Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication     

Experiments on orientation of the intertidal barnacle Tesseropora rosea (Krauss)

The intertidal barnacle Tesseropora rosea (Krauss) exhibits very marked directionality of orientation on sloping and vertical surfaces. The barnacles are orientated so that their cirral fans face the water current; this pattern was found within 1 wk after settlement. Adults also showed similar orientation to the water current at each site.Three manipulative experiments were done in the field to determine the consequences of incorrect orientation by the barnacles. Correct orientation was necessary for T. rosea to maintain its tissue weight, but experimental rotation of barnacles through 90° or 180° made no difference to rates of shell growth, mortality or weight of egg masses. Furthermore, this species was unable to re-orientate once it had metamorphosed, and orientation of newly-settled barnacles was not influenced by that of adults.     

Behavioural components of prey-selection by dogwhelks, Nucella lapillus (L.), feeding on barnacles, Semibalanusbalanoides (L.), in the laboratory

Adult dogwhelks, maintained on mussels for 60 days before experimentation to reduce prior effects of ingestive conditioning and handling skills appropriate to barnacles, and thence deemed “inexperienced”, preferred the largest barnacles presented to them. Juvenile and adult dogwhelks, maintained on barnacles and deemed experienced, preferred barnacles of intermediate sizes that were correlated with the sizes of the dogwhelks. “Inexperienced” dogwhelks penetrated barnacles significantly more often by drilling than by prising, whereas experienced dogwhelks did the reverse. A predominant tendency to prise open barnacles was developed by previously “inexperienced” dogwhelks after they had eaten six to eight consecutive prey. Larger dogwhelks prised open greater proportions of larger barnacles than did smaller dogwhelks. Experienced adult dogwhelks prised open all barnacles of ≈2-mm opercular diameter but only 20% of those ≈ 6 mm in diameter. The preferred barnacles, 4–5 mm in opercular diameter, were of a size that the experienced adult dogwhelks could prise open in 50% of attacks.Dogwhelks inspected barnacles by crawling over them for ≈20 min when the prey were subsequently rejected and ≈30 min when subsequently eaten. Penetration time and ingestion time were linear functions of barnacle diameter, total handling time ranging from 4–26 h for barnacles of 2.5–2.7 mm opercular diameter. Experienced dogwhelks handled barnacles faster than “inexperienced” dogwhelks, largely because they used the quicker method of penetration by prising whenever possible. The yield of flesh per unit handling time was an accelerating function of barnacle size and, with experience, could be increased by a factor of ≈1.2 for barnacles of 7 mm opercular diameter to ≈2.0 for barnacles of 2 mm opercular diameter. The preference of experienced dogwhelks for barnacles somewhat smaller than the largest available, may have reflected the greater frequency with which these prey could be penetrated by the quicker method of prising. Minimizing handling time could be important in nature if there is a risk to feeding dogwhelks of being displaced by competitors. In the laboratory, 12% of dogwhelks where thus displaced.Barnacles of ≈5 mm opercular diameter were estimated to be slightly more profitable than mussels of ≈17.5-mm shell length to a dogwhelk experienced with barnacles, but less profitable to a dogwhelk that has recently fed extensively on mussels. Dogwhelks, therefore, might switch between diets predominately of barnacles or of mussels if prey of comparable profitabilities change drastically in their relative abundance on the shore.     

Induction of Settlement and Metamorphosis of the Veliger Larvae of the Nudibranch,Onchidoris bilamellata

Summary

Onchidoris bilamellata veligers were reared in the laboratory on a combination of phytoñagellates and diatoms. They attained metamorphic competence after a period of 28 to 32 days at 11°C, or 60 to 80 days at a temperature averaging 7.5° C.

Experimental evidence suggests that settlement is stimulated by a diffusible chemical emanating from living barnacles, whereas metamorphosis is induced by a chemical or mechanochemical cue, which is also associated with barnacles. Settlement and metamorphosis are considered to be separable events in O. bilamellata. The settlement response is reversible and can be repeated; it involves a characteristic behavioral repertoire including descent to the bottom, foot contortions and crawling on the pedal sole. Settlement occurs only in seawater that contains, or had previously contained, living barnacles. Metamorphosis is irreversible and involves the resorption of the velum, loss of the larval shell, and incorporation of the visceral mass into the cephalopedal mass. Metamorphosis is triggered only when physical contact with living or dead barnacles is made (dead barnacles refers to shell and tissue fragments which are only effective in inducing metamorphosis when they are used in combination with seawater that had previously contained living barnacles).

Settlement and metamorphosis in O. bilamellata is compared with that of other nudibranch species, and its unique settlement response is discussed.     

Ecology of the intertidal snail Littorina acutispira Smith

Littorina acutispira Smith, a minute gastropod of < 3 mm shell height, lives at great densities in pools and on rock-surfaces at the highest levels of sea-shores in New South Wales. Populations from pools and dry areas were sampled on two shores for 18 months to investigate seasonal changes in density, size-structure, rates of growth and reproductive biology of the snails. Densities of snails increased between February and May, due to an influx of juveniles, and then declined until the following February, when they increased again. The decrease in density was due to the death of the largest snails at the end of summer, and the mortality of medium-sized snails between June and January. Longevity was estimated as 1–2 yr, but most individuals died by ≈ 16 months from settlement on the shore. Newly-settled snails grew to merge in size with those of the previous year's population by winter. During the summer months, the rate of growth of snails from a sheltered shore was greater than that of snails on a shore exposed to wave-action. Laboratory experiments revealed that this could be attributed to the presence of better quality food, or food in greater abundance on the sheltered shore, compared with the exposed shore.During winter months, but not during the summer, snails from dry areas grew more slowly than those from pools. An experiment demonstrated that some snails from dry areas might be able to compensate for reduced periods of feeding by being able to feed faster when submersed. This could not explain the differences in natural rates of growth.L. acutispira bred from October–November to March–April. Spawning in the laboratory was greatest during late summer (January to March). The percentage of mature oocytes in the gonads was small in winter and increased in early summer. Among the largest-sized snails, females outnumbered males. Two experiments, on unsexed and pre-sexed snails, demonstrated that the biased sex-ratio of the largest snails was due to faster growth by females.There was a greater density of snails on the exposed shore, which was correlated with the presence of barnacles. When barnacles were removed from experimental areas, the density of the snails declined within 24 h. This suggested that barnacles provided a refuge from wave-shock, rather than shelter from desiccation or high temperature. In laboratory experiments, snails were exposed to higher temperatures and less humidity than they would normally encounter on the shore. There was negligible mortality of small or large snails after 24 h of these conditions.This minute species grows quickly, recruits annually and has a short life-span. This type of life-history is discussed in comparison with similar small species from other habitats.     

Settlement in different-sized patches by the gregarious intertidal barnacle Chamaesipho tasmanica Foster and Anderson in New South Wales

Most models on settlement of open marine invertebrate populations are based on space-limitation. These models, however, do not recognise that free space may not drive the demography of populations when larval numbers are small or when larval supply varies along a gradient in the habitat. They also do not incorporate the effects of larval choice when settling. It has been hypothesised that, in gregarious barnacles, the effects of adult conspecifics, rather than available free space, may play a primary role in settlement. That is, cues from adults along perimeters of patches, rather than space available, may enhance colonisation. This study therefore aimed to distinguish between these separate influences on populations of Chamaesipho tasmanica, a gregarious barnacle characterised by relatively few larvae arriving to settle each year. Patches of 6, 3 and 1.5 cm diameter were cleared within aggregations of barnacles at three heights (Low, Mid, Upper) of Chamaesipho's distribution at two sites and during 2 years of settlement. Total numbers of settlers in each year were manipulated to determine the separate influences on settlement due to availability of substratum or the effects of conspecific adults. To test for the effects of available free space, numbers of settlers per unit area were analysed. To test for gregarious effects due to the presence of adults, numbers of settlers per unit perimeter were analysed. While available substratum was found not to affect settlement of this barnacle, gregarious settlement in response to adults at perimeters of patches was thought to be confounded by differential larval supply and differential conspecific cues among heights on the shore. Results from this study therefore have important implications for survival of gregarious populations following disturbances, especially in species where larval supply is poor.     

Recruitment density can determine adult morphology and fecundity in the barnacle, Semibalanus balanoides

Although consequences of the settlement preference of larvae have been well documented, the consequence of these settlement choices on subsequent mortality, morphology and fecundity has been little studied. The aim of this study was to determine the relationship between recruit and adult density and to determine the effect of recruitment on adult morphology and egg tissue mass. This study follows 48,718 barnacles (Semibalanus balanoides) from recruitment at the end of the settling season to reproductively mature adults at a field site in the Clyde Sea (UK). Overall survivorship of the recruits to adulthood was 8.5%, although survivorship was up to 42% on low density settlement panels. In low density colonies (< 10 recruits cm-2), recruitment density was related to adult density (P < 0.001), whereas no relationship was found for higher density colonies. A shell morphology index measured at adulthood was related to recruitment density for low density recruited colonies (P < 0.001) but not high density colonies. Using ANCOVA, variations between the colonies in shell and egg tissue mass were not explained by mass of somatic tissue. However, egg mass was explained by recruitment density (P < 0.01). These results show that adult density is not a reliable indicator of the previous population density of the colony. Moreover, there are marked differences in population development between colonies with high and low recruit densities in terms of impact upon shell morphology and egg production. The dynamics that operate between recruits at the end of the settlement season and sexually mature adults to create the patterns elucidated in this paper, and other literature, remain unclear.     

Direct and indirect effects of Littorina littorea (L.) on barnacles growing on mussel beds in the Wadden Sea

On the extensive sedimentary tidal flats of the Wadden Sea, beds of the blue mussel Mytilus edulis represent the only major hard substratum and attachment surface for sessile organisms. On this substratum, the barnacle Semibalanus balanoides is the most frequent epibiont. In summer 1998, it occurred on over 90% of the large mussels (>45 mm shell length) and the dry weight of barnacles reached 65% of mussel dry weight. However, the extent of barnacle overgrowth is not constant and differs widely between years. Periwinkles (Littorina littorea) may reach densities >2000 m^–2 on intertidal mussel beds. Field experiments were conducted to test the effect of periwinkle grazing on barnacle densities. An experimental reduction of grazing and bulldozing pressure by periwinkles resulted in increased recruitment of barnacles, while barnacle numbers decreased with increasing snail density. The highest numbers of barnacles survived in the absence of L. littorea. However, a lack of periwinkle grazing activity also facilitated settlement of ephemeral algae which settled later in the year. Field experiments showed that the growth rate of barnacles decreased in the presence of these ephemeral algae. Thus, L. littorea may reduce initial barnacle settlement, but later may indirectly increase barnacle growth rate by reducing ephemeral algae. It is suggested that periwinkle density may be a key factor in the population dynamics of S. balanoides on intertidal mussel beds in the Wadden Sea.     

Bewuchsuntersuchungen auf Natursteinsubstraten im Gezeitenbereich des Nordsylter Wattenmeeres: Sessile und hemisessile Tiere

Six different types of test substrates, arranged in order of texture from smooth (Solnhofen limestone, Bunter sandstone, and basalt) to rough surfaces (Middle Triassic limestone, granite, and basaltic lava), were exposed in tidal zone of the wadden sea near the harbour of List (Island of Sylt, North Sea). The test substrates were fixed to panels at the midtide to high-water level, the midtide to low-water level, and 75 cm below the latter (sublittoral level). Animal settlement was primarily influenced by abiotic factors at the two higher levels. In the sublittoral zone, however, influences of biotic factors (competitors and predators) predominated. Therefore, the physical quality of the chosen substrates more decisively affected the growth of settling animals above the midtide to low-water mark than in the deeper zone. At the midtide to high-water levelBalanus balanoides only settled on the rough surface of the Middle Triassic limestone and in the troughs of the Bunter sandstone; barnacles attached to the even surface of the latter, were destroyed by wave beating. They died on the rough surface of the dark-coloured granite, when this stone was warmed during low-water. At the midtide to low-water level, the barnacles survived best on the rough surface of the Middle Triassic limestone and the granite. Only the larvae ofB. crenatus andB. improvisus preferred to settle on the even surface of the Bunter sandstone and the basalt; but there they died from wave beating or desiccation. The surface of the basaltic lava, formed by sharp-edged pores, was unsuitable for settling of barnacles. Colonies of Hydrozoa covered each substrate with a somewhat rough surface structure, even the basaltic lava. At the sublittoral level, differences in settling between the single substrates disappeared more and more. Nevertheless, the three species of barnacles showed the same preferences in settling, as they did at the higher levels. In August,Asterias rubens destroyed all barnacles and thus restricted the lower limit of barnacle settlement at the low-water mark. The distribution of epibiotic organisms is dependent upon the density of their living substrates, directly attached to the stone surfaces. During the short time of their growth, hemisessile young,Mytilus edulis settled on thready forms like algae, or on raised areas like the top of barnacle shells. Therefore, young mussels could be found on stones, that already carried a compact cover ofEnteromorpha sp. or a dense settlement of adult barnacles. The polychaetePolydora ciliata rarely burrowed directly into stony substrates (Middle Triassic limestone, Solnhofen limestone). It settled primarily between barnacle shells where it was sheltered from wave beating and at the same time profited from the current produced by the filter-feeding organs of the barnacles. The density of this polychaete was directly proportional to the density of the barnacles. AfterA. rubens had destroyed the barnacles in the sublittoral zone,P. ciliata disappeared too. At the low-water mark, however, theP. ciliata population between living and active barnacles increased. Hence, the lower limit ofP. ciliata — as those of barnacles and mussels — was fixed by this predator.

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Herrn Professor W. E. Ankel zur Vollendung des 80. Lebensjahres in Dankbarkeit gewidmet.     

Predatory behaviour of juvenile shore crab Carcinus maenas (L.)

Juvenile shore crabs Carcinus maenas (L.) were observed feeding on rock barnacles Semibalanus balanoides (L.) on a Bay of Fundy rocky shore. This previously unreported predatory behaviour was further investigated in the laboratory. When given a choice of three common and abundant gastropods, Nucella lapillus (L.), Littorina littorea (L.), and Littorina obtusata (L.), and the rock barnacle Semibalanus balanoides, juvenile shore crabs of both sexes ate mainly barnacles and consumed proportionately more barnacles than gastropods compared with adults, which ate mainly gastropods. The rock barnacle is an abundant and readily available food source which may be important in sustaining the juvenile crab through periods of moults and rapid growth. As the shore crab attains a certain age (size), it must forage lower on the shore as gastropods become more important in its diet.     

Vicious circle in the intertidal: Facilitation between barnacle epibionts, a shell boring polychaete and trematode parasites in the periwinkle Littorina littorea

We studied interactions between three organisms associated with a common gastropod of northern Atlantic shores, the periwinkle Littorina littorea: barnacle epibionts Balanus crenatus, a shell boring polychaete Polydora ciliata, and tissue invading trematodes which use the periwinkles as first intermediate host. Snails collected shortly after barnacle settlement with > 50% cover of barnacles had significantly higher infestation of shell boring worms compared to unfouled snails, while trematode infestation was similar. The result was the same at two sites, and we conclude that the worm P. ciliata facilitates barnacle fouling on snails. The reverse was also the case. In an experiment with 14 weeks of exposure, snail treatments with barnacle epibionts had a significantly higher P. ciliata load than unfouled and cleaned snails. Again, trematode infestations were similar. The reciprocal positive interactions between barnacle epibionts and shell boring worms on snail houses is regarded as a case of facultative mutualism. On the other hand, for the snail basibiont, both barnacles and shell boring worms exert strongly negative effects by reducing fecundity, growth, and survival, resulting in a vicious circle for the snails. The combined effects of these associated organisms may rival in importance any competitive or predacious effects on the host L. littorea.     

Factors influencing initial larval settlement: temporal, spatial and surface molecular components

The impact of initial surface chemistry on settlement of barnacle, bryozoan, and hydroid larvae was assessed. Temporal (on a scale of weeks to months) and fine scale spatial (centimeter to meter) variation in settlement were quantified. Four arrays of silanized glass surfaces, deployed at ≈2-wk intervals, were monitored after 1 and 3 days of immersion. Settlement of all larval types exhibited strong temporal variation. There was a 25, 22- and 18-fold difference between the highest and lowest Day 1 settlement for barnacles, bryozoans and hydroids, respectively. Bryozoan and hydroid settlement was spatially variable, barnacle settlement was not. Barnacle and bryozoan settlement was influenced by inttial surface chemistry, hydroid settlement was not. For barnacles, there was a 2-fold difference between total settlement over the four arrays on untreated glass and diphenyl-silanized surfaces; for bryozoans, there was a 51-fold difference, and in the opposite direction. There was a negative correlation between barnacle and bryozoan settlement with respect to surface. This result was independent of the presence of the other species. Bryozoan settlement was also spatially and temporally quantified in a separate, single surface 56-h array, and total bryozoan settlement ·h⁻¹ was found to be correlated with light intensity. Patterns of initial colonization were strongly influenced by surface chemistry and spatial temporal variation in larval supply. These effects on initial colonization may influence subsequent community development.     

Fluctuating asymmetry of <Emphasis Type="Italic">Amphibalanus</Emphasis> (<Emphasis Type="Italic">Balanus</Emphasis>) <Emphasis Type="Italic">amphitrite</Emphasis> (Cirripedia: Thoracica) in association with shore height and metal pollution

The level of fluctuating asymmetry (FA), which is defined as random deviations from perfect bilateral symmetry in the morphological traits of an organism, increases with increasing developmental instability, and it may be used as an indicator of environmental and/or genetic stresses. This study attempted to relate FA levels in the opercular plates of the barnacle Amphibalanus (Balanus) amphitrite with shore height and body trace metal concentrations. Barnacles were collected from both low and mid shores at six coastal locations with various degrees of marine pollution in Hong Kong. Four opercular traits, namely scutum length, tergum length, scutum width, tergum width were measured in the specimen while concentrations of five common trace metals (Cd, Cu, Cr, Mn and Zn) were determined in their body tissues using inductively coupled plasma-atomic emission spectrophotometry. Among the four traits, only tergum length and scutum width fulfilled the assumption of FA while their measurement errors were low. Across all sites, mid-shore A. amphitrite consistently exhibited a significantly higher FA level in scutum width than that in low-shore barnacles, but FA levels in tergum length were similar between the two shore heights. These results suggested that FA was trait-dependent, and mid-shore barnacles were possibly under high physical stress, such as desiccation and high temperature, leading to high developmental instability. Although no positive association was observed between FA and metal contamination in low-shore A. amphitrite, FA level in tergum length significantly increased with decreasing body concentration of manganese (Mn) in these barnacles. Such a negative relationship may be explained by the mechanism of bioaccumulation and physiological role of Mn in A. amphitrite with respect to the formation of barnacle shell plates. Handling editor: T. P. Crowe     

The distribution of Oligochaeta on an exposed rocky shore in southeast Ireland

The distribution of oligochaetes was studied on a wave-exposed, granite shore at Carnsore Point, County Wexford. Habitats sampled were crevices and shallow surface cracks in rock, Lichina pygmaea turf, a Mytilus edulis bed, barnacles, Laurencia pennatifida turf, Corallina officinalis turf and pools. Lichina, mussels, barnacles greater than 8 mm in height, Laurencia and Corallina were sampled quantitatively.Three species of Tubificidae and nine species of Enchytraeidae were recorded including two new species described from this locality and five which are undescribed. Oligochaetes were present in all the habitats sampled but were rare in pools and numbers were low on barnacle-covered rock except where barnacles were more than 8 mm in height. The number of species increased with decreasing tide level and was highest in lower shore crevices. Highest densities were recorded from Corallina turf and in barnacles > 8 mm. Lumbricillus semifuscus was the most widespread species, occurring in crevices and mats at all tide levels; other species had more restricted distributions. Grania species were confined to Corallina turf. Only three species of oligochaete were recorded from cracks where they were almost the only fauna present. Species diversity and density were influenced by physical structure of the habitat, particularly the amount of retained sediment.The oligochaetes are members of a rich cryptofauna in habitats which provide them with organic matter and moisture and protect them from environmental extremes and wave damage and from predators during immersion. Reasons for the scarcity of oligochaete records from exposed rocky shores and the high proportion of new species in this study are discussed.     

Function of the flexible periostracal hairs in Trichotropis cancellata (Mollusca, Gastropoda)

Abstract. The marine snail Trichotropis cancellata possesses hairy projections of periostracum (outer shell layer) whose function is unknown. Although rigid shell projections in molluscs have been studied extensively, the selective advantage of flexible extensions of periostracum is less clear. None of the functions proposed previously for periostracum (e.g., protection from erosion and boring) are promoted when it is drawn into hair-like projections. We investigated hypothetical functions that may be served by flexible periostracal hairs, including predator deterrence, alteration of flow vectors to promote feeding or affect turbulence dynamics during freefall, and providing a differential substratum for epibionts. Our laboratory results indicate that crabs, Cancer oregonensis , and sea stars, Pycnopodia helianthoides , consumed snails with the periostracum removed more often than snails with an intact hairy periostracum. However, in both predatory species, some individuals showed no significant preference, and another crab species ( Cancer productus ) did not strongly discriminate based on the shell periostracum. Field studies showed no difference in the rate of predation on hairy- versus smooth-shelled snails. The hairs did not alter flow around the shells consistently in laboratory flume experiments. Additionally, hairy- and smooth-shelled kleptoparasitic snails grew at rates that were statistically indistinguishable, while hairy, suspension-feeding snails grew more slowly. The hairs did not impact the orientation of a snail after a falling event or the time to righting after a fall. The presence of the hairs did deter settlement by barnacles. We conclude that the hairy periostracum acts as a slight deterrent to crab and sea star predators and as a stronger deterrent to the settlement of large calcareous epibionts, such as barnacles, that would increase the weight the snail must bear and potentially increase drag.     

Roles of larval supply and behavior in determining settlement of barnacles in a temperate mangrove forest

Recruitment is often a major influence on the spatial distribution of populations of benthic marine invertebrates, but the contributions of different components of recruitment are not well known, with the added complication that the relative importance of various life-history processes may be scale-dependent. Previously, we have shown that over a large scale across a mangrove (Avicennia marina) forest in southeastern Australia, settlement of the barnacle Elminius covertus explained its patterns of recruitment, which in turn explained the distribution of adults on mangrove pneumatophores. Post-settlement mortality had little influence on this pattern. In contrast, small-scale vertical distributions of adult barnacles along individual pneumatophores were determined by the pattern of recruitment, which differed from the pattern of settlement, so post-settlement mortality determined the vertical patterns of adults.

In this study, we tested whether larval supply and/or settlement behavior influence the observed settlement patterns of E. covertus across a forest (from seaward to landward zones). We also tested whether larval supply could explain the vertical settlement patterns along the pneumatophores. A pumping system was used to collect cypris larvae from seaward, mid and landward zones of a mangrove forest and an adjacent, unvegetated shore and from three heights above the sediment surface. We also used transplantation of wooden stakes bearing microbial films and barnacle recruits between horizontal zones of the forest to determine whether settlement was influenced by these films or recruits.

Both cyprid supply and cyprid behavior were important factors in determining the patterns of settlement of E. covertus across the forest. Cyprid supply was a result of three-fold differences in immersion times of different (landward, mid and seaward) zones across the forest and a decrease in density of cyprids in the water column from the seaward zone of the forest to the landward sections. In the absence of mangroves immediately adjacent to the forest, there was no temporally consistent difference in cyprid density across the shore and even the differences in immersion time did not produce consistent differences in cyprid supply across the shore. Wooden substrata that had been immersed at seaward sections of the forest attracted consistently more settlers than substrata immersed initially at other sections of the forest and settlement could be induced beyond the normal distribution of adults of E. covertus by stakes transplanted from the seaward zone.

The vertical settlement pattern could not be explained by the supply of cyprids, suggesting that larval behavior must determine the vertical settlement pattern.     

Surface wettability as a determinant in the settlement of the barnacle Balanus Improvisus (DARWIN)

Several studies have shown that the initial surface wettability, is of importance in the settlement of macrofouling larvae such as barnacles, bryozoans and hydroids in the field as well as in laboratory assays. In this study we present results from laboratory assays using hydrophilic and hydrophobic polystyrene (PS) and cyprid larvae of Balanus improvisus (Darwin). The results obtained differ markedly from those reported for the barnacle Balanus amphitrite (Darwin), where a high surface wettability seemed to be preferred for settlement. Our results show that a surface with intermediary wettability (hydrophilic PS) reduced settlement by 38% as compared to surfaces of low wettability (hydrophobic PS) during an 8-day period. During the experiment, the wettability in the hydrophilic PS dishes was not significantly changed as measured by advancing contact angle with mQ water. Over an 8-day period wettability of the hydrophobic PS dishes approached that of the hydrophilic PS surfaces. We further conducted experiments with highly hydrophilic and highly hydrophobic methylsilane-treated glass surfaces with known chemistry. In this experiment, the settlement of cyprid larvae was completely inhibited by the high wettability surfaces. Contact angle measurements revealed that the wettability during the length of the experiment of the hydrophilic glass surfaces was not significantly altered. We conclude by these experiments that even an intermediate wettability can significantly affect the overall settlement success of the barnacle B. improvisus. The mechanism by which the settlement is impeded might be biologically mediated through the recognition by cyprid larvae of the molecular composition of the surface when the cyprid reverts to the settlement phase, i.e. when swimming behaviour is abandoned in favour of surface exploration, or it is mediated by physicochemical forces acting between the surface and the larval body or the larval antennules.     

THE ACTION OF COPPER IN ANTIFOULING PAINTS

Experiments on the number of barnacle cyprids found on plane surfaces and on the surface of small pits coated with compositions containing different amounts of cuprous oxide show that the presence of copper does not deter the cyprids from attempting to settle. Copper-oxide paints do not inhibit settlement appreciably; their efficiency as antifouling agents is caused by the toxic action of copper on cyprids and young spat after initial attachment.
The cementing of the shell to the substratum in young barnacles which have survived metamorphosis appears to be hindered by the presence of copper paints so that they are easily dislodged.     

The relation between cypris ultrastructure and metamorphosis in male and female Sacculina carcini (Crustacea,Cirripedia)

Summary To elucidate current controversies on sex in rhizocephalan barnacles, broods of Sacculina carcini, infesting the shore crab Carcinus maenas, were raised to cyprids in the laboratory and followed through settlement and metamorphosis. Free-swimming cyprids were studied by transmission electron microscopy and occur in male and female morphological types, which differ in the structure of carapace cuticle, antennular cuticle, antennular glands, and the cells suspected of being the stem cells during metamorphosis. These dissimilarities are in addition to the already known differences in cypris size, in number of antennular sense organs, and in substrata settled on by morphological males and females. Metamorphosing males (trichogons) and females (kentrogons) are illustrated in interference phase-contrast micrographs. The morphological differences between male and female cyprids are directly related to their dissimilar metamorphosis. Hence, cyprids of male morphology are anatomically incapable of metamorphosing into kentrogons, while cyprids of female morphology cannot metamorphose into trichogons. The determination of sex in rhizocephalan barnacles is discussed.The results refute the hypothesis that sex in Sacculina carcini is determined environmentally, e.g., by the substratum encountered by the cyprids at settlement. It is concluded that sex is determined already in the free-swimming larvae and, most probably, already in the ovary. This agrees with the mode of sex determination in other species of the Rhizocephala Kentrogonida.     

Characterisation of the bacteria associated with barnacle,Balanus amphitrite,shell and their role in gregarious settlement of cypris larvae

The acorn barnacle Balanus amphitrite (syn. Amphibalanus amphitrite) is a model organism to investigate pelago-benthic transitions in marine invertebrates. A driver for larval settlement in this organism is the need to attach close to conspecifics, to allow reproduction to take place. Adult barnacles are covered by microbial biofilms and the contribution of these biofilms to conspecific recognition is not fully understood. Little information is available on microbial communities associated with B. amphitrite. We compared biofilm communities from the barnacle shell surface with those from the surrounding rocks using the culture-independent methods of quantitative PCR and denaturing gradient gel electrophoresis. Quantification of the relative abundances of higher bacterial taxa showed that barnacles hosted a greater proportion of α-Proteobacteria compared to rock-associated biofilms (p < 0.01). Differences in relative abundances of other taxa were not observed but DGGE profiling suggested that differences were present at lower taxonomic levels. The capacity of these communities to influence larval settlement was assessed by growing multispecies biofilms on artificial medium, obtained by extracting nutrients from adult barnacles. Biofilms composed of shell-associated bacteria were capable of promoting conspecific settlement by 67% compared to control surfaces (p < 0.05), while rock-associated communities showed contrasting effects. A taxonomic comparison of settlement-stimulating and -inhibiting bacteria was performed by DGGE and band sequencing. All partial 16S rRNA genes sequenced were similar to members of the Vibrio and Pseudoalteromonas genera, suggesting that larvae can detect and respond to variations in the composition of microbial biofilms at low taxonomic levels. Our results indicate that barnacle larvae may be able to detect parentally-associated biofilms and use this information to settle close to members of its own species.