Effect of light and substratum structural complexity on microhabitat selection by the snail Helix aspersa muller

Terrestrial snails choose their microhabitat according to anumber of environmental factors. We evaluated the effect oflight intensity and substratum complexity on microhabitat preferenceof the terrestrial snail Helix aspersa using a multi-factorialdesign. The snails were offered two levels of light intensityand two types of structural complexity, hence 16 treatmentsin total were used: 12 in which choice was offered and 4 inwhich no choice was offered. The snails preferred ambient lightover dim light, regardless of substratum complexity, and complexover smooth substrata, regardless of light intensity. The levelof one factor did not affect the response to the other. Thus,the results revealed a preference for microhabitats with greaterlight intensity and that were structurally complex, and a rejectionof dimly lit microhabitats with smooth substrates. (Received 17 March 2006; accepted 21 September 2006)     

Patch selection by juvenile black surfperch (Embiotocidae) under variable risk: Interactive influence of food quality and structural complexity

The influence of risk on the selection of foraging patches by young-of-year black surfperch, Embiotoca jacksoni Agassiz, was investigated by laboratory and field experiments. These foragers harvest crustacean prey from a variety of benthic algal substrata. In field environments, patch types vary in two ways. First, substrata differ in structural complexity and probably afford different degrees of protection from predators. Second, substratum types vary in prey richness. There was no correlation between structural complexity and prey richness, and either or both factors could be a component of foraging patch value. Each patch is small and individual foragers are simultaneously confronted with arrays of patches encompassing the full range of variation in structure and prey richness. Furthermore, a major predator of young-of-year black surfperch, the kelp bass, Paralabrax clathratus (Girard), is patchily distributed in space and time. Thus similar arrays of patch types can be characterized by different levels of overall risk. Risk to foragers is dependent on light level as well as the presence and density of predators.The interplay between food quality and shelter potential in influencing patch choice was examined under different regimes of risk. Both laboratory and field experiments indicated patch preference was based primarily on food quality. However, the physical structure of a patch did become a component of patch choice as risk increased. The relative value of physical structure under high risk was dependent on the prey richness of a patch; food-poor substrata with high shelter potential remained unfavored even in situations of high risk.     

Habitat selection of juvenile banana prawns, Penaeus merguiensis de Man: Testing the roles of habitat structure, predators, light phase and prawn size

The effects of fish predators, light phase, habitat structure and prawn size on the habitat preferences of juvenile Penaeus (Fenneropenaeus) merguiensis de Man were examined with laboratory experiments. The behaviour of juvenile P. merguiensis within habitats of different structural complexity was also examined. Experiments were carried out in a tank (1.8 m diameter) divided into four habitats representing: bare substratum, leaf litter (little vertical structure), mangrove pneumatophores (regular vertical structure) and mangrove woody debris (heterogeneous vertical structure). The location of 10 prawns was monitored over 270 min (135 min light and 135 min dark), with different prawns five times for each combination of prawn size class, and predator (no predator; Arius graeffei Kner and Steindachner and Lates calcarifer Bloch).In the absence of predators and during the light phase, when observations on prawn behaviour were made, swimming was the most common behaviour (of seven mutually exclusive behavioural categories) with few differences in behaviour between sizes. All size classes of juvenile P. merguiensis selected vertical structure (mangrove debris and pneumatophores) over low vertical structure (leaf litter and bare substratum), in both light and dark conditions and in the presence or absence of predators. When L. calcarifer was present, the selection by prawns of the mangrove-debris habitat increased significantly. This was attributed to an increase in predation risk in the other habitats. L. calcarifer rarely pursued prey amongst the mangrove-debris structure, compared to habitats with less heterogeneous vertical structure (pneumatophores, leaf litter and bare substratum).     

Structure of the organs of attachment of brachiolaria larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) (Echinodermata: Asteroidea)

The structure of the brachiolar arms and adhesive disk of the brachiolaria larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) was determined from light microscopy and from scanning and transmission electron microscopy. The structure of these organs was very similar in both species.The brachiolar arms are comprised of a stem region terminating in a crown of adhesive papillae which are made up of a variety of secretory cell types. Principal among these are elongated cells producing very electron-dense secretory particles, which are released at the free cell surface attached to cilia. Secretory particles appear to be important in temporary attachment of the brachiolar arms to the substratum. Ciliary sense cells, possibly used in the recognition of specific substrata are located at the tip of adhesive papillae.The adhesive disk is comprised of large cells packed with secretory droplets and elongated intracellular fibres. In the attached adhesive disk, secretory droplets are lost, having formed the cement that attaches the disk to the substratum. It appears that adhesive papillae lateral to the adhesive disk hold the disk in position close to the substratum during secretion and hardening of the cement. The intracellular fibres are the principal anchoring structures running from microvilli (locked into the attachment cement) on the surface of the disk to the underlying connective tissue of the attachment stalk.     

Effect of the substratum in the recruitment and survival of the introduced barnacle Balanus glandula (Darwin 1854) in Patagonia, Argentina

The barnacle Balanus glandula was introduced in Argentina in the 1970s, and today it dominates the high intertidal level in most Argentinean rocky shores. The aim of this work was to evaluate the effect of the type of substrata and intertidal height on a population of Balanus glandula by conducting field surveys and one-year field experiments in which we combined different substrata (hardness: hard and soft, and texture: smooth and rough) at two intertidal heights (mid and high). In natural populations, the highest density of adults and recruits occurred on soft-rough substratum and in the high intertidal. The different textures were important only on the soft substrata and high intertidal, and the density of barnacles of the soft-rough substrata was higher than soft and smooth ones. The most suitable experimental substratum was the soft-rough of the high intertidal, which had the highest recruitment, survival and final density of barnacles at the end of the experiment. In contrast, the hard and smooth of the high and middle intertidal were the least suitable in all cases. Although the recruitment of B. glandula occurred throughout the year, it was higher in the high intertidal, and it showed a recruitment peak in the winter and a second in the summer. While most studies on this barnacle investigated the effects of granite or other volcanic hard substrata, our study also focused on soft substrata. The effects of soft substrata are particularly important because soft sedimentary rocks characterise the southern Atlantic coast of South America and the presence of soft rocks appears to optimize the success of Balanus glandula.     

Substratum preferences and planulae settling of two red sea alcyonaceans: Xenia macrospiculata Gohar and Parerythropodium fulvum fulvum (Forskl)

The settling behaviour and substratum preferences of the planulae of the Red Sea soft corals Xenia macrospiculata Gohar and Parerythropodium fulvum fulvum (Forskl) were examined in the laboratory. The planulae of the two species have a short pelagic phase and they tend to settle immediately upon leaving the parent colonies. Mucous secretion is used by the larvae for crawling and adhering to the substratum. They exhibit an aggregated pattern of settlement. The developing polyps are found in depressions or pits of the substratum. The planulae preferentially settle on rough substrata and avoid smooth surfaces. They search for substrata covered with an organic coating, composed of turf or crustose coralline algae. Such substrata create better conditions for larval settlement and metamorphosis. The planulae of P. f. fulvum exhibit a striking preference for upside-down attachment on undersides of the substrata, while Xenia macrospiculata utilizes both substratum faces for settlement. Light intensity seems insignificant in determining attachment sites. The findings of the experiments correspond well with the distributional patterns of juveniles of the two species as found in the natural environment. The specific requirements for settling of both species increase their chances of successful development and thus enhance their survival.     

First record of the association between Malmgreniella cf. variegata (Polychaeta, Polynoidae) and Ophionereis annulata (Echinodermata, Ophionereididae) in the Mexican Pacific

An association between the scale-worm Malmgreniella cf. variegata and the ophiuroid Ophionereis annulata is reported for first time in the Mexican Pacific. The relationship was found in five localities across a total of 22 sites where O. annulata was collected. The polynoids collected were similar to the Western Atlantic scale-worm Malmgreniella variegata, which was previously recorded as commensal of O. annulata on the Pacific side of Panama, although they differ in colour pattern and the presence of notochaetae with a unidentate tip. The prevalence of infestation was 5 % and the intensity was 1.20. Experimental observations suggest that the behaviour of the scale-worm on O. annulata could be affected by light conditions, and that the commensal had a preference for its host over other ophiuroid species. More studies should be carried out to determine the taxonomic status of M. cf. variegata in the Eastern Pacific and to further elucidate the commensal–host relationship between this scale-worm and O. annulata.     

Taphonomy of dense ophiuroid accumulations from the Middle Triassic of Poland

The taphonomy of three Middle Triassic (Muschelkalk) monospecific ophiuroid taphocoenoses, comprising Aspiduriella similis (Eck), from different regions in Poland (Holy Cross Mountains, Upper Silesia and North-Sudetic Basin) has been investigated. The majority of specimens (88.5% of a total of 428 individuals) are partially disarticulated, having only proximal and median portions of their arms preserved (Taphonomic Group 2). Pristine specimens, with only faint traces of disarticulation (Taphonomic Group 1), as well as those that preserve the disc only, or which have only proximal stumps of arms preserved (Taphonomic Group 3), are much rarer (6.5% and 5%, respectively). Moreover, most specimens (76.4%) are oral side up. Only 19.2% of specimens are preserved in life position, and a small fraction (4.4%) are preserved oblique to bedding. All ophiuroids studied occur in thin, pelitic layers devoid of any trace fossils. Associated body fossils, such as bivalves, gastropods or crinoids, are very rare. Taking into account ophiuroid taphonomy, as well as sedimentological characteristics of the thin layer (burial layer), it is clear that all assemblages were transported prior to burial. The predominance of articulated skeletons indicates that the burial event (obrution event) was not only rapid, but also single – subsequent events would have destroyed the previously buried ophiuroids. Storm-related resuspension of fine-grained material from nearshore environments which covered the ophiuroids is the most likely burial agent. The fact that in all regions the ophiuroid taphofacies studied is identical suggests that the three assemblages underwent the same burial history, and that a single event might have been responsible.     Muschelkalk , ophiuroids , Poland , taphonomy , Triassic .     

Colonization and substratum preference of an introduced burrowing crustacean in a temperate estuary

Habitat use in marine invertebrates is often influenced by multiple abiotic and biotic factors. Substratum composition is one factor known to have a dramatic effect on habitat selection. The Australasian burrowing isopod (Sphaeroma quoianum, H. Milne Edwards 1840) is a common introduced species in many estuaries on the Pacific coast of North America. S. quoianum burrows into a variety of firm substrata including marsh banks (composed of peat, clay, and/or mud), wood, friable rock, and Styrofoam floats. In some areas, isopods achieve high densities and may accelerate the rate of shoreline erosion and damage marine structures; thus, understanding the substratum preference of this species may be important for conservation and management efforts. Field experiments were conducted in Coos Bay, Oregon to examine substratum preference, burrowing rates, and the life stage of colonizers. In three experimental trials (Fall 2005, Spring 2006, Fall 2006), replicates of four intertidal substrata (marsh banks, decayed wood, sandstone, Styrofoam) were deployed near intertidal populations of S. quoianum. The numbers of burrows created in each substratum were enumerated weekly or daily (depending on trial). After the trials were completed, the total numbers of isopods inhabiting each substratum were counted. In weeks, S. quoianum extensively burrowed the substrata but exhibited a distinct preference for decayed wood. Significantly more isopods were present in wood than the other substrata at the end of the experiments and rates of burrowing were greatest in wood, although significance varied across time in one trial. Nearly 90% of colonizing isopods were under 5 mm in length suggesting that juvenile isopods primarily colonize intertidal substrata. Differences between burrow densities measured in the field and the results from these preference trials may indicate other factors, such as relative availability of substrata, recruitment and dispersal limitations, and possible gregarious behavior also influence local isopod densities.     

Development of the nervous system in the brittle star Amphipholis kochii

There are several studies of neural development in various echinoderms, but few on ophiuroids, which develop indirectly via the production of pluteus larvae, as do echinoids. To determine the extent of similarity of neuroanatomy and neural development in the ophiuroids with other echinoderm larvae, we investigated the development of the nervous system in the brittle star Amphipholis kochii (Echinodermata: Ophiuroidea) by immunohistochemistry. Immunoreactive cells first appeared bilaterally in the animal pole at the late gastrula stage, and there was little migration of the neural precursors during A. kochii ontogeny, as is also the case in echinoids and holothuroids. On the other hand, neural specification in the presumptive ciliary band near the base of the arms does occur in ophiuroid larvae and is a feature they share with echinoids and ophiuroids. The ophiopluteus larval nervous system is similar to that of auricularia larvae on the whole, including the lack of a fine network of neurites in the epidermis and the presence of neural connections across the oral epidermis. Ophioplutei possess a pair of bilateral apical organs that differ from those of echinoid echinoplutei in terms of relative position. They also possess coiled cilia, which may possess a sensory function, but in the same location as the serotonergic apical ganglia. These coiled cilia are thought to be a derived structure in pluteus-like larvae. Our results suggest that the neural specification in the animal plate in ophiuroids, holothuroids, and echinoids is a plesiomorphic feature of the Ambulacraria, whereas neural specification at the base of the larval arms may be a more derived state restricted to pluteus-like larvae.     

The interplay of substrate nature and biofilm formation in regulating Balanus amphitrite Darwin, 1854 larval settlement

The settlement of marine larvae is influenced by a wide range of physical and biological factors. It is still poorly known how the nature of substrate and the biofilm can interact in regulating settlement patterns of invertebrate larvae. Here we use laboratory experiments focused on settlement behaviour of the barnacle Balanus amphitrite. The aim of this work is to understand whether: (i) the nature of substratum can affect biofilm formation and its structure, (ii) the nature of substratum can affect B. amphitrite larval settlement, (iii) the age of the biofilms and the nature of substrate can interact in influencing larval settlement.Four kinds of substrata (marble, quartz, glass, and cembonit) were biofilmed under laboratory conditions for 5, 10 and 20 days at the temperature of 28 °C. Settlement response was investigated with 5-day-old cyprids. Biofilms were quantitatively and qualitatively analysed by scanning electron microscopy. The settlement of B. amphitrite larvae significantly differed among substrata; also, the patterns of development of biofilm assemblages changed with substrate. In addition, the larval attractiveness of different substrates tends to disappear with biofilm age.     

Preservation of tube feet in an ophiuroid (Echinodermata) from the Lower Devonian Hunsrück Slate of Germany and a redescription of<Emphasis Type="Italic">Bundenbachia beneckei</Emphasis> and<Emphasis Type="Italic">Palaeophiomyxa grandis</Emphasis>

The preservation of non-mineralized tissues in the fossil record is extremely rare. The Lower Devonian Hunsrück Slate of Germany has long been known for the preservation of non-mineralized tissues in pyrite but whether or not these remnants represent true soft tissues has been questioned. This is especially true for struetures visible only on radiographs that are too delicate for excavation by traditional methods. Here we report the discovery of well-preserved pyritized tube feet in six fully prepared specimens of the protasterid brittle starBundenbachia beneckei from the Hunsrück Slate. This discovery represents the first report of fossilized ophiuroid tube feet in the fossil record. The successful excavation of the delicate tube feet was made possible by improved airbrasive techniques developed by German fossil collectors. The relatively large size of the fossil tube feet inBunden-bachia beneckei is consistent with earlier inferences on size based on the presence of large podial basins. Protasterid ophiuroids lack the specialized arm musculature and articulations that provide increased flexibility and strength to the arms of modern ophiuroids with typically reduced tube feet. How-ever, tube foot form and perhaps function inBundenbachia might have been similar to those of living asteroids in which large tube feet are used primarily for locomotion and food-manipulation thus compensating for a lack of specialized arm musculature and articulation. Hence, feeding and life mode of protasterid ophiuroids was not necessarily limited to sedentary, infaunal microphagy as traditionally suggested. Two Hunsrück protasterid ophiuroids,Bundenbachia benecki andPalaeophiomyxa grandis are redescribed and compared.      

A new brittle star (Ophiuroidea: Ophiodermatina) from the Early Triassic Paris Biota (Bear Lake County,Idaho, USA)

Knowledge on the morphology of Early Triassic ophiuroids is very limited, in spite of the relatively high number of described species. This hampers attempts to use morphology-based phylogenetic analyses in order to explore the early diversification of the ophiuroid modern clades between the latest Paleozoic and the early Mesozoic. Here, a new ophiuroid from the Early Triassic of southern Idaho (USA) is described and assigned to the new taxon Shoshonura brayardi Thuy nov. gen., nov. sp. It is part of an exceptional Early Triassic faunal assemblage recently described as the Paris Biota. On account of the arm morphology and the shape of the arm spines and arm spine articulations, Shoshonura brayardi nov. gen., nov. sp. is considered a basal member of the suborder Ophiodermatina.     

Brittle stars from the upper Cenomanian of the Preafrican platform: First ophiuroid remains for the Cretaceous of Algeria

While Late Cretaceous ophiuroids are relatively well known in Europe, these faunas have been much less studied in North Africa. With the exception of some Tunisian assemblages preliminary described at the turn of the 21st century, nothing is known about the Cretaceous brittle stars of the southwestern Tethyan margin. The present paper seeks to bring the first data about hitherto unknown ophiuroids recently found in the early upper Cenomanian succession of the eastern side of the Preafrican trough (Menaguir section, Algeria). This “community” of brittle stars comprises at least eight species. Most of them are probably new, but have not been formally named here. These are representatives of the families Hemieuryalidae, Amphiuridae, Ophiodermatidae, Ophiacanthidae, Ophiopezidae and probably also Ophiomyxidae and Ophiobyrsidae. Almost all vertebrae are zygospondylous; no streptospondylous vertebrae indicate the absence of the order Euryalida here. Most of the ophiuroids belong to the orders Amphilepidida and Ophiacanthida. Ophiotitanos serrata, Ophiomyxa? aff. jekerica, Ophiojagtus? sp. and some other taxa resembling ophiuroid assemblages from the Late Cretaceous of central, western and northern Europe. With respect to the late Cenomanian age, the depth of the sea and the taxonomic composition, there are some similarities with ophiuroids of the Bohemian Cretaceous Basin. The mid-ramp subtidal facies suggests that brittle stars lived here in a warm, euphotic and probably shallow sea.     

Brittle-star bioluminescence functions as an aposematic signal to deter crustacean predators

The behaviour of three species of nocturnally active, visually orienting crabs (Portunus sebae, P. spinnimanus and P. ordwayi) was observed to determine whether the luminescent signals produced by Ophiopsila riisei (Echinodermata: Ophiuroidea) function as an aposematic deterrent against crustacean predators. In repeated experimental trials, crabs showed more rapid rejection of luminescent unpalatable ophiuroids than of non-luminescent controls. After five trials, unpalatable luminescent prey were rejected three times as quickly as either unpalatable or palatable non-luminescent controls. Over the course of the trials, crabs damaged fewer luminescent ophiuroids than non-luminescent controls. Furthermore, blind crabs (that could not perceive the luminescent flashes) caused significantly more damage to luminescent brittle-stars than did crabs with intact eyes (that could see the light signals). All three species of crab ophiuroids versus non-luminescent controls. Ophiopsila riisei survived the majority of crab attacks, and crabs can therefore learn to reject unpalatable luminescent prey without killing them. This suggests that individual selection may be an important mechanism for the evolution of aposematic luminescent signals in this, and probably other, luminescent species.     

Diel variation in predator abundance, predation risk and prey distribution in shallow-water estuarine habitats

Predation by visual predators is often affected by light conditions and may therefore exhibit strong diel variation. The dominant predators on grass shrimp, Palaemonetes pugio, are finfish predators that are thought to locate their prey by visual cues. We examined the response of grass shrimp to diel variation in predation risk in the nearshore shallow waters of the Chesapeake Bay. We used diel shoreline seines to assess the relative abundance of predators. We assessed the relative risk of predation with shrimp tethered at refuge (30 cm) and nonrefuge (60 cm) depths. To measure grass shrimp response to predation risk, we used dipnets to monitor habitat use. Four predominantly visual predators dominated the shoreline seine catches, Fundulus heteroclitus, Micropogonias undulatus, Morone americana and Morone saxatilis. Total predator abundance had a diel component, with dramatic nighttime decreases in total abundance, whereas guild composition and relative abundance remained unchanged. Relative predation risk for tethered shrimp exhibited significant time by habitat interaction. During the day, depth negatively affected survivorship of tethered shrimp while at night overall survivorship increased and there was no effect of depth. Shrimp habitats use reflected diel predation risks. Abundances in the near shore were highest during the day with decreased abundances at night. Together, the seine and tethering data highlight the importance for a refuge (e.g., shallow water) from predation during the daytime and a relaxation of predation pressure at night.     

Size‐dependent microhabitat use and intraspecific competition in Cottus gobio

An electrofishing survey of daytime shelter microhabitat use of bullhead Cottus gobio in a southern English chalk stream revealed positive selection for moderate water velocity, vegetation cover and coarse substrata. Water depth, other forms of cover, shade and substratum embeddedness had no significant influence on the distribution of fish. Microhabitat use was size‐dependent, with patches occupied by adult fish containing coarser substrata and less blanket weed ( Cladophora algae) than those occupied by smaller juvenile conspecifics. Differences in substratum use between size‐classes were less pronounced in parts of the stream shaded by the tree canopy. In laboratory tanks stocked at low fish density, both juveniles and adults favoured use of cobbles over pebbles. The response of fish to increased conspecific density was size‐dependent; juveniles reduced use of the coarse substratum whereas adults maintained their predominance in this habitat. An apparently greater shift by juveniles when in the presence of adults was significant at α = 0·10 only, as was an apparent reduction in interactions between size‐classes under low light intensity. The displacement of small juvenile fish from the preferred cobble substratum is consistent with the hypothesis that intraspecific competition contributes to the size‐related microhabitat shift observed in the field. Although there was a tendency for the strength of competition to be reduced at low light levels, the mechanism by which tree canopy cover affects microhabitat use remains uncertain.     

Settlement and post-settlement processes limit the abundance of the geniculate coralline alga Calliarthron on subtidal walls

Cover of geniculate coralline algae is significantly greater on subtidal platforms than on vertical walls in a central California kelp forest. To determine the cause(s) of this pattern, settlement and growth of Calliarthron were compared on horizontal and vertical substrata attached to platforms or walls. Recruitment on horizontal substrata attached to reef walls indicated that spores were abundant along the walls. Densities of recruits were significantly smaller on vertical substrata attached to walls than on horizontal substrata attached to platforms, indicating that Calliarthron settlement is reduced on vertical substrata. Growth of basal crusts and the densities of fronds were also significantly greater on horizontal substrata attached to platforms than on vertical substrata attached to walls. Irradiance reaching wall surfaces was less than irradiance reaching platform surfaces. We suggest that light reaching wall surface is low enough to affect Calliarthron growth. These results indicate that the low abundance of Calliarthron on subtidal walls is a result of poor recruitment and slow growth.     

Habitat complexity modifies the impact of piscivores on a coral reef fish population

Patterns in juvenile mortality rates can have a profound affect on the distribution and abundance of adult individuals, and may be the result of a number of interacting factors. Field observations at Lizard Island (Great Barrier Reef, Australia) showed that for a coral reef damselfish, Pomacentrus moluccensis, juvenile mortality (over 1 year) varied between 20 and almost 100% among sites. Correlative data showed that juvenile mortality increased as a function of initial densities (recruitment), predator densities and the availability of preferred coral substrata. A multiple regression showed that these three variables together did not explain significantly more variation in mortality than the single factor showing the strongest relationship. This appeared to be because recruitment, predator densities and preferred coral substrata were all highly correlated, suggesting that one, two or all of these factors may be influencing juvenile mortality rates. One hypothesis was that density-dependent mortality in juveniles was the result of an interaction between predators (which appear to aggregate at high-recruitment sites) and the availability of preferred substrata (predator refuges). We tested this hypothesis by using both laboratory and field experiments to see whether fish predation could significantly alter survivorship of this damselfish, and whether this impact was dependent upon the coral substratum. The laboratory experiment was designed to test the effects of three common predators (Pseudochromis fuscus, Cephalopholis boenak and Thalassoma lunare) and three different coral substrata that varied in their complexity (Pocillopora damicornis, Acropora nasuta and A. nobilis) on the survival of juvenile Pomacentrus moluccensis. There was a significant interaction between predator species and microhabitat in determining survival. Pseudochromis fuscus and C. boenak were both significantly better at capturing juvenile damselfish than T. lunare. Juvenile survivorship was significantly better when they were given the more complex corals, Pocillopora damicornis and A. nasuta, compared with those given the open-structured species A. nobilis. This pattern reflects habitat selection in the field. Predators differed in their strike rates and the proportion of strikes that were successful, but all exhibited greater success at prey capture where A. nobilis was provided as shelter. The interaction between the effect of predator species and microhabitat structure on damselfish survival was tested in the field for a cohort of juvenile Pomacentrus moluccensis. We examined juvenile survival in the presence and absence of two predators that co-occur on natural patch reefs (C. boenak and Pseudochromis fuscus). The experimental patch reefs we used for this purpose were constructed from both high complexity (Pocillopora damicornis) and low complexity (A. nobilis) coral substrata. Both juveniles and predators were translocated to reefs at natural densities. The effects of predation were clearly dependent upon the microhabitat. Reefs of the high-complexity coral with predators supported the same high numbers of Pomacentrus moluccensis as the reefs with no resident predators. However, damselfish abundance was significantly lower on low-complexity reefs with resident predators, relative to the other treatments. Background rates of loss were high, even on preferred coral in the absence of the manipulated predator, suggesting that transient predators may be even more important than the residents. We suggest that adult abundances in this species were strongly influenced by the densities of different predators and the availability of preferred refuges. Received: 3 April 1997 / Accepted: 26 August 1997     

Tidal activity pattern and feeding behaviour of the ophiuroid Ophiocoma scolopendrina on a Kenyan reef flat

Ophiocoma scolopendrina exhibits a distinctive pattern of feeding activity on intertidal reef platforms off Kenya. With the first wave of the flooding tide, dense aggregations of these ophiuroids (up to 320 m⁻²) engage in a 1–2 min burst of surface-film feeding, vigorously sweeping the air-water interface and associated sea foam with the ventral surface of 2–4 arms. Suspension feeding (with arms extended in the water column) is the primary feeding mode throughout the rest of the tidal cycle (involving 25–65% of the population at a time), while bottom feeding (with arms extended along the substratum) is infrequent (<10%). Field experiments showed that surface-film feeding is regulated by water depth and can be triggered by suspended particles. This feeding mode appears to be an adaptation to the intertidal habitat, enabling the ophiuroids to exploit a nutrient-rich surface film during a temporal refuge (low tide) from fish predation. Dense populations of O. scolopendrina may represent an important trophic link between producers of particulate organic material and higher-level consumers in coral reef environments.Tara Oak and Robert E. Scheibling contributed equally to this paper. The order of authorship is alphabetical