such as wind-generated water movements and cues from other predators may prevent depth choices similar to those seen under controlled conditions. These results illustrate biotic and abiotic differences between riverine and lacustrine zones in a large reservoir, and contrast with Daphnia depth segregation and migration patterns in natural lakes.

Larval feeding habits of Diaphus garmani and Myctophum asperum (Pisces: Myctophidae) in the transition region of the western North Pacific

We examined the feeding habits of the 2 dominant myctophid larvae, Diaphus garmani and Myctophum asperum, in the transition region of the western North Pacific. The incidence of feeding by these 2 species was ~10 times higher during the day than at night (66 to 71% vs. 5 to 6%), indicating that both larvae are daytime visual feeders. The prey of the 2 species were quite different: D. garmani depended mainly on appendicularian houses and copepod nauplii, while M. asperum fed mainly on ostracods and polychaetes. Prey size ingested increased with larval development, while niche breadth was independent of larval size, and did not change during development for either species. The average number of prey consumed by both larvae was 1 to 2 per gut, although an underestimate of prey number for D. garmani is possible owing to rapid digestion of fragile appen- dicularian houses. The number of prey was positively correlated with body length for D. garmani, but not for M. asperum. Competition for prey among these myctophid larvae and co-existing Japanese anchovy Engraulis japonicus larvae is unlikely because of their diet and habitat depth segregation.

Overlapping features can be parsed on the basis of rapid temporal cues that produce stable emergent percepts

When two overlapping displays alternate rapidly, it is difficult to resolve the temporal coincidence of objects, parts, or features. However, under certain conditions (at least for luminance-based stimuli) rapid temporal coincidence can be detected on the basis of stable emergent percepts in which parts that oscillate in phase appear more strongly grouped than (or appear distinct from) parts that oscillate out of phase. These emergent percepts appear as depth segregation, enhanced slow orientation rivalry, and oriented shimmer (a new phenomenon that cannot be explained in terms of conventional apparent motion or temporal contrast illusions). These percepts resulted in up to an eightfold decrease in the coincidence detection threshold (alternations as fast as 20 ms/frame or 25 Hz) relative to control conditions that did not yield them; these sensitivity enhancements are unlikely to be due to temporal probability summation. The results provide psychophysical evidence that temporal-phase information can contribute to the parsing of overlapping patterns.

Life-history traits of Plesionika martia (Decapoda: Caridea) from the eastern-central Mediterranean Sea

Life-history traits of Plesionika martia (Milne Edwards, 1883) were studied through data collected during six seasonal trawl surveys carried out in the Io- nian Sea (eastern-central Mediterranean) between July 1997 and September 1998. P. martia was found at be- tween 304 and 676 m depth, with the highest density in the 400-600 m range. Intraspecific, size-related depth segregation was shown. Recruitment occurred in sum- mer at the shallowest depths. Juveniles moved to the deepest grounds as they grew. The largest female and male were 26 and 25 mm carapace length, respectively. The sex ratio was slightly in favour of females at depths >400 m. Although a seasonal spawning peak was shown, the reproduction appears to be rather prolonged throughout the year. Females with ripe gonads were found from spring to autumn. Ovigerous females with eggs in late maturity stage were found year round. Large females could spawn more than one time within their annual reproductive cycle. The size at first maturity (50% of the ovigerous females) was 15.5 mm CL. Av- erage brood size of eggs with a well-developed embryo was 2,966±1,521. Iteroparity, low fecundity and large egg size patterns were observed. Brood size increased according to the carapace length. Two main annual groups were found in the field population of the Ionian Sea. Estimates of the Von Bertalanffy growth parame- ters are: L¥=30.5 mm, k=0.44 year -1 in females; L¥=28.0 mm, k=0.50 year -1 in males. A negative all- ometry was detected mostly in the ovigerous females. The life cycle of P. martia is discussed in the light of life- history adaptations shown in other deep-water shrimp species.

Depth segregation phenomenon in &lt;i&gt;Donax trunculus&lt;/i&gt; (Bivalvia: Donacidae) populations of the Algarve coast (southern Portugal)

A study was undertaken to evaluate the existence of depth segregation between size classes of Donax trunculus populations that occur along the Algarve coast (southern Portugal). Five collecting areas were sampled in the Ria Formosa barrier islands, namely in the Culatra, Barreta and Armona Islands, between March and June 1998. The study was based on size frequency distribution analysis and shell biometric relationships. The size frequency distribution showed the predominance of smaller individuals (< 25 mm) in the renewal area (0.5-1.0 m depth) and the prevalence of larger individuals (? 25 mm) at greater depths (4.0-6.0 m depth). The biometric study revealed the existence of significant differences in the type of growth between size classes in relation to depth distribution and burrowing activity. On the basis of these results, some specific management measures were proposed for this important bivalve fishery, taking into account the distinct fishing strategies and operations (manual and boat-operated dredging) performed at different depths.

Segregation and speciation in the Neogene planktonic foraminiferal clade Globoconella

The origin of the Neogene planktonic foraminifer Globorotalia (Globoconella) pliozea in the subtropical southwest Pacific has been attributed to its isolation resulting from intensification of the Subtropical Divergence (Tasman Front). Oxygen isotopic analyses suggest that, although the Subtropical Divergence may have played a role, the evolution of Gr. (G.) pliozea was facilitated by depth segregation of Gr. (G.) conomiozea morphotypes (low and high conical) during an interval of near-surface warming and increasing thermal gradient. Oxygen isotopic analyses suggest that low conical morphotypes of Gr. (G.) conomiozea inhabited greater depths than high conical morphotypes. Low conical forms of Gr. (G.) conomiozea are considered ancestral to the low conical species, Gr. (G.) pliozea. Oxygen isotopes indicate that Gr. (G.) pliozea inhabited greater depths than its ancestor, Gr. (G.) conomiozea.These data are consistent with depth-parapatric and depth-allopatric models, but not with a sympatric model of speciation. In the allopatric model, reproduction at different water depths acts as a barrier between morphotypes. In the parapatric model, clinal variation along a depth gradient acts as a barrier between morphotypes living at the limits of the gradient. Depth segregation in both models results in genetic isolation and evolutionary divergence. Our data support a correlation between morphological evolution and habitat changes in the Globoconella clade, implying separation of populations as a driving force for morphological evolution.Ecological segregation of morphotypes and species may be related to morphology (height of the conical angle), based on the data from Gr. (G.) conomiozea and Gr. (G.) pliozea. However, morphological differences alone do not necessarily produce depth differences. Large morphological differences between Gr. (G.) pliozea and closely related Gr. (G.) puncticulata did not result in isotopic and therefore depth differences between these species. These species coexisted at the same water depths for nearly 1 m.y. Thus, it is unlikely that the extinction of Gr. (G.) pliozea in the middle Pliocene resulted from competition with Gr. (G.) puncticulata, as previously suggested.

Young Infants' Perception of Illusory Contours in Dynamic Displays

Ninety-six 4-month-old infants were habituated to one of three computer-generated displays depicting two rod parts above and below an occluding box. In the first display, the surfaces and boundaries of the rod and box were specified by dense surface texture. Their depth segregation was specified by accretion and deletion of background texture and motion shear. In the second display, the unity of the rod parts and box, and their depth segregation, were specified only by illusory contours. In the third display, the boundaries of the rod and box were specified by illusory contours, perceptible only via spatiotemporal integration of accretion and deletion of sparse-background-texture elements. Infants appeared to perceive object unity, and segregate the rod and box surfaces, in all three displays, indicating use of illusory contours to perceive bounded surfaces in depth. The results suggest a cognitive contribution to perception of some illusory contours, abilities which seem to be present by at least 4 months of age.

Stereoscopic segregation of transparent surfaces and the effect of motion contrast.

Stereoscopic segregation in depth was studied using two superimposed frontoparallel surfaces displayed in dynamic random dot stereograms. The two patterns were positioned symmetrically in front of and behind a binocular fixation point. They were either stationary, or they could move relative to each other. Sensitivity for segregation was established by adding gaussian distributed disparity noise to the disparities specifying the two planes, and finding the noise amplitude that gave threshold segregation performance. Observers easily segregate the two surfaces for disparity differences between approximately 6 and 30-40 arcmin. Motion contrast, which by itself provides no cue to perform the task, greatly improves sensitivity for segregation. Noise tolerance rises by a factor of two or more when the patterns move at different speeds, or in different (frontoparallel) directions. The effect increases with directional difference, but the optimal directional difference deviated from 180 deg. The optimal speed varies with disparity difference. Thus, motion and disparity must interact in order to resolve the two transparent planes.

Luminance contrast affects motion coherency in plaid patterns by acting as a depth-from-occlusion cue.

Moving plaid patterns composed of component gratings that differ in luminance contrast tend not to cohere perceptually. Plaid patterns configured to mimic one occlusive grating overlying another also fail to cohere. We hypothesized that plaids constructed of components with different luminance contrasts fail to cohere because these components are interpreted as occlusive surfaces lying in different depth planes. It is known that when depth-from-occlusion and depth-from-binocular disparity cues support the same depth-ordering, both segregation in depth and motion non-coherency are more likely to be perceived than when these two cues conflict. We exploited this interaction and tested our hypothesis by introducing horizontal binocular disparity between two superimposed component gratings of different luminance contrasts. We found that both depth segregation and motion non-coherency were much more likely when the high-contrast grating was stereoscopically in front of the low-contrast grating. From these results we infer that luminance contrast acts as a depth-cue in plaid patterns, with higher contrast gratings appearing to lie in front of lower contrast gratings. Perceptual motion coherency parallels these depth-ordering judgments. We conclude that luminance contrast affects motion coherency by acting as a depth-from-occlusion cue.

Mating of Snow Crab Chionoecetes Opilio (Ο. Fabricius, 1788) (Decapoda, Majidae) in the Fjord of Bonne Bay, Newfoundland

Pairs of male and female snow crabs, Chionoecetes opilio, in mating courtship embrace were collected by SCUBA diving between 1984 and 1991, and with tangle nets between 1987 and 1993 in the Bonne Bay fjord, Newfoundland. A Nephrops trawl was also used to study the snow crab population size structure between 1988 and 1993. Males in mating courtship embrace were all identified as hard-shell morphometrically mature. Although large hard-shell juvenile males were present in the population, they did not participate in the reproduction. Comparison of the size structure of males in mating pairs and in the population also show that small mature male crabs can be excluded from mating by larger dominant mature males. The size structure between females in mating pairs and in the population was not significantly different. A depth segregation by size on the very steep slope of the fjord was observed for male snow crabs in mating pairs, as smaller males were found above 50 m. There was no correlation between male and female size in mating couples, but males were always larger than females. We conclude that there is a strong intra-specific competition between males during mating in nature.

Antiphase flicker induces depth segregation.

We examined the influence of the temporal phase of flickering stimuli on perceptual organization. When two regions of a uniform random-dot field are flickered in temporal alternation with the same flicker rate, one of the regions appears to lie in front of the other. Within the range of temporal frequencies used in the present experiments, depth perception was maximal between 5 and 31.3 Hz. Which region of the two is perceived as lying in front is different from person to person and sometimes fluctuates within the same subject, but when two regions are of different sizes, the smaller region tends to be perceived in front for longer than the larger region. The depth segregation was not due to a luminance difference, because the average temporal luminance of the regions was kept equal. Strikingly, the illusory depth segregation is perceived even between two adjacent regions whose densities of dots, sizes, shapes, and flicker rates are identical. This result suggests that a difference of temporal phase between two flickering regions is crucial for this new depth perception.

Stereoscopic Transparency and Segregation in Depth

We previously described the spatiotemporal requirements for binocular correlation in stereopsis using sinusoidal gratings-in-depth (Lankheet and Lennie, 1996 Vision Research36 527 – 538). The use of smooth sinusoidal surfaces emphasised the effects of spatial and temporal integration. Binocular correlation, however, depends not only on integration, but also on segregation at depth discontinuities. In the present experiments we therefore investigated segregation-in-depth, using random dot stereograms that depicted two transparent frontoparallel planes positioned symmetrically on either side of a binocular fixation point. Sensitivity for segregating the two planes was established by adding Gaussian distributed disparity noise to the disparities specifying the planes, and finding the noise amplitude that rendered transparency just detectable. The stimuli consisted of dynamic random-dot displays (dot lifetime 4 frames, at a frame rate of 67 Hz), generated in real time by a Macintosh computer, displayed on a television monitor, and viewed through a stereoscope. We used a method of constant stimuli and a 2AFC procedure. Two transparent planes were presented in one interval, and a single plane, with Gaussian distributed disparity values spanning the same range, was presented in the other. Segregation of stationary patterns was optimal for disparity differences of about ±9 min arc. Differences smaller than ±3 min arc and larger than about ±18 min arc could not be resolved. Motion contrast between the two patterns greatly facilitated segregation in depth. The facilitating effect increased with the difference in motion directions. The optimal speed varied with the difference in disparity.

Functional Aspects of Border-Signalling Mechanisms

Multiple border-signalling mechanisms have been proposed in some models of object segregation. We investigated whether distinct functions of different border mechanisms could be isolated in human psychophysical tasks. The display was a low-spatial-frequency (1.5 cycles deg−1) sinusoidal grating. Two types of centre-surround stimuli were produced by (i) increasing or decreasing the mean luminance of a central patch of the grating to create a signed luminance border (LB), or (ii) by shifting the grating within the central patch 180° to create a sign-reversing border (PSB) without a change in mean luminance. Grating contrast was kept constant at 20%. Observers performed a spatial 2AFC task in each condition. The PSB stimulus served as the comparison stimulus. Test stimuli were either LB or PSB+LB. The mean luminance of the central test patches varied over trials to create border contrasts of −15% to 15% Michelson contrast. In separate sessions, subjects made four types of comparisons: which of the two central areas appeared (1) to differ more in depth from its surround; (2) to have greater modulation contrast of the sinusoid; (3) lighter/darker, (4) to have higher border contrast. Results depended strongly upon test border type and suggested that the mechanism responding to each has a distinct and different perceptual function, and the two may interact. Our results support the hypothesis that surface lightness is calculated by a sign-preserving mechanism, whereas a phase-insensitive mechanism is involved in percepts relating to segregation in depth.

Depth profiling by ARXPS in surface analysis.

A method of nondestructive depth profiling in near surface regions of solids is described. Models have been discussed from which algorithms for evaluation of measured data are obtained. The algorithms, based on standard profiles with free parameters, have been adjusted to the data resulting from angle resolved XPS (ARXPS) by means of least squares fits. Depth profile analyses and segregation studies were performed on Pt-Ni and Fe-S specimens.

Reproductive Biology of the Black Dogfish,Centroscyllium Fabricii, Collected From Waters off Western Greenland

The reproductive biology of 1124 male (165–760 mm TL) and 1476 female (175–898 mm TL) black dogfish,Centroscyllium fabricii, was examined. The sharks were caught off western Greenland with bottom otter trawls. Size at maturity was about 550 mm TL in males and 650 mm TL in females. Ovarian ova did not continue to develop during gestation. The sex ratio of embryos was 1:1. Litter size ranged from 4 to 40, with a mean of 16–4. The total number of mature ova, fertilized ova, or embryos tended to increase with size of the females. Development of black dogfish embryos is dependent solely on yolk reserves. Near-term embryos (152–192 mm TL) had completely absorbed their external yolk sacs. The smallest free-living specimens collected in this study were 165 mm TL for males and 175 mm TL for females. The black dogfish does not seem to have a well-defined breeding season. This species displayed a well-defined pattern of depth segregation by size. The sex ratios in five shallow depth strata (500–599, 600–699, 700–799,800–899, and 900–999 m) were 1:1, but those of three deeper depth strata (1000–1099,1100–1199, and 1200–1299 m) contained a significantly greater number of females than males. Four abnormal hermaphrodites possessed gonads containing both testicular and ovarian tis-sues. Two specimens had normal female reproductive organs (e.g. ovaries, oviducts, nidamental glands, and uteri) as well as claspers.

Social and reproductive behavior of a captive group of walleye pollock,Theragra chalcogramma

The social and reproductive behavior of a group of four male and seven female walleye pollock,Theragra chalcogramma, were observed in a large tank. Pollock spent most of their time swimming in a loose aggregation near the surface. Males descended from the aggregation more often than females to follow and make physical contact with other males as well as with females. The difference between males and females in the frequency of diving in our tank is consistent with the reported pattern of depth segregation of the sexes in natural pollock spawning aggregations. The frequency of social interactions increased when pollock became reproductively active and was higher at night and during twilight when most of the spawning occurred. Male interactions with females most frequently involved physical contact, while male interactions with other males were more often limited to following. There was no indication that male-male interactions result in the formation of stable social dominance relationships that determine priority of access to mates, as has been suggested previously for walleye pollock. Rather, following and contact interactions appear to promote male identification of potential mates and encounters with ripe females. The possible functional significance of male social interactions is discussed in relation to reports on natural walleye pollock spawning aggregations.

Size at maturity, reproductive cycle, fecundity, and depth segregation of the deep sea squaloid sharks Centroscymnus owstoni and C. coelolepis in Suruga Bay, Japan.

Specimens of the deep sea sharks, Centroscymnus owstoni and C. coelolepis, were collected from Suruga Bay, Japan to examine their reproductive habits. Size at maturity for both species was about 700mm TL in males and 1000mm TL in females. Neither species has a well defined breeding season. The reproductive cycle appears to extend over one calendar year. The ratio of males to females was approximately 1:1 for C. owstoni and 1: 2 for C. coelolepis. Litter size ranged from 16 to 28 for C. owstoni and from 15 to 29 for C. coelolepis. Total number of mature ova, fertilized ova and embryos tended to increase with size of the female. Females of both species displayed a well defined pattern of depth segregation by stage of maturity. The catch depths of pregnant females for both species were shallower than those of non-pregnant ones. However, pregnant females with full-term embryos were not caught in shallow waters of Suruga Bay.