Colonial Hydroid Research Articles

Reproductive ecology of fire corals in the northern Red Sea

AbstractScleractinian corals, the principal architects of coral reefs, face substantial threats from ongoing and anticipated climate change and other anthropogenic disturbances. This underscores the significance of investigating alternative reef-building organisms and their contribution to reefs’ resilience. Among these alternatives, colonial hydrozoans of the genus Millepora, commonly called ‘fire corals’, play substantial roles in contributing to coral reef structure and functionality by depositing calcareous exoskeletons. Despite the ecological importance of fire corals, fundamental knowledge gaps remain regarding their biology and reproductive ecology. Here, we present a comprehensive study on the population dynamics, reproductive ecology, phenology, and sex ratio of the three Red Sea fire corals—Millepora dichotoma, M. exaesa, and M. platyphylla—in the Gulf of Aqaba and Eilat. The abundance of M. dichotoma and M. exaesa seems to have remained consistent over the last 50 years across two of the three depth zones surveyed, indicating their potential resistance or resilience. However, at the third depth surveyed, their abundance appears to have decreased. Our extensive in-situ monitoring of breeding events over six consecutive years has revealed a previously undetected relationship between Millepora species breeding events and the lunar cycle. Histological analyses indicated remarkably short reproductive cycles of only 2–3 weeks, recurring multiple times within a single season, in both M. dichotoma and M. exaesa, which is a unique reproductive aspect compared to other reef-building corals. These results highlight the high reproductive and resilience potential of Millepora species. Consequently, fire corals could assume a more substantial role as keystone species in changing environments and future reefs, emphasizing their importance in reef conservation and management.

Integral Effect of Interaction of Parts of a Noncentralized Biosystem by the Example of Magistral Hydroplasma Flow Formation in the Shoots of Colonial Hydroid Dynamena pumila (L., 1758)

Integral Effect of Interaction of Parts of a Noncentralized Biosystem by the Example of Magistral Hydroplasma Flow Formation in the Shoots of Colonial Hydroid Dynamena pumila (L., 1758)

The genome of the colonial hydroid Hydractinia reveals that their stem cells use a toolkit of evolutionarily shared genes with all animals.

Hydractinia is a colonial marine hydroid that shows remarkable biological properties, including the capacity to regenerate its entire body throughout its lifetime, a process made possible by its adult migratory stem cells, known as i-cells. Here, we provide an in-depth characterization of the genomic structure and gene content of two Hydractinia species, Hydractinia symbiolongicarpus and Hydractinia echinata, placing them in a comparative evolutionary framework with other cnidarian genomes. We also generated and annotated a single-cell transcriptomic atlas for adult male H. symbiolongicarpus and identified cell-type markers for all major cell types, including key i-cell markers. Orthology analyses based on the markers revealed that Hydractinia's i-cells are highly enriched in genes that are widely shared amongst animals, a striking finding given that Hydractinia has a higher proportion of phylum-specific genes than any of the other 41 animals in our orthology analysis. These results indicate that Hydractinia's stem cells and early progenitor cells may use a toolkit shared with all animals, making it a promising model organism for future exploration of stem cell biology and regenerative medicine. The genomic and transcriptomic resources for Hydractinia presented here will enable further studies of their regenerative capacity, colonial morphology, and ability to distinguish self from nonself.

First Record of Sarsia tubulosa (M. Sars, 1835) (Cnidaria, Hydrozoa) on Red King Crabs in the Coastal Barents Sea

Sarsia tubulosa (M. Sars, 1835), a colonial hydrozoan of the family Corynidae, is common in the Barents Sea. This study provides new evidence that the species acts as an epibiont of the red king crab Paralithodes camtschaticus, a non-indigenous but remarkably abundant decapod crustacean of immense commercial value in the coastal regions of the sea. This finding adds to the current collection of identified epibionts on the red king crab and highlights the prospect of ongoing symbiotic relationships between the crab and the regional fauna. We also provide photographic evidence of S. tubulosa colonies and a comparative morphological evaluation with another closely related corynid, Coryne hincksi Bonnevie, 1898, previously found on the red king crab. The main differences between these two species are the location of the gonophores (reproductive organs) on the hydranths and the final stage of their development: in the form of free-floating medusae (S. tubulosa) or remaining sessile as sporosacs (C. hincksi). Other distinguishing features include variations in hydranth morphology, number and arrangement of tentacles, and structural development of gonophores.

Seasonal abundance of the nudibranch Dondice jupiteriensis on the colonial hydroid Eudendrium carneum

Benthic colonial hydroids often serve as a host to a large variety of invertebrate taxa. The colonial hydroid Eudendrium carneum Clarke, 1882 is host to a variety of epifauna, including the recently described nudibranch Dondice jupiteriensis García—Méndez & Valdés, 2022. Like many species of heterobranch sea slugs, little is known about the natural history and population biology of D. jupiteriensis. Here we describe a year—long study recording the population density and size structure of D. jupiteriensis on E. carneum colonies in Tampa Bay, FL USA. We found that the D. jupiteriensis population persists year—round but is the densest during the summer months. This coincides with the time period when individual slugs were the smallest, suggesting the population increase in the summer is likely due to larval recruitment. Additionally, 13 other species of heterobranch sea slugs were found inhabiting E. carneum over the course of the study, although none of them were as numerous as D. jupiteriensis. This study provides the first available information on the ecology and population biology of the recently described nudibranch D. jupiteriensis.

Morphological, Genetic, and Physiological Effects of Nutrient Manipulation on a Colonial Marine Hydroid.

AbstractThe availability of environmental nutrients is an existential constraint for heterotrophic organisms and is thus expected to impact numerous biochemical and physiological features. The continuously proliferative polyp stage of colonial hydroids provides a useful model to study these features, allowing genetically identical replicates to be compared. Two groups of colonies of Eirene sp., defined by different feeding treatments, were grown by explanting the same founder colony onto cover glass. Colonies of both treatments were allowed to grow continuously by explanting them onto new cover glass as they reached the edge of the existing surface. The nutrient-abundant polyps grew faster and produced more clumped or "sheet-like" colonies. Compared to the founder colony, the nutrient-abundant colonies exhibited more mutations (i.e., single-nucleotide polymorphisms) than the nutrient-scarce colonies. Nevertheless, these differences were not commensurate with the differences in growth. Using a polarographic electrode, we found that the nutrient-abundant colonies exhibited lower rates of oxygen uptake relative to total protein. The probe 2',7'-dichlorodihydrofluorescein diacetate and fluorescent microscopy allowed visualization of the mitochondrion-rich cells at the base of the polyps and showed that the nutrient-abundant colonies exhibited greater amounts of reactive oxygen species than the nutrient-scarce colonies. Parallels to the Warburg effect-aerobic glycolysis, diminished oxygen uptake, and lactate secretion-found in human cancers and other proliferative cells may be suggested. However, little is known about anaerobic metabolism in cnidarians. Examination of oxygen uptake suggests an anaerobic threshold at a roughly 1-mg/L oxygen concentration. Nutrient-abundant colonies may respond more dramatically to this threshold than nutrient-scarce colonies.

How Tolerant Are Hydroids to Climate-Change-Induced Acute Spikes in Sea Water Temperature? A Case Study of Arctic Dynamena pumila (L., 1758)

The temperature of the water surface layer in the Arctic may increase significantly in the coming decades. To what extent will shallow-water fauna be affected by warming? We investigated this issue using an example of one species of colonial hydroid, Dynamena pumila. We judged its reaction to warming via its pulsation activity and the growth of stolons. Pulsations of the coenosarc in colonial hydroids are a sensitive indicator of the body’s reaction to the influence of environmental factors. We tested the ability of D. pumila colonies to survive and adapt to existing at 25 °C for five days. After raising the temperature from 14 °C to 25 °C, colony growth and the pulsation of stolon growth tips on the first day increased and then decreased during the day. In the following days, the growth pulsations almost ceased, the colonies stopped growing, and their coenosarcs began to exfoliate from their perisarcs. However, by the fourth day, this process slowed down, and the colonies existed in an economy mode of experiencing unfavourable conditions. The thermal shock continued in the experiment for five days. Then, after the temperature dropped from 25 °C to 15–16 °C, all the colonies recovered within five days and continued to grow.

Optimization of plastic polymers for shellfish aquaculture infrastructures: in situ antifouling performance assessment

Biofouling poses a significant challenge to bivalve aquaculture affecting both the target culture and/or the immersed infrastructure. In suspended bivalve cultures (e.g., oysters and scallops), biofouling accumulation leads to additional labor demands and increased costs for the maintenance of underwater structures. Given that the inherent properties of materials used in farming infrastructure influence the formation of fouling communities, evaluating how these materials perform under diverse environmental conditions can help the industry select the most effective materials for preventing or minimizing biofouling growth. This study evaluates the impact of aquaculture material and environmental conditions on biofouling, focusing on two commonly used plastic polymers in marine aquaculture: polyamide (PA) and high-density polyethylene (PE). Both untreated and color-additive treated polymers were tested for their response to fouling development. Performance was gauged by total fouling wet weight and the extent of fouling-induced mesh occlusion. Experimental panels were deployed for 4 months (from May to September 2021) in estuarine (oyster farm) and marine (port) environments on the northern coast of Portugal. The marine sites exhibited greater fouling species diversity, while higher biofouling loads were found in the subtidal estuarine area. Within 3 months, complete mesh occlusion occurred mainly due to colonial hydroids (Obelia dichotoma) in the subtidal site. In contrast, panels deployed in the intertidal estuarine area had lower fouling biomass and mesh occlusion. Notably, significant differences between polymer types and treatments were only evident in the estuarine intertidal area, with long air exposure during low tide. White panels outperformed orange ones in fouling biomass, and PA panels outperformed PE panels in mesh occlusion. These differences were attributed to the settlement and growth of the acorn barnacle Austrominius modestus, known to favor dark-colored and less hydrophilic surfaces. Considering that oyster production in intertidal areas is one of the most important aquaculture industries globally, these findings offer valuable insights into material selection and characteristics that can mitigate fouling loads and their associated impacts. These results could also be relevant for other forms of bivalve aquaculture where infrastructure-related biofouling presents a challenge.

Ultrastructural and immunocytochemical evidence of a colonial nervous system in hydroids.

As the sister group to all Bilateria, representatives of the phylum Cnidaria (sea anemones, corals, jellyfishes, and hydroids) possess a recognizable and well-developed nervous system and have attracted considerable attention over the years from neurobiologists and evo-devo researchers. Despite a long history of nervous system investigation in Cnidaria, most studies have been performed on unitary organisms. However, the majority of cnidarians are colonial (modular) organisms with unique and specific features of development and function. Nevertheless, data on the nervous system in colonial cnidarians are scarce. Within hydrozoans (Hydrozoa and Cnidaria), a structurally "simple" nervous system has been described for Hydra and zooids of several colonial species. A more complex organization of the nervous system, closely related to the animals' motile mode of life, has been shown for the medusa stage and a few siphonophores. Direct evidence of a colonial nervous system interconnecting zooids of a hydrozoan colony has been obtained only for two species, while it has been stated that in other studied species, the coenosarc lacks nerves. In the present study, the presence of a nervous system in the coenosarc of three species of colonial hydroids - the athecate Clava multicornis, and thecate Dynamena pumila and Obelia longissima - was studied based on immunocytochemical and ultrastructural investigations. Confocal scanning laser microscopy revealed a loose system composed of delicate, mostly bipolar, neurons visualized using a combination of anti-tyrosinated and anti-acetylated a-tubulin antibodies, as well as anti-RF-amide antibodies. Only ganglion nerve cells were observed. The neurites were found in the growing stolon tips close to the tip apex. Ultrastructural data confirmed the presence of neurons in the coenosarc epidermis of all the studied species. In the coenosarc, the neurons and their processes were found to settle on the mesoglea, and the muscle processes were found to overlay the nerve cells. Some of the neurites were found to run within the mesoglea. Based on the findings, the possible role of the colonial nervous system in sessile hydroids is discussed.

Integral effect of a non-centralized biosystem parts interaction with the example of major hydroplasma flow formation in the shoots of colonial hydroid <i>Dynamena pumila</i> (L., 1758)

The emergence of the main hydroplasmic flows (HFs) in the shoot of colonial hydroids (Hydrozoa), flowing from it into the stolon and capable of transporting food particles along stolons to the opposite end of the non-centralized organism, is described. We developed a simple method for minute-by-minute visual registration of the direction of particles movement in hydroplasm in each shoot shaft module for 1.5 hours. The HFs entering the shoot are shown to be rhythmic (15.1 ± 5.0 min), but if the outgoing HFs are usually filled with food particles, the returning HFs, as a rule, contain few particles. There is more chaos in the pulsations of hydrants on the shoot than in the stem of the shoot, i.e. the period of receipt of HF from hydrants to the stem of the shoot varies significantly. Previously, it has been assumed that outgoing HFs are formed only under the influence of incoming ones, as a response to coenosarc stretching when hydroplasma enters the shoot (under the pressure of other pulsators in the colonial organism). However, it turned out that from the appearance of the incoming to the beginning of the outgoing HF, 5 minutes elapsed as an average. During this time, hydroplasma fills not only the stem of the shoot, but also the hydrants, which contract in response, although not simultaneously. The hydrants squeeze out the hydroplasma during compression into the stem of the shoot, after which the coenosarc of the shoot begins to shrink. It turned out that when taking into account the pulsations of hydrants for 3 minutes, a clear periodicity is registered, coinciding with the periodicity of HF emanating from the shoot, resulting from compression of the coenosarc of the stem. At the same time, hydroplasma cannot enter the hydrants until the end of the long phase of their compression. Otherwise, the movements of the hydroplasma would be limited only by the escape space. Thus, in a non-centralized system such as colonial hydroids organism, the coordination of pulsations of hydrants and coenosarc occurs with significant backlash. In the coenosarc, as a whole, the pulsations are rhythmic with minor fluctuations, while the hydrants are sources of many little rhythmic pulsations (with large variations in the period).Therefore, the contribution of hydrants to the formation of the main HF does not appear at once, but for several minutes in the interval between the incoming and outgoing flows.

Establishing bilateral symmetry in hydrozoan planula larvae, a review of siphonophore early development.

Siphonophores are colonial hydrozoans, characterized by complex colony organization and unparalleled zooid functional specialization. Recent genomic studies have offered an evolutionary perspective on how this morphological complexity arose, but a molecular characterization of symmetry breaking in siphonophore embryonic development is still largely missing. Here, bringing together historical data on early development with new immunohistochemical data, we review the diversity of developmental trajectories that leads to the formation of bilaterally symmetric planula larvae in siphonophores. Embryonic development, up to the planula stage, is remarkably similar across siphonophore phylogeny. Then, with the appearance of the lateral endodermal thickening ( = ventral endoderm), larval development diverges between taxa, differing in the location and patterning of the primary buds, chronology of budding, establishment of growth zones and retention of larval zooids. Our work also uncovers a number of open questions in siphonophore development including homology of different zooids, mechanisms underlying formation and maintenance of spatially restricted growth zone(s), and molecular factors establishing a secondary dorsal-ventral axis in planulae. By discussing siphonophore development and body axes within the broader cnidarian context, we then set the framework for future work on siphonophores, that is finally achievable with the advent of culturing methods.

Localization of Multiple Jellyfish Toxins Shows Specificity for Functionally Distinct Polyps and Nematocyst Types in a Colonial Hydrozoan.

Hydractinia symbiolongicarpus is a colonial hydrozoan that displays a division of labor through morphologically distinct and functionally specialized polyp types. As with all cnidarians, their venoms are housed in nematocysts, which are scattered across an individual. Here, we investigate the spatial distribution of a specific protein family, jellyfish toxins, in which multiple paralogs are differentially expressed across the functionally specialized polyps. Jellyfish toxins (JFTs) are known pore-forming toxins in the venoms of medically relevant species such as box jellyfish (class Cubozoa), but their role in other medusozoan venoms is less clear. Utilizing a publicly available single-cell dataset, we confirmed that four distinct H. symbiolongicarpus JFT paralogs are expressed in nematocyst-associated clusters, supporting these as true venom components in H. symbiolongicarpus. In situ hybridization and immunohistochemistry were used to localize the expression of these JFTs across the colony. These expression patterns, in conjunction with known nematocyst type distributions, suggest that two of these JFTs, HsymJFT1c-I and HsymJFT1c-II, are localized to specific types of nematocysts. We further interpret JFT expression patterns in the context of known regions of nematogenesis and differential rates of nematocyst turnover. Overall, we show that JFT expression patterns in H. symbiolongicarpus are consistent with the subfunctionalization of JFT paralogs across a partitioned venom system within the colony, such that each JFT is expressed within a specific set of functionally distinct polyp types and, in some cases, specific nematocyst types.

GNL3 is an evolutionarily conserved stem cell gene influencing cell proliferation, animal growth and regeneration in the hydrozoan Hydractinia.

Nucleostemin (NS) is a vertebrate gene preferentially expressed in stem and cancer cells, which acts to regulate cell cycle progression, genome stability and ribosome biogenesis. NS and its paralogous gene, GNL3-like (GNL3L), arose in the vertebrate clade after a duplication event from their orthologous gene, G protein Nucleolar 3 (GNL3). Research on invertebrate GNL3, however, has been limited. To gain a greater understanding of the evolution and functions of the GNL3 gene, we have performed studies in the hydrozoan cnidarian Hydractinia symbiolongicarpus, a colonial hydroid that continuously generates pluripotent stem cells throughout its life cycle and presents impressive regenerative abilities. We show that Hydractinia GNL3 is expressed in stem and germline cells. The knockdown of GNL3 reduces the number of mitotic and S-phase cells in Hydractinia larvae of different ages. Genome editing of Hydractinia GNL3 via CRISPR/Cas9 resulted in colonies with reduced growth rates, polyps with impaired regeneration capabilities, gonadal morphological defects, and low sperm motility. Collectively, our study shows that GNL3 is an evolutionarily conserved stem cell and germline gene involved in cell proliferation, animal growth, regeneration and sexual reproduction in Hydractinia, and sheds new light into the evolution of GNL3 and of stem cell systems.

Biochemical and Toxinological Characterization of Venom from Macrorhynchia philippina (Cnidaria, Hydrozoa)

Macrorhynchia philippina is a colonial benthic hydroid from the Class Hydrozoa (Phylum Cnidaria) distributed in the tropical and subtropical marine waters from Atlantic Ocean, Indo-Pacific, and Mozambique. Its colonies somewhat resemble plants, causing confusion in the bathers who accidentally touch the animal. Acute burning/local pain, edema, erythema, and pruritus were symptoms already described, but its venom composition is unknown, as well as the participation of toxins for the symptom's development. Thus, herein, we show the biochemical composition and toxic effects of M. philippina venom. Colonies were collected and processed for histological analysis; alternatively, they were immersed into methanol containing 0.1% acetic acid for venom attainment, which was analyzed by mass spectrometry and submitted to edema and nociception evaluation in mice, hemolysis and antimicrobial assays in vitro. Before the molecule's extraction, it was possible to see the inoculation structures (hydrocladiums and hydrotheca) containing venom, which was released after the immersion of the animal in the solvents. The venom was composed mainly by low molecular mass compounds, able to cause significant reduction of the paw withdrawal latency from the hot plate test, 30 minutes after the injection. Moreover, significant edema was observed 10 and 30 minutes after the injection, indicating the activity of at least two inflammatory mediators. The venom caused no hemolytic activity but reduced the growth of A. baumannii and K. pneumoniae strains. This study is the first biochemical description of M. philippina venom, with molecules that cause fast inflammatory and painful effects, characteristic of the envenomation.

Venom system variation and the division of labor in the colonial hydrozoan Hydractinia symbiolongicarpus.

Cnidarians (jellyfish, hydroids, sea anemones, and corals) possess a unique method for venom production, maintenance, and deployment through a decentralized system composed of different types of venom-filled stinging structures called nematocysts. In many species, nematocyst types are distributed heterogeneously across functionally distinct tissues. This has led to a prediction that different nematocyst types contain specific venom components. The colonial hydrozoan, Hydractinia symbiolongicarpus, is an ideal system to study the functional distribution of nematocyst types and their venoms, given that they display a division of labor through functionally distinct polyps within the colony. Here, we characterized the composition and distribution of nematocysts (cnidome) in the different polyp types and show that the feeding polyp (gastrozooid) has a distinct cnidome compared to the reproductive (gonozooid) and predatory polyp (dactylozooid). We generated a nematocyst-specific reporter line to track nematocyst development (nematogenesis) in H. symbiolongicarpus, and were able to confirm that nematogenesis primarily occurs in the mid-region of the gastrozooid and throughout stolons (tubes of epithelia that connect the polyps in the colony). This reporter line enabled us to isolate a nematocyst-specific lineage of cells for de novo transcriptome assembly, annotate venom-like genes (VLGs) and determine differential expression (DE) across polyp types. We show that a majority of VLGs are upregulated in gastrozooids, consistent with it being the primary site of active nematogenesis. However, despite gastrozooids producing more nematocysts, we found a number of VLGs significantly upregulated in dactylozooids, suggesting that these VLGs may be important for prey-capture. Our transgenic Hydractinia reporter line provides an opportunity to explore the complex interplay between venom composition, nematocyst diversity, and ecological partitioning in a colonial hydrozoan that displays a division of labor.

Integrating siphonophores into marine food‐web ecology

AbstractSiphonophores are a clade of understudied colonial hydrozoans (Cnidaria) that are abundant predators in oceanic ecosystems, with species present across the water column. We (1) synthesize current knowledge about siphonophore trophic ecology and predator–prey interactions, (2) analyze siphonophore‐prey networks to compare food‐web topology between shallow and deep‐pelagic habitats, (3) discuss contemporary techniques that will allow for more integrative studies of siphonophore feeding ecology, and (4) and present a vision for future research. We found distinct diet differences between siphonophore species, indicating that siphonophores occupy multiple trophic niches and prey on a diversity of taxa. Our results suggest that siphonophore‐prey networks may be more specialized in the deep pelagic than in the epipelagic, suggesting potential trophic differences between depth habitats. This study highlights niche differentiation and trophic complexity among siphonophores and demonstrates the importance of gelatinous zooplankton in shaping food web structure in pelagic ecosystems.

Pleustonic colonies of cnidarians (Physalia physalis, Porpita porpita and Velella velella) found along the coastal belt of Sri Lanka

Three species of the free-floating, colonial hydrozoans namely Physalia physalis L., Porpita porpita L. and Velella velella L., found along the coastal belt of Sri Lanka are reported here with their morphological descriptions and respective abundance. The study was conducted from March 2017 to April 2018, implementing monthly coastal surveys at 26 sampling stations along the coastal belt of Sri Lanka. The abundance of stranded colonies of those species was assessed using line transects and the specimens were identified taxonomically. Of the porpitids found in this study, V. velella was reported for the first time in Sri Lankan waters while P. porpita was re-reported trustworthy as its initial record was just a sighting. The siphonophore P. physalis was also re-reported and it always represented the lowest abundance among the stranded colonies of the three species. The highest abundance of all the species was from May to August on the southwest coast of Sri Lanka, and from November to February on the northeast coast. There was a significant size difference in colonies of each species between the two coasts.

Morphogenetic Polyvariance in the Colonial Hydroid Dynamena pumila (L.)

The formation of aberrant modules (internodes) of the shoot was found sporadically in the colonial hydroid Dynamena pumila from the family Sertulariidae, which is distinguished by the symmetrical two-row position of the hydrotheca on the shoot stem. Despite rare significant deviations from the norm in the structure of some modules, the following modules in the shoots are formed normally. All variants of deviation from the norm (called morphovariations) are classified and, in general, are represented by four groups according to the possible reasons for their occurrence. The morphotypes are the most interesting morphological variations and look like similar modules in other genera of hydroids. Considering the position of the aberrant modules on the shoots of one colony, we can confidently infer that they were formed at different times, which undermines the assumption that deviations from the norm in morphogenesis were caused by environmental factors. All morphovariations are characterized by certain occurrences. Consequently, the described phenomenon is not limited to phenotypic plasticity, polymorphism, or sequential changes of phenotypes in the life cycle and, therefore, is singled out as a special phenomenon called morphogenetic polyvariance.

The Hydractinia allorecognition system.

Hydractinia symbiolongicarpus is a colonial hydroid and a long-standing model system for the study of invertebrate allorecognition. The Hydractinia allorecognition system allows colonies to discriminate between their own tissues and those of unrelated conspecifics that co-occur with them on the same substrate. This recognition mediates spatial competition and mitigates the risk of stem cell parasitism. Here, I review how we have come to our current understanding of the molecular basis of allorecognition in Hydractinia. To date, two allodeterminants have been identified, called Allorecognition 1 (Alr1) and Allorecognition 2 (Alr2), which occupy a genomic region called the allorecognition complex (ARC). Both genes encode highly polymorphic cell surface proteins that are capable of homophilic binding, which is thought to be the mechanism of self/non-self discrimination. Here, I review how we have come to our current understanding of Alr1 and Alr2. Although both are members of the immunoglobulin superfamily, their evolutionary origins remain unknown. Moreover, existing data suggest that the ARC may be home to a family of Alr-like genes, and I speculate on their potential functions.

Effects of soft marine fouling on wave-induced forces in floating aquaculture cages: Physical model testing under regular waves

Previous experiments on the fouling/current and fouling/wave interactions were mostly limited to two dimensional (2D) net panels or isolated twines. The current paper deals with the wave-induced forces/moments in three dimensional (3D) physical models of a gravity net-cage. The colonial hydroid Ectopleura larynx, a common fouling organism in the aquaculture industry, was artificially modelled and considered in the current experiments. A clean cage and cages with different fouling lengths were tested under deep-water regular wave trains. The time series of the wave-induced forces/moments in the surge, sway, roll and pitch degrees of freedom were recorded and analysed. The experimental results showed that the surge force amplitude increased up to 72% by the fouling, but the sway force amplitude decreased up to 26%. The asymmetry of the surge and sway force waveforms increased by the fouling, respectively. The effects of the fouling on the force/moment response amplitude operators (RAO) were also studied. Pixel histogram analysis was performed to define equivalent net solidities of the fouled nets. In general, the tests under regular waves were found to underestimate the soft fouling effects on the wave-induced forces/moments, as compared to those carried out under irregular waves.