2+-free artificial seawater or in different calcium channel blockers (verapamil, diltiazem) and calmodulin inhibitor (trifluoperazine) solutions. All of these results strongly suggest that T. helgolandica produces its light flashes via activation of nicotinic cholinergic receptors and a calcium-dependent intracellular mechanism involving L-type calcium channels.
The vast majority of pelagic bioluminescent organisms emit a blue light with emission maxima (λ ) ranging from 450 to 490 nm. Among the known outliers, the tomopterids (Annelida: Polychaeta) are usually described as yellow-emitters (λ = 565-570 nm) for which bioluminescence functions as a specific recognition signal. Here, we report the first data regarding the colours emitted by four different tomopterid species, Tomopteris pacifica, T. carpenteri, T. septentrionalis and T. planktonis. Surprisingly, T. planktonis is a blue-emitter (λ = 450 nm). Our pharmacological results on T. planktonis support cholinergic control, as recently demonstrated in the yellow-emitter, T. helgolandica. Moreover, as revealed by epifluorescence microscopy, the light seems to be produced in both species from the same yellow-pigmented parapodial glands. Despite these similarities, tomopterids express an unexpected diversity of bioluminescent colour patterns. This leads us to reassess the ecological value of bioluminescence within this group.
To resist hydrodynamic forces, two main underwater attachment strategies have evolved multiple times in aquatic animals: glue-like “bioadhesive secretions” and pressure-driven “suction attachment”. In this review, we use a multi-level approach to highlight convergence in underwater attachment mechanisms across four different length-scales (organism, organ, microscopic and molecular). At the organism level, the ability to attach may serve a variety of functions, the most important being: (i) positional maintenance, (ii) locomotion, (iii) feeding, (iv) building, and (v) defense. Aquatic species that use bioadhesive secretions have been identified in 28 metazoan phyla out of the 34 currently described, while suction organs have a more restricted distribution and have been identified in five phyla. Although biological adhesives are highly diverse, it is possible to categorize them into four main types according to the time scale of operation: permanent, temporary, transitory, and instantaneous adhesion. At the organ level some common principles have evolved independently in different biological lineages: for example, animals with single-unit attachment organs can be distinguished from those with multi-unit organs. Fundamental design elements can also be recognized for both types of attachment mechanisms. Suction attachment systems comprise a circular or elliptical attachment disc, a sealing rim to prevent leakage and a mechanism to lower the internal pressure. Bioadhesive-producing organs, on the other hand, usually contain a glandular tissue associated with connective tissues or other types of load-bearing support structures and muscles that facilitate locomotion or mechanical detachment. At the microscopic level, similar designs and organizations appear once again to have emerged independently in different phylogenetic lineages. Independent of the taxon and type of adhesion, there are species in which the biosynthesis, packaging and release of adhesive secretions takes place at the level of a single type of secretory cell, whereas in others these secretions are produced by two or more secretory cell types. Duo-gland adhesive systems involved in temporary adhesion present an additional level of complexity as they also exhibit de-adhesive secretory cells. Yet, strikingly similar cellular organizations have been reported in highly disparate species. In the case of biological suction organs, regions of the organ that contact the substratum are highly textured with stiff microstructures. Although clearly non-homologous in different animals, these microstructures are thought to enhance friction on rough surfaces. At the molecular level, proteins are the main organic constituent of adhesive secretions in aquatic animals. We compared the global amino acid compositions of bioadhesives using principal component analysis to show that homologous adhesives from phylogenetically related species cluster together, and there is little overlap between taxonomic groups. However, several non-permanent adhesives are grouped together even though they belong to disparate phyla, indicating convergence in amino acid composition. We also investigated relatedness among individual adhesive proteins using a sequence similarity-based clustering analysis. While many proteins appear taxon-specific, some have clear sequence homologies based on shared protein domains between phylogenetically distant organisms. However, it is highly probable that these domains, which are also present in many non-adhesive proteins, were convergently acquired from ancestral proteins with unrelated general functions. We herein present morphological, structural, and molecular convergences between different attachment mechanisms in aquatic animals that likely arose in response to shared functional and selective pressures.
In contrast to most mesopelagic bioluminescent organisms specialised in the emission and reception of blue light, the planktonic annelid Tomopteris helgolandica produces yellow light. This unusual feature has long been suggested to serve for intraspecific communication. Yet, this virtually admitted hypothesis has never been tested. In this behavioural study of spectral colour sensitivity, we first present an illustrated repertoire of the postures and action patterns described by captive specimens. Then video tracking and motion analysis are used to quantify the behavioural responses of singled out worms to photic stimuli imitating intraspecific (yellow) or interspecific (blue) bioluminescent signals. We show the ability of T. helgolandica to react and to contrast its responses to bioluminescent-like blue and yellow light signals. In particular, the attractive effect of yellow light and the variation of angular velocity observed according to the pattern of yellow stimuli (flashes versus glows) support the intraspecific communication hypothesis. However, given the behavioural patterns of T. helgolandica, including mechanically induced light emission, the possibility that bioluminescence may be part of escape/defence responses to predation, should remain an open question.
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