Evidence for Host Selectivity and Specialization by Epizoic Chelonibia Barnacles Between Hawksbill and Green Sea Turtles

Epibionts are organisms that utilize the exterior of other organisms as a living substratum. Many affiliate opportunistically with hosts of different species, but others specialize on particular hosts as obligate associates. We investigated a case of apparent host specificity between two barnacles that are epizoites of sea turtles and illuminate some ecological considerations that may shape their host relationships. The barnacles Chelonibia testudinaria and Chelonibia caretta, though roughly similar in appearance, are separable by distinctions in morphology, genotype, and lifestyle. However, though each is known to colonize both green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) sea turtles, C. testudinaria is >5 times more common on greens, while C. caretta is >300 times more common on hawksbills. Two competing explanations for this asymmetry in barnacle incidence are either that the species’ larvae are spatially segregated in mutually exclusive host-encounter zones or their distributions overlap and the larvae behaviorally select their hosts from a common pool. We indirectly tested the latter by documenting the occurrence of adults of both barnacle species in two locations (SE Florida and Nose Be, Madagascar) where both turtle species co-mingle. For green and hawksbill turtles in both locations (Florida: n = 32 and n = 275, respectively; Madagascar: n = 32 and n = 125, respectively), we found that C. testudinaria occurred on green turtles only (percent occurrence – FL: 38.1%; MD: 6.3%), whereas the barnacle C. caretta was exclusively found on hawksbill turtles (FL: 82.2%; MD: 27.5%). These results support the hypothesis that the larvae of these barnacles differentially select host species from a shared supply. Physio-biochemical differences in host shell material, conspecific chemical cues, external microbial biofilms, and other surface signals may be salient factors in larval selectivity. Alternatively, barnacle presence may vary by host micro-environment. Dissimilarities in scute structure and shell growth between hawksbill and green turtles may promote critical differences in attachment modes observed between these barnacles. In understanding the co-evolution of barnacles and hosts it is key to consider the ecologies of both hosts and epibionts in interpreting associations of chance, choice, and dependence. Further studies are necessary to investigate the population status and settlement spectrum of barnacles inhabiting sea turtles.

Ocean Acidification Triggers Cell Signaling, Suppress Immune and Calcification in the Pacific Oyster Larvae

Elevated carbon dioxide levels in ocean waters, an anthropogenic stressor, can alter the chemical equilibrium of seawater through a process called ocean acidification (OA). The resultant reduction of pH can be detrimental during the early developmental stages of the commercially important edible Pacific oyster Crassostrea gigas; the ability of larvae to join a population is likely to be compromised by declining ocean pH. Given this threat, it is important to study the molecular mechanisms that these organisms use to overcome OA stress at the gene expression level. Here, we performed transcriptome profiling in oyster larvae following exposure to ambient (8.1) and reduced (7.4) pH during the pre-settlement growth period (i.e., 18 d post fertilization) using RNA-seq with Illumina sequencing technology. In total, 1,808 differentially expressed genes (DEGs) were identified, 1,410 of which were matched by BLAST against the Swiss-Prot database. Gene ontology classification showed that most of these DEGs were related to ribosomal, calcium ion binding, cell adhesion and apoptotic processes. Pathway enrichment analysis revealed that low pH (7.4) enhanced energy production and organelle biogenesis but prominently suppressed several immune response pathways. Moreover, activation of the MAPK signaling pathway was observed along with inhibition of the Wnt, VEGF, and ErbB pathways, highlighting the fact that the initiation of stress responses is given priority over larval development or shell growth when the larvae cope with low pH. In conclusion, our study demonstrated a unique gene expression profiling approach in studying oyster larval responses to OA, which not only provides comprehensive insights into the mechanisms underlying oyster tolerance to CO2-driven decreases in ocean pH but also supplies a valuable genomic resource for further studies in this species.

Texture of the Freshwater Shells from the Unionidae Family Collected in the Czech Republic Investigated by X-ray and Neutron Diffraction

Bivalve shells exhibit extreme mechanical resistance despite using a minimal amount of material. The shells thus represent an inspiration and a source of information for environmental, geological, and engineering sciences. In this study, two species of freshwater shells from the Unionidae family, collected in the Czech Luznice River, were investigated with respect to their crystallographic preferred orientation by means of X-ray and neutron diffraction. The observed texture was found to be of a strongly uniaxial type, with the strength increasing along the shell growth direction. The c-axis of aragonite does not change during growth and its alignment remains perpendicular to the outer surface of the shell

Selected life history traits of Leptaxis simia (Gastropoda: Hygromiidae) established in the laboratory

Among ten adult specimens of the Madeiran endemic land snail Leptaxis simia kept in the laboratory, a single clutch of 110 eggs was reared, and the mortality, growth and reproductive capacity of hatchlings over a 1240- day period were monitored. Of 70 hatchlings, 34 survived to complete shell growth at around 640 days. Growth was rapid, and mortality high in the early stages; growth slowed and mortality was very low as full size was approached. Snails kept singly failed to lay any eggs. Among those with potential mates, only three clutches were produced, one of which yielded no hatchlings. Clutches were produced at least five months after shell growth was complete. Mortality increased sharply after the onset of sexual maturity, and the oldest snail survived for three and a half years. The apparently very strictly semelparous mode of reproduction, delayed sexual maturity and relatively long period of juvenile growth are discussed in relation to other species, and to the practical problems of rearing potentially endangered species in captivity.

Five hundred million years to mobility: directed locomotion and its ecological function in a turtle barnacle

Movement is a fundamental characteristic of life, yet some invertebrate taxa, such as barnacles, permanently affix to a substratum as adults. Adult barnacles became ‘sessile’ over 500 Ma; however, we confirm that the epizoic sea turtle barnacle, Chelonibia testudinaria , has evolved the capacity for self-directed locomotion as adults. We also assess how these movements are affected by water currents and the distance between conspecifics. Finally, we microscopically examine the barnacle cement. Chelonibia testudinaria moved distances up to 78.6 mm yr −1 on loggerhead and green sea turtle hosts. Movements on live hosts and on acrylic panels occasionally involved abrupt course alterations of up to 90°. Our findings showed that barnacles tended to move directly against water flow and independent of nearby conspecifics. This suggests that these movements are not passively driven by external forces and instead are behaviourally directed. In addition, it indicates that these movements function primarily to facilitate feeding, not reproduction. While the mechanism enabling movement remained elusive, we observed that trails of cement bore signs of multi-layered, episodic secretion. We speculate that proximal causes of movement involve one or a combination of rapid shell growth, cement secretion coordinated with basal membrane lifting, and directed contraction of basal perimeter muscles.

Bivalves rapidly repair shells damaged by fatigue and bolster strength

ABSTRACT Hard external armors have to defend against a lifetime of threats yet are traditionally understood by their ability to withstand a single attack. Survival of bivalve mollusks thus can depend on the ability to repair shell damage between encounters. We studied the capacity for repair in the intertidal mussel Mytilus californianus by compressing live mussels for 15 cycles at ∼79% of their predicted strength (critically fracturing 46% of shells), then allowing the survivors 0, 1, 2 or 4 weeks to repair. Immediately after fatigue loading, mussel shells were 20% weaker than control shells that had not experienced repetitive loading. However, mussels restored full shell strength within 1 week, and after 4 weeks shells that had experienced greater fatiguing forces were stronger than those repetitively loaded at lower forces. Microscopy supported the hypothesis that crack propagation is a mechanism of fatigue-caused weakening. However, the mechanism of repair was only partially explained, as epifluorescence microscopy of calcein staining for shell deposition showed that only half of the mussels that experienced repetitive loading had initiated direct repair via shell growth around fractures. Our findings document repair weeks to months faster than demonstrated in other mollusks. This rapid repair may be important for the mussels’ success contending with predatory and environmental threats in the harsh environment of wave-swept rocky coasts, allowing them to address non-critical but weakening damage and to initiate plastic changes to shell strength. We highlight the significant insight gained by studying biological armors not as static structures but, instead, as dynamic systems that accumulate, repair and respond to damage.

STUDY OF THE SHELL MATTER OF MODERN AND PLEISTOCENE MOLLUSCS OF THE GENUS DIDACNA

One of the ways to study the reactions of marine invertebrates to the external effects of changes in temperature and salinity is the biogeochemical analysis of skeletal parts, which are consistently built up during ontogenesis and record a variety of information about these changes. The most studied shells of mollusks, sea urchin shells and skeletal parts of corals. Information about the chemical composition of modern and fossil mollusk shells is widely used in solving geological and biological problems, including determining the temperature and salinity of ancient marine basins, studying the diagenesis of carbonate sediments, and the biochemical evolution of invertebrates. X-ray diffraction analysis of the shell matter of didacnae belonging to the Cardiidae showed an aragonite composition. The quantitative determination of elements in mollusk shells by microprobe analysis of spot scanning and spectrometric method is carried out. Samples were taken in successive layers of shell growth within the annual ring, and the seasonal dynamics of strontium changes were detected. For Didacna, strontium is the main element-indicator of seasonal and ontogenetic growth, is included in the crystal lattice of aragonite and forms strong compounds in the process of shell formation during the life of these bivalves. The variability of seasonal, ontogenetic, and taxonomic differences in a number of indicator elements in living and Pleistocene bivalves of the genus Didacna was studied.

Larval metamorphosis is inhibited by methimazole and propylthiouracil that reveals possible hormonal action in the mussel Mytilus coruscus

Larval metamorphosis in bivalves is a key event for the larva-to-juvenile transformation. Previously we have identified a thyroid hormone receptor (TR) gene that is crucial for larvae to acquire “competence” for the metamorphic transition in the mussel Mytilus courscus (Mc). The mechanisms of thyroid signaling in bivalves are still largely unknown. In the present study, we molecularly characterized the full-length of two iodothyronine deiodinase genes (McDx and McDy). Phylogenetic analysis revealed that deiodinases of molluscs (McDy, CgDx and CgDy) and vertebrates (D2 and D3) shared a node representing an immediate common ancestor, which resembled vertebrates D1 and might suggest that McDy acquired specialized function from vertebrates D1. Anti-thyroid compounds, methimazole (MMI) and propylthiouracil (PTU), were used to investigate their effects on larval metamorphosis and juvenile development in M. coruscus. Both MMI and PTU significantly reduced larval metamorphosis in response to the metamorphosis inducer epinephrine. MMI led to shell growth retardation in a concentration-dependent manner in juveniles of M. coruscus after 4 weeks of exposure, whereas PTU had no effect on juvenile growth. It is hypothesized that exposure to MMI and PTU reduced the ability of pediveliger larvae for the metamorphic transition to respond to the inducer. The effect of MMI and PTU on larval metamorphosis and development is most likely through a hormonal signal in the mussel M. coruscus, with the implications for exploring the origins and evolution of metamorphosis.

Investigating controls of shell growth features in a foundation bivalve species: seasonal trends and decadal changes in the California mussel

Marine bivalve mollusc shells can offer valuable insights into past oceanographic variability and seasonality. Given its ecological and archaeological significance, Mytilus californianus (California mussel) presents the opportunity to examine seasonal and decadal changes recorded in its shell over centuries to millennia. While dark–light growth bands in M. californianus shells could be advantageous for reconstructing past environments, uncertainties remain regarding shell structure, environmental controls of dark–light band formation, and the amount of time represented by a dark–light pair. By analyzing a suite of M. californianus shells collected in 2002, 2003, 2019, and 2020 from Bodega Bay, California, we describe the mineralogical composition, establish relationships among growth band pattern, micro-environment, and collection season, and compare shell structure and growth band expression between the archival (2002–2003) and modern (2019–2020) shells. We identified three mineralogical layers in M. californianus: an outer prismatic calcite layer, a middle aragonite layer, and a secondary inner prismatic calcite layer, which makes M. californianus the only Mytilus species to precipitate a secondary calcite layer. Within the inner calcite layer, light bands are strongly correlated with winter collection months and could be used to reconstruct periods with moderate, stable temperatures and minimal upwelling. Additionally, modern shells have significantly thinner inner calcite layers and more poorly expressed growth bands than the archival shells, although we also show that growth band contrast is strongly influenced by micro–environment. Mytilus californianus from northern California is calcifying differently, and apparently more slowly, than it was 20 years ago.