Gene-flow patterns in Atlantic and Mediterranean populations of the Lusitanian sea star Asterina gibbosa

In this study, the population structure of the Lusitanian sea star Asterina gibbosa was assessed using amplified fragment length polymorphism (AFLP). One hundred and twenty-two AFLP loci were analysed in 159 individuals from eight populations from across the species' range and revealed high levels of genetic diversity, with all individuals but two harbouring a unique banding pattern. As reported for other marine invertebrates, we found high levels of genetic differentiation between the Atlantic and Mediterranean basins, suggesting that the Strait of Gibraltar represents a major barrier to dispersal for this sea star. Our assignment studies suggest that, in the Atlantic, a measurable degree of gene flow occurs between populations, which could result in the isolation-by-distance pattern of differentiation observed in this basin. In contrast, no evidence of contemporary gene flow was found in the Mediterranean, suggesting contrasting patterns of dispersal of Asterina gibbosa in the Atlantic and Mediterranean basins.     

The coeloms in a late brachiolaria larva of the asterinid sea star Parvulastra exigua: deriving an asteroid coelomic model

Morris, V.B., Selvakumaraswamy, P., Whan, R., and Byrne, M. 2011. The coeloms in a late brachiolaria larva of the asterinid sea star Parvulastra exigua: deriving an asteroid coelomic model. —Acta Zoologica (Stockholm) 92 : 266–275. The coeloms and their interconnexions in a late pre‐metamorphic brachiolaria larva of a sea star are described from the series of images in the frontal, transverse and sagittal planes obtained by confocal laser scanning microscopy. A larval, brachial coelom connects with the coeloms of the adult rudiment that lie posteriorly. The connexion is through the anterior coelom, which lies over the head of the archenteron, to the right anterior coelom and then to the left posterior coelom through the ventral horn of the left posterior coelom. The right posterior coelom is a separate coelom. The hydrocoele is on the larval left side separated from other coeloms except for a connexion to the anterior coelom. On the larval right side, the anterior coelom and right anterior coelom connect with the pore canal that opens to the exterior at the hydropore. From these coeloms, we derived an asteroid coelomic model comprising the larval left and right coeloms linked over the head of the archenteron by a common anterior coelom. The asymmetry of the hydrocoele and the left posterior coelom on the left side linked through the common anterior coelom to the right side, with the external opening, translates into the oral and aboral coeloms of the adult stage. The coelomic model has application in the search for morphological homology between the echinoderm classes and the deuterostome phyla.     

Ultrastructure of spermatogenesis in the sea star,Asterina minor

The ultrastructural features of spermatogenesis were investigated in the hermaphroditic sea star Asterina minor. The primordial germ cells in the genital rachis contain small clusters of electron-dense material (nuage material) and a stack of annulate lamellae. They also have a flagellum and basal body complex situated close to the Golgi complex. After the development of the genital rachis into the ovotestis, spermatogenic cells increase in number and differentiation begins. Nuage material is observed in spermatogonia, but it gradually disappears in spermatocytes. The annulate lamellae do not exist beyond the early spermatogonial stage. By contrast, a flagellum and basal body complex are found throughout spermatogenesis. The Golgi-derived proacrosomal vesicles appear in the spermatocyte and coalesce to form an acrosomal vesicle in the early spermatid. The process of acrosome formation is as follows: (1) a lamella of endoplasmic reticulum (ER) continuous with the outer nuclear membrane encloses the posterior portion of the acrosomal vesicle; (2) the vesicle attaches to the cell membrane with its anterior portion; (3) periacrosomal material accumulates in the space between the acrosomal vesicle and the ER; (4) the nucleus proper changes its features to surround the acrosome; (5) amorphous, electron-dense material is deposited under the electron-dense disk; and (6) the nucleus forms a hollow opposite the electron-dense material.     

Development of tetranucleotide microsatellite markers for the cushion star, Asterina gibbosa, and cross-species amplification

We describe nine polymorphic tetranucleotide microsatellite loci from the starfish, Asterina gibbosa. Loci were isolated from a partial genomic library that had been enriched for AAAC repeat sequences. Number of alleles per locus ranged from two to 14 in a sample of 85 individuals from three populations (two from Spain and one from the UK). Observed and expected heterozygosities per population ranged from 0.000 to 0.400 and from 0.040 to 0.784, respectively. All loci presented significant heterozygote deficits in one or more populations. Eight of these loci were amplified and variable in A. pancerii and A. phylactica. These loci will be used to study population structure in A. gibbosa.     

The combined effect of temperature and salinity on development of the sea star Asterina pectinifera

The effect of various combinations of temperature, which increases from 14°C up to 25°C in the summer season, and salinity, which varies from 34 to 12‰ in the early stages of development of the sea star Asterina (= Patiria) pectinifera (Müller et Troschel) from Vostok Bay, Sea of Japan, was studied. The most vulnerable process in the early ontogenesis of A. pectinifera is its embryonal development, which is completed successfully within narrow ranges of temperature (20–22°C) and salinity (34–26‰). The ability of gametes to fertilize was retained in wider ranges of temperature and salinity. The dipleurula was the most responsive of the larval stages; the resistance of blastula, bipinnaria, and brachiolaria at ages of 12.5 and 15.5 days was almost the same for fluctuations of temperature from 14 up to 25°C and salinity from 34 to 18 and 16‰ Settling of the brachiolaria and completion of metamorphosis were also responsive to variations in the environmental factors. Settling of the larvae was faster at 17°C without illumination (on the 22nd–24th days of development) than at 22°C with the day-night mode (27th–28th day of development). The lack of light apparently had a positive effect on the settling of the brachiolaria.     

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea, Echinodermata) during the perimetamorphic period

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.     

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea,Echinodermata) during the perimetamorphic period

Abstract

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.     

The attachment complex of brachiolaria larvae of the sea star <Emphasis Type="Italic">Asterias rubens</Emphasis> (Echinodermata): an ultrastructural and immunocytochemical study

The attachment complex of brachiolaria larvae of the asteroid Asterias rubens comprises three brachiolar arms and an adhesive disc located on the preoral lobe. The former are used in temporary attachment and sensory testing of the substratum, whereas the latter is used for permanent fixation to the substratum at the onset of metamorphosis. Brachiolar arms are hollow structures consisting of an extensible stem tipped by a crown of dome-like ciliated papillae. The papilla epidermis is composed of secretory cells (type A, B and C cells), non-secretory ciliated cells, neurosecretory-like cells and support cells. Type A and B secretory cells fill a large part of the papilla epidermis and are always closely associated. They presumably form a duo-gland adhesive system in which type A and B cells are respectively adhesive and de-adhesive in function. The adhesive disc is an epidermal structure mainly composed of secretory cells and support cells. Secretory cells produce the cement, which anchor the metamorphic larva to the substratum until the podia are developed. The relatedness between the composition of the adhesive material in the brachiolaria attachment complex and in the podia of adults was investigated by immunocytochemistry using antibodies raised against podial adhesive secretions of A. rubens. Type A secretory cells were the only immunolabelled cells indicating that their temporary adhesive shares common epitopes with the one of podia. The attachment pattern displayed by the individuals of A. rubens during the perimetamorphic period—temporary, permanent, temporary—is unique among marine non-vertebrate Metazoa.     

Characterization of O-GlcNAcylation in starfish (Asterina pectinifera) development from fertilization to bipinnaria larva

Though O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) of nucleocytoplasmic proteins has been found in many multicellular organisms, its presence or absence in Echinodermata is unknown. Here we report the occurrence of O-GlcNAcylation in starfish (Asterina pectinifera) oocytes and the apparent O-GlcNAcylation pattern in starfish early development. O-GlcNAcylation might participate in the regulation of starfish development at the mid-blastula stage and thereafter.     

Lysosomes and yolk formation in the oocytes of Asterina gibbosa (Echinodermata,Asteroidea)

Summary

Primary lysosomes appear in the oocytes of A. gibbosa at the end of previtellogenesis. The lysosomes fuse with the cisternae of the endoplasmic reticulum and give rise to yolk globules containing acid phosphatase. The yolk globules then grow by fusion.     

Feeding repellence of Antarctic and sub-Antarctic benthic invertebrates against the omnivorous sea star Odontaster validus

Antarctic and sub-Antarctic benthic invertebrates are subjected to intense predation by mobile macroinvertebrates. Accordingly, chemical protection as well as other defensive mechanisms are expected to be common in organisms inhabiting these ecosystems. In order to evaluate anti-predation activities and allocation of chemical defenses within the anatomy of marine benthic Antarctic and sub-Antarctic invertebrates, 55 species were tested for feeding repellence against the sea star Odontaster validus, a common eurybathic sympatric predator. The invertebrates tested were collected from the deep waters of two poorly surveyed areas in terms of chemical ecology studies: the eastern Weddell Sea (Antarctica) and the vicinities of Bouvet Island (sub-Antarctica). Experiments were conducted at the Spanish Antarctic Base in Deception Island. In the feeding deterrence experiments, shrimp pieces were treated with crude lipophilic fractions obtained from each species, and were offered to the sea stars. A total of 29 species (53 %) from 7 different phyla (Porifera, Cnidaria, Chordata, Bryozoa, Echinodermata, Mollusca, and Annelida) showed feeding repellence against O. validus, and are therefore chemically protected against this keystone predator. Furthermore, 25 species were dissected into parts to investigate the possible allocation of defensive compounds. Some of the results obtained from these analyses support the prediction that the most exposed/vulnerable tissues concentrate chemical defenses to avoid predation against the sea stars. In summary, the results obtained in our survey support the hypothesis that deep-water Antarctic and sub-Antarctic benthic invertebrates are well protected chemically against sympatric predators, similarly to what has been reported in previous studies investigating shallow-water Antarctic species.     

Evolution of development in the sea star genus Patiriella: clade-specific alterations in cleavage

Examination of early development in five species of the Patiriella sea star species complex indicates that the ancestral-type radial holoblastic cleavage (Type I) is characteristic of P. regularis and P. exigua, whereas cleavage in species from the calcar clade followed multiple alternatives (Types II-IV) from holoblastic to meroblastic. Considering that invariant radial cleavage is thought to play a role in embryonic axis formation in echinoderms, we documented the details of blastomere formation in Patiriella sp. and followed development of the embryos. In Type II cleavage, the first and second cleavage planes appeared simultaneously at one pole of the embryo, dividing it directly into four equally sized blastomeres. In Type III cleavage, the first and second cleavage planes appeared simultaneously, followed promptly by the third cleavage plane, dividing the embryo directly into eight equally sized blastomeres. In Type IV cleavage, numerous furrows appeared simultaneously at one end of the embryo, dividing it into 32-40 equally sized blastomeres. Confocal sections revealed that embryos with cleavage Types II-IV were initially syncytial. The timing of karyokinesis in embryos with Types II and III cleavage was similar to that seen in clutch mates with Type I cleavage. Karyokinesis in embryos with Type IV cleavage, however, differed in timing compared with Type I clutch mates. Alteration in cleavage was not associated with polarized distribution of maternally provided nutrients. For each cleavage type, development was normal to the competent larval stage. Although variable blastomere configuration in the calcar clade may be linked to possession of a lecithotrophic development, other Patiriella species with this mode of development have typical cleavage. The presence of variable cleavage in all calcar clade species indicates that phylogenetic history has played a role in the distribution of this embryonic trait in Patiriella. The plasticity in early cleavage in these sea stars indicates that this aspect of early development is not constrained against change and that there are many ways to achieve multicellularity.     

Terminology and functional morphology of attachment structures in pelmatozoan echinoderms

Brett, Carlton E. 1981 12 IS: Terminology and functional morphology of attachment structures in pelmatozoan echinoderms. Lethaia . Vol. 14, pp. 343–370. Oslo. 1SSN 0024–1164.
Fixation on the seafloor is a fundamental, though little studied, aspect of the biology and distributional ecology of pelmatozoan echinoderms. Morphology of pelmatozoan attachment structures (holdfasts) and their component appendages is summarized herein and a detailed terminology proposed. Three major modes of fixation are differentiated in living and fossil stemmed echinoderms - primary, composite and secondary attachments - in addition to the direct thecal attachment in stemless forms. Specific holdfast morphologies, which characterize certain major taxa (families and orders) of pelmatozoans, may limit the available life-modes in these echinoderms. Significantly, numerous lineages of echinoderms have independently developed secondary attachment modes associated with the evolution of flexible tapered stem ends and/or true cirri. Cirri are rare in early Paleozoic pelmatozoans but appeared in most major groups by the late Paleozoic. * Pelmatozoa. attachment structures, holdfasts, terminology, functional morphology .     

Adaptations of Anomopoda crustaceans (Cladocera) to the benthic mode of life

Crustaceans of the order Anomopoda inhabit both pelagic and littoral zones of fresh water bodies. Different adaptations to the benthic mode of life in representatives of different families and genera of this order are described. They include incomplete molting, the presence of a mucous film on the body surface, protection of the head from frontal damage, large lateral projections on the valves and additional sculpture on the head and valves, well-developed lateral armature of the postabdomen, the presence of projections on the postabdomen preanal margin, specialization of setae and spines of the second antenna, especially large exopodites on limbs IV and V, a reduced compound eye, and a large, well-developed ocellus. Similar adaptations to the benthic mode of life in the Anomopoda and Ostracoda are discussed.     

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral–aboral axis, involving local mesoderm–ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left–right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal–vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom–vestibule interaction that establishes the adult oral–aboral axis.     

Adaptations to migration in birds: behavioural strategies, morphology and scaling effects

The annual life cycle of many birds includes breeding, moult and migration. All these processes are time and energy consuming and the extent of investment in any one may compromise the others. The output from breeding is of course the ultimate goal for all birds, while the investment in moult and migration should be selected so that lifetime fitness is maximized. In particular, long-distance migrants breeding at high latitudes face severe time pressures, which is a probable reason why natural selection has evolved efficient behaviours, physiological and morphological adaptations allowing the maximum possible migration speed. Optimal migration theory commonly assumes time minimization as an overall strategy, but the minimization of energy cost and predation risk may also be involved. Based on these assumptions, it is possible to derive adaptive behaviours such as when and at which fuel load a stopover site should be abandoned. I review some core components of optimal migration theory together with some key predictions. A review of accumulated empirical tests of the departure rule indicates that time minimization is an important component of the overall migration strategy, and hence gives support to the assumption about time-selected migration. I also briefly discuss how the optimal policy may be implemented by the bird by applying a set of simple rules. The time constraints on migrants increase with increasing body size. Some consequences of this are discussed.     

Contribution of sea ice microbial production to Antarctic benthic communities is driven by sea ice dynamics and composition of functional guilds

Organic matter produced by the sea ice microbial community (SIMCo) is an important link between sea ice dynamics and secondary production in near‐shore food webs of Antarctica. Sea ice conditions in McMurdo Sound were quantified from time series of MODIS satellite images for Sept. 1 through Feb. 28 of 2007–2015. A predictable sea ice persistence gradient along the length of the Sound and evidence for a distinct change in sea ice dynamics in 2011 were observed. We used stable isotope analysis (δ¹³C and δ¹⁵N) of SIMCo, suspended particulate organic matter (SPOM) and shallow water (10–20 m) macroinvertebrates to reveal patterns in trophic structure of, and incorporation of organic matter from SIMCo into, benthic communities at eight sites distributed along the sea ice persistence gradient. Mass‐balance analysis revealed distinct trophic architecture among communities and large fluxes of SIMCo into the near‐shore food web, with the estimates ranging from 2 to 84% of organic matter derived from SIMCo for individual species. Analysis of patterns in density, and biomass of macroinvertebrate communities among sites allowed us to model net incorporation of organic matter from SIMCo, in terms of biomass per unit area (g/m²), into benthic communities. Here, organic matter derived from SIMCo supported 39 to 71 per cent of total biomass. Furthermore, for six species, we observed declines in contribution of SIMCo between years with persistent sea ice (2008–2009) and years with extensive sea ice breakout (2012–2015). Our data demonstrate the vital role of SIMCo in ecosystem function in Antarctica and strong linkages between sea ice dynamics and near‐shore secondary productivity. These results have important implications for our understanding of how benthic communities will respond to changes in sea ice dynamics associated with climate change and highlight the important role of shallow water macroinvertebrate communities as sentinels of change for the Antarctic marine ecosystem.     

The functional morphology and attachment mechanism of pandarid adhesion pads (Crustacea: Copepoda: Pandaridae)

The attachment mechanism of pandarid adhesion pads is described from observations of their externally ridged structure and internal construction in three species; Pandarus bicolor Leach, 1816, Dinemoura latifolia (Steenstrup and Lutken, 1861) and Echthrogaleus coleoptratus (Guerin-Meneville, 1837). The host's external skin morphology was also examined, since parasite attachment mechanism and host surface can be considered as components of a single system.

The results emphasise the importance of the physical nature of the pad's surface. This is inferred from the compliance of the cuticle and subsurface structure, and the presence of cuticular ridging. The pads probably prevent pandarids from being dislodged by hydrodynamic drag, by increasing overall adhesion. It is proposed that this is achieved in different ways, by two types of adhesion pad identified here, distinguishable by their external structure and location. Type I pads are suggested to remove interfacial water and increase surface contact by one of two contrasting methods. The ridges may act as tyre treads, by channelling water from the contact surface. Alternatively, the channels between ridges may be hydrophobic and behave as dewetting structures, preventing water from entering in the same way that troughs between surface nodules function to produce superhydrophobicity on lotus leaves. Type I adhesion pads are also suggested to aid attachment by hindering the process of peeling, by which they are thought to be removed by hydrodynamic drag. Type II pads are more likely to function as one-way frictional attachments. Both types of pad appear to be attached passively, since they lack muscles inserting into them. The adhesive mechanism of each, which functions most effectively on hard surfaces, may explain why pads are absent or reduced on pandarids which parasitise the softer, unscaled surfaces of hosts.

Pandarids predominantly parasitise the skin and fins of fast-swimming sharks. This may be because the scales are characteristically smaller in these species and are more easily encircled by the primary attachment appendages, the maxillipeds.

This is thought to be the first published report to reveal frictional attachment structures from the Crustacea, which have convergently evolved in many terrestrial Arthropoda.     

Expression of Hox4 during development of the pentamerous juvenile sea star, Parvulastra exigua

Expression of Hox4 during development of the bilateral larva and pentameral juvenile sea star was investigated in Parvulastra exigua. The role of Hox4, possibly the anterior-most gene in the echinoderm Hox cluster, in the formation of the echinoderm adult body plan has not been examined previously. In the larva of P. exigua, PeHox4 is expressed in the developing coeloms—the anterior and the right and left coeloms that generate the aboral and oral coeloms of the juvenile. At the rudiment stage, PeHox4 was expressed in the five primary lobes of the hydrocoel that give rise to primary podia, the foundation of the adult body plan. This suggests a role for this gene in the development of the echinoderm body plan. In contrast to other bilaterians, Hox4 was not expressed in the developing asteroid central nervous system.