Inhibition and induction of barnacle settlement by natural products present in octocorals

Living on fire: Deactivating fire coral polyps for larval settlement and symbiosis in the fire coral-associated barnacle Wanella milleporae (Thoracicalcarea: Wanellinae)

Symbiosis is increasingly recognized as being an important component in marine systems, and many such relationships are initiated when free-swimming larvae of one partner settle and become sedentary on a host partner. Therefore, several crucial questions emerge such as the larva's mechanism of locating a host, selection of substratum and finally settlement on the surface of its future partner. Here, we investigated these mechanisms by studying how larvae of the fire coral-associated barnacle Wanella milleporae move, settle and establish symbiosis with their host, Millepora tenera. Cyprids of W. milleporae possess a pair of specialized antennules with bell-shaped attachment discs that enable them to explore and settle superficially on the hostile surface of the fire coral. Intriguingly, the stinging polyps of the fire coral remain in their respective pores when the cyprids explore the fire coral surface. Even when cyprids come into contact with the nematocysts on the extended stinging polyps during the exploratory phase, no immobilization effects against the cyprids were observed. The exploratory phase of Wanella cyprids can be divided into a sequence of wide searching (large step length and high walking speed), close searching (small step length and low speed) and inspection behavior, eventually resulting in permanent settlement and metamorphosis. After settlement, xenogeneic interactions occur between the fire coral and the newly metamorphosed juvenile barnacle. This involved tissue necrosis and regeneration in the fire coral host, leading to a callus ring structure around the juvenile barnacle, enhancing survival rate after settlement. The complex exploratory and settlement patterns and interactions documented here represent a breakthrough in coral reef symbiosis studies to show how invertebrates start symbiosis with fire corals.

Warm temperature alters the chemical cue preference of Acropora tenuis and Heliopora coerulea larvae

Larvae released into the water column rely on chemical cues from the benthos for successful settlement. However, larval preference for substrates may be affected by rising seawater temperature brought about by global climate change. In this study, we examined the effect of elevated temperature on chemical cue preference by larvae of the scleractinian coral, Acropora tenuis, and the octocoral, Heliopora coerulea, collected from northwestern Philippines. At ambient temperature (28 °C), both H. coerulea and A. tenuis larvae showed preference for substrates containing either crustose coralline algae or crude ethanolic extracts from conspecific or congeneric corals. In contrast, at higher temperature (30 °C), greater preference was shown for substrates containing the crude extract from conspecific or congeneric corals. These results demonstrate that elevated temperature can change larval substrate preference, which will have downstream impacts on crucial biological processes, such as larval settlement and recruitment.

Sea whip coral Leptogorgia virgulata in the Mid-Atlantic Bight: Colony complexity, age, and growth

Sea whip coral Leptogorgia virgulata are a common structural component of both natural and artificial hard-bottom reef habitats in the mid-Atlantic region and may serve as essential habitat for commercially valuable species. However, they are slow-growing, easily damaged, and especially vulnerable to damage by passive fishing gear such as pots and traps. Despite their potential importance, until recently, sea whips have been generally understudied in this region. We examined the colony complexity, length, age, and growth of sea whips from four artificial reef sites in the mid-Atlantic region to gain a better understanding of their biology in the area. There were no significant differences in the bifurcation (R_b) and tributary to source (T/S) ratios between sites, with the R_b ≈3 for all sites, indicating similar complexity between sites. The total length distribution was 8.3 cm to 85.3 cm, and 50% of corals in the range of 34.2–56.4 cm. Age, estimated from annual growth ring counts, ranged from 2 to 15 y, with 50% of corals in the range of 6 to 8 y. The large proportion of middle-sized and middle-aged corals suggests episodic recruitment. Age-length keys showed the trend of age increasing with total coral length, and a von Bertalanffy growth model demonstrated size-dependent growth following the equation: E[L—t] (cm) = 86.1(1−e^{−0.14(t−1.44)}). This is the first study providing such data for sea whips in the coastal mid-Atlantic region, and the baseline created will be a useful reference to study changes over time.

Love at First Taste: Induction of Larval Settlement by Marine Microbes

Marine biofilms are composed of many species of bacteria, unicellular algae, and protozoa. Biofilms can induce, inhibit, or have no effect on settlement of larvae and spores of algae. In this review, we focus on induction of larval settlement by marine bacteria and unicellular eukaryotes and review publications from 2010 to September 2019. This review provides insights from meta-analysis on what is known about the effect of marine biofilms on larval settlement. Of great interest is the impact of different components of marine biofilms, such as bacteria and diatoms, extracellular polymeric substances, quorum sensing signals, unique inductive compounds, exoenzymes, and structural protein degradation products on larval settlement and metamorphosis. Molecular aspects of larval settlement and impact of climate change are reviewed and, finally, potential areas of future investigations are provided.

A review of gorgonian coral species (Cnidaria, Octocorallia, Alcyonacea) held in the Santa Barbara Museum of Natural History research collection: focus on species from Scleraxonia, Holaxonia, Calcaxonia - Part II: Species of Holaxonia, families Gorgoniidae and Plexauridae

Gorgonian coral specimens from the Holaxonia, families Gorgoniidae and Plexauridae held in the collection of the Santa Barbara Museum of Natural History (SBMNH) were reviewed and evaluated for species identification. The specimens were collected from within, and adjacent areas of, the California Bight. The SBMNH collection has encompassed within it a large percentage of specimens collected by the Allan Hancock Foundation (AHF) 'Velero' Expeditions of 1931-1941 and 1948-1985. This historic collection displays an emphasis on species belonging to the Holaxonia, particularly the gorgoniids and plexaurids; thus, this second part presents a thorough discussion of well-known genera from within the California Bight, with more extensive discussions of several genera that have historically, and currently, led to confusion (and thus, misidentification). A brief discussion of a California Bight grouping, referred to within as the "red whips," is presented; this grouping encompasses several species with very similar colony appearance across a number of genera. Two species, the gorgoniid Leptogorgiachilensis (Verrill, 1868) and the plexaurid Chromoplexauramarki (Kükenthal, 1913) each required the designation of a neotype from within the collection. A new species in the genus Eugorgia Verrill, 1868, a whip or thread-like form belonging to the family Gorgoniidae, is described. One additional plexaurid genus (Placogorgia) is discussed, a genus not commonly reported for the California Bight region. This is the first comprehensive work, in three parts, focusing on all species of gorgonian coral known to inhabit the California Bight. This paper, Part II of the full work, continues the systematic review of all species represented in the Santa Barbara Museum of Natural History research collection begun in Part I.

Chemical ecology and the search for marine antifoulants : Studies of a predator-prey symbiosis

The gorgonian octocoralLeptogorgia virgulata (Phylum Coelenterata, Class Anthozoa) is rarely overgrown by fouling organisms and is avoided by most predators. Laboratory experiments suggest that secondary metabolites and calcium carbonate spicules interact synergistically to provideL. virgulata with an effective defense against predatory fish. In spite of these defenses,L. virgulata is consumed by the symbiotic, trophically specialized gastropodNeosimnia uniplicata, which closely mimics the appearance of the gorgonian.Neosimnia uniplicata is readily eaten by fish and appears not to acquire an effective antipredator defense from its gorgonian host. Extracts ofLeptogorgia virgulata andNeosimnia uniplicata strongly inhibited the settlement of the barnacleBalanus atnphitrite. Bioassay-directed purification of the more potent antifouling agents fromL. virgulata led to the isolation of two previously described, diterpenoid hydrocarbons, known as pukalide and epoxypukalide. A third inhibitor of barnacle settlement, whose structure is presently unknown, was obtained from bothL. virgulata andN. uniplicata. When assayed for ability to inhibit barnacle settlement, these three compounds possessed EC50 values ranging from 19 to 55 ng/ml. These secondary metabolites may prevent the overgrowth ofL. virgulata by fouling organisms in nature. The allelochemicals ofL. virgulata, N. uniplicata, and other marine organisms may provide nonpolluting alternatives to existing, commercial antifoulants based on derivatives of tri-n-butyltin.

Antifouling agents from marine spongeLissodendoryx isodictyalis carter

The spongeLissodendoryx isodictyalis is an odorous, encrusting, blue-gray sponge found on subtidal flats in North Carolina waters. The strong odor ofL. isodictyalis, coupled with the observation that it is rarely overgrown by fouling organisms, suggested that this sponge may produce metabolites with potent antifouling activity. Ethyl acetate extracts ofL. isodictyalis inhibit larval settlement of the barnacleBalanus amphitrite in laboratory assays at 10 ng/ml. Barnacle settlement bioassays of isolated preparative TLC fractions show thatL. isodictyalis produces at least two pungently scented, antifouling agents with EC50 values of less than 85 μg/ml and less than 250 μg/ml, respectively. The most potent agent inhibits settlement at or below a concentration of 400 ng/ml and kills approximately 25 % of settlement-stage barnacle larvae at 400 μg/ml. The other agent causes 100% mortality of larvae at concentrations greater than 400 μg/ml and inhibits settlement at approximately 40 μg/ml. These metabolites ofL. isodictyalis may inhibit overgrowth of the sponge in nature.

Bacterial-barnacle interaction: Potential of using juncellins and antibiotics to alter structure of bacterial communities

In preparation for studies using natural products to probe interactions between bacterial consortia and settlement stage barnacles, we isolated 16 strains of bacteria associated with barnacles and examined: (1) effects of films of bacterial isolates on barnacle settlement, and (2) bacteriostatic effects of juncellins and standard antibiotics. Bacteria were isolated from the biofilm associated withBalanus amphitrite. On the basis of morphological and biochemical characteristics, bacteria were classified into five major groups:Aeromonas, Alcaligenes, Flavobacterium, Pseudomonas, andVibrio. Barnacle settlement was inhibited by allVibrio films and 64% of the other isolates. No film stimulated barnacle settlement. Juncellins were approximately as potent as standard antibiotics for all bacterial species tested.Vibrio spp. were most resistant to juncellins.

Two new species of the gorgonian inhabiting barnacle, Conopea (Crustacea, Cirripedia, Thoracica), from the Gulf of Guinea

Two new species of Conopea (Say 1822) are described from the Gulf of Guinea: Conopea saotomensis sp. n.and Conopea fidelis sp. n. These two new species were collected from the historically isolated volcanic islands of São Tomé and Príncipe. The relationship between Conopea saotomensis sp. n., Conopea fidelis sp. n.and two other Atlantic barnacle species, Conopea calceola (Ellis 1758) and Conopea galeata (Linnaeus 1771), is examined. The methods employed are the construction of a molecular phylogeny using mitochondrial COI and nuclear H3 gene sequence data along with morphological comparisons of calcareous and cuticular body parts. It is found that Conopea saotomensis sp. n., Conopea fidelis sp. n.and Conopea calceola are most closely related to each other but the relationship among them is unresolved. Gorgonian hosts are identified. Preliminary observations show species level host specificity for Conopea fidelis sp. n.

Antimicrobial activity in the common seawhip, Leptogorgia virgulata (Cnidaria: Gorgonaceae)

Antimicrobial activity was examined in the gorgonian Leptogorgia virgulata (common seawhip) from South Carolina waters. Extraction and assay protocols were developed to identify antimicrobial activity in crude extracts of L. virgulata. Detection was determined by liquid growth inhibition assays using Escherichia coli BL21, Vibrio harveyii, Micrococcus luteus, and a Bacillus sp. isolate. This represents the first report of antimicrobial activity in L. virgulata, a temperate/sub-tropical coral of the western Atlantic Ocean. Results from growth inhibition assays guided a fractionation scheme to identify active compounds. Reverse-phase HPLC, HPLC-mass spectrometry, and 1H and 13C NMR spectroscopy were used to isolate, purify, and characterize metabolites in antimicrobial fractions of L. virgulata. Corroborative HPLC-MS/NMR evidence validated the presence of homarine and a homarine analog, well-known emetic metabolites previously isolated from L. virgulata, in coral extracts. In subsequent assays, partially-purified L. virgulata fractions collected from HPLC-MS fractionation were shown to contain antimicrobial activity using M. luteus and V. harveyii. This study provides evidence that homarine is an active constituent of the innate immune system in L. virgulata. We speculate it may act synergistically with cofactors and/or congeners in this octocoral to mount a response to microbial invasion and disease.

A comparative study of the non-acidic chemically mediated antifoulant properties of three sympatric species of ascidians associated with seagrass habitats

The present study investigated aspects of the antifoulant properties of three sympatric species of ascidians found in seagrass habitats of the Gulf of Mexico, Southern Atlantic Ocean, and Caribbean. Field observations in Saint Joseph Bay, Florida indicate that all three species are common and that the tunic of the solitary ascidian Molgula occidentalis is often heavily fouled, while the outer surfaces of both the colonial ascidians Amaroucium stellatum and Botryllus planus are free of fouling organisms. Antifoulant activities of a suite of increasing hydrophilic organic extracts prepared from the tunic of M. occidentalis and whole colonies of A. stellatum and B. planus were measured using both sympatric microbial (bacteria) and macroinvertebrate (cyprid larvae of Balanus amphitrite) fouling organisms in laboratory bioassays. In addition, field antifoulant assays were conducted by combining organic extracts with controlled-release resin and subsequently coating this material on to acrylic rods deployed in the field for a 72 h period. Extracts of the tunic of M. occidentalis generally did not inhibit bacterial growth. The exception was the methanol extract, which inhibited growth in one of the six marine bacteria tested. Moreover, only the highest concentrations of hexane and methanol tunic extracts tested prevented attachment of cyprid larvae. Field assays revealed no antifoulant activity on rods coated with resin containing extracts of M. occidentalis. Inhibition of both microbial growth and cyprid settlement were much more pronounced in whole-organism extracts of the two colonial ascidians. Most potent were the aqueous methanol extracts of colonies of B. planus and A. stellatum which inhibited growth in five of the six marine bacteria tested. In addition, hydrophilic and lipophilic extracts of the colonial ascidians significantly inhibited attachment of cyprid larvae, in many instances across a wide range of extract concentrations. Field antifoulant assays indicated that extracts of both colonial ascidians inhibited settlement of bryozoans and barnacles. The findings indicate that the colonial ascidians B. planus and A. stellatum possess chemical antifoulant properties. In contrast, the solitary ascidian M. occidentalis appears to either tolerate fouling or possess other non-chemical mechanisms to cope with the risks associated with epibiont overgrowth.

Chemical control of bacterial epibiosis and larval settlement of Hydroides elegans in the red sponge Mycale adherens

The sponge, Mycale adherens, usually occurs within the fouling community of Hong Kong waters, yet its body surface is rarely fouled by other macro-organisms. In this study, sponge-associated bacteria were isolated using enrichment culture techniques and compared with indigenous bacterial isolates from an inanimate reference site in the close vicinity. Bacterial isolates were identified phylogenetically by 16S rRNA gene sequence analysis. The comparison between culturable bacterial communities from the sponge and indigenous benthic bacteria revealed differences both in the total number of isolates and their phylogenetic affiliation. Laboratory bioassays utilizing monospecies bacterial films revealed that a significant portion of sponge-associated bacteria had either an inhibitory or neutral effect on larval settlement of the fouling polychaete, Hydroides elegans. In contrast to natural biofilms, which harbor ca 65% of bacteria with at least some sort of inductive effect on H. elegans, statistical analysis showed that only 25% of sponge-associated bacteria were classified as "inductive" strains while the remaining 75% were classified as "non-inductive" strains. Waterborne metabolites of sponges affected the larvae of H. elegans in a concentration-dependent manner by either exerting a toxic or an anti-settlement effect. Organic solvent extracts of sponge tissue weakly inhibited growth of bacterial strains isolated from marine biofilms. A potential antifouling mechanism in the sponge M. adherens is discussed.

Natural product antifoulants: One perspective on the challenges related to coatings development

Fouling of surfaces by abiotic and biotic substances has molecular, microbial, and macro-organismal levels of organization. Fouling involves molecular bonding and biological adhesives. Existing commercial solutions to fouling are antifouling or foul-release. Antifouling uses broad-spectrum biocides which kill foulers by virtue of oxidation or toxic metal ions. Foul-release coatings are dimethyl silicone polymers that foul, but clean easily. The best foul-release coatings also contain additives that kill organisms. Environmentally unacceptable consequences of toxic antifouling coatings, especially those based on organotins, have prompted interest in antifoulants found in living organisms. Laboratories worldwide now use bioassays with target fouling organisms to direct purification and identification of antifoulant compounds. Natural antifoulants are common and include toxins, anesthetics, surface-active agents, attachment and/or metamorphosis inhibitors and repellents. Development of commercial coatings using natural products is blocked by cost, the time horizon to meet government regulations and performance standards based upon coatings with unacceptable environmental impacts. If blocks are removed, the potential for environmentally acceptable solutions that combine natural products with organic biocides is high.

Signal transduction in barnacle settlement: Calcium re-visited

The search for marine natural product antifoulants is being hampered by problems associated with conventional settlement assays. Yet it has been recognised that the study of how chemical cues are perceived by fouling organisms may offer clues to settlement inhibitors and may identify novel biochemical assays for antifoulants based on signal transduction pathways. Here the role of calcium in barnacle settlement is re-examined. A requirement for calcium in settlement of the cypris larva of Balanus amphitrite has been confirmed; settlement was inibited in low calcium, and calcium-free, seawater. Although 10 mM (19.27 mM) excess calcium had no effect and higher concentrations were inhibitory, a 5 mM excess stimulated settlement. Stimulation is proposed to be effected by an increase in intracellular calcium. The release of calcium from intracellular pools with thapsigargin (but not cyclopiazonic acid) induced settlement and an antagonist of intracellular calcium, TMB-8, generally inhibited settlement. Nevertheless, the calcium ionophore A23187 did not induce settlement at the concentrations tested. Consequently, the relative importance of external calcium and intracellular pools to increased intracellular calcium has yet to be determined. Pharmacological manipulations of calcium channels with organic and inorganic channel blockers strongly indicate calcium channel involvement in barnacle settlement. The data are summarised in an hypothetical scheme for signal transduction at settlement and are compared to those obtained for other marine invertebrate larvae.

Seasonal variation in the effects of hard substratum biofilming on settlement of marine invertebrate larvae

The effect of season on "biofilming";, as a cue for the settlement of marine invertebrate larvae, was investigated in a long-term field study during the years 1992-1994. The series of settlement experiments was conducted in a tidal rapid on the west coast of Scotland, and involved manipulations of artificial panels. Biofilming of substrata, whilst excluding larval settlement, was achieved by the enclosure of panels within tight-fitting (but removable) mesh screens so that the number of settlers on filmed and unfilmed substrata were counted in the initial absence of other incumbent post-larvae. Depending on larval species, the effects of biofilming were found to be either facilitatory or inhibitory. Significant within- and between-species seasonal differences in the settlement responses were detected, and a reversal of the effect of biofilming on larval settlement response, from inhibitory to facilitatory and vice versa, was noted with season in the case of some taxonomic groups and species (e.g. Tubulipora sp., Plagioecia sp., Electra pilosa (L.)). The present study emphasizes the need for extended field studies of larval responses to environmental cues, when the focus of interest is in drawing general inferences about naturally occurring behavioural patterns at settlement.