Correction: Active microrheology determines scale-dependent material properties of Chaetopterus mucus.

[This corrects the article DOI: 10.1371/journal.pone.0176732.].

Active microrheology determines scale-dependent material properties of Chaetopterus mucus.

We characterize the lengthscale-dependent rheological properties of mucus from the ubiquitous Chaetopterus marine worm. We use optically trapped probes (2-10 µm) to induce microscopic strains and measure the stress response as a function of oscillation amplitude. Our results show that viscoelastic properties are highly dependent on strain scale (l), indicating three distinct lengthscale-dependent regimes at l1 ≤4 µm, l2≈4-10 µm, and l3≥10 µm. While mucus response is similar to water for l1, suggesting that probes rarely contact the mucus mesh, the response for l2 is distinctly more viscous and independent of probe size, indicative of continuum mechanics. Only for l3 does the response match the macroscopic elasticity, likely due to additional stiffer constraints that strongly resist probe displacement. Our results suggest that, rather than a single lengthscale governing crossover from viscous to elastic, mucus responds as a hierarchical network with a loose biopolymer mesh coupled to a larger scaffold responsible for macroscopic gel-like mechanics.

Evidence from polychromatism and bioluminescence that the cosmopolitan ophiuroid Amphipholis squamata might not represent a unique taxon.

Individuals of the cosmopolitan ophiuroid Amphipholis squamata were collected from eight stations. Eleven colour varieties were described and their distribution was non-random among stations. This suggests that the varieties differ in ecophysiologic tolerance and that their geographical distribution is modulated by environmental conditions. Varieties also differed in bioluminescence. Contrary to kinetics, intensity of light production varied among co-occurring varieties, meaning that they have similar bioluminescent reactions but a different amount of bioluminescent reagent. Light intensity differed in absolute value among stations but the rank position of each variety relative to others remained constant from one station to another. The 'colour-bioluminescence' link appeared clearly fixed (the same level of bioluminescence for the same variety) and is suggested to be of genetic origin. The species 'A. squamata' may then be a mosaic of genetically different entities (the varieties) rather than a unique cosmopolitan taxonomic entity.

Alteration of bioluminescence in Amphipholis squamata (Ophiuroidea: Echinodermata) by heavy metals contamination: a field study.

The ophiuroid Amphipholis squamata (Echinodermata) is a bioluminescent species whose light production varies with physico-chemical parameters of the medium. Individuals collected in the bay of Portman along a gradient of heavy metal contamination show different patterns of light production: the ones from the highest contaminated area showing a bioluminescence weaker and slower than those from the lowest contaminated area. Individuals that were transferred for 3 days from the lowest to the highest contaminated area displayed a light production that became weaker and slower. It is suggested that the decrease of the bioluminescent capability due to heavy metal pollution could indirectly affect the ophiuroid ecological success (bioluminescence is associated with defense functions in ophiuroids.

Evidence of seasonal variation in bioluminescence of Amphipholis squamata (Ophiuroidea, Echinodermata): effects of environmental factors.

The bioluminescence of Amphipholis squamata was assessed from freshly collected individuals for 16 successive months, and from individuals maintained in the laboratory under various experimental conditions of salinity, temperature and photoperiodic regime. Field investigations showed that bioluminescence intensity and kinetics varied seasonally, with the light produced being brighter and faster in winter and summer. The seasonal variation was not correlated with changes of ambient salinity. However, it was correlated with changes in temperature, the luminescence being brighter and faster in coldest and warmest seasons, and with the changes of photoperiod, the luminescence being brighter and faster in seasons with shortest and longest day length. Laboratory investigations also demonstrated that luminescence was not affected by salinity conditions. Conversely, luminescence was affected by temperature, the light production being brighter and faster in warmer conditions (in agreement with field observations) and dimmer and slower in colder conditions (in disagreement with field observations). Light production was also affected by photoperiod since experimental changes of natural light:dark regime caused the bioluminescence to decrease. Considering that photoperiod guides the biology of A. squamata and that reproduction takes place during coldest months in the species, an endogenous factor of neurophysiological nature linked to the ophiuroid reproductive cycle is proposed to induce the luminescence to peak in winter. This was confirmed by the fact that seasonal variation of luminescence was different between adult and juveniles, the latter showing no winter peak of luminescence. It is suggested that the luminescence normally associated with defense could also be part of an intraspecific visual signal related to individuals aggregating for reproduction during winter.

Cytological changes during bioluminescence production in dissociated photocytes from the ophiuroid Amphipholis squamata (Echinodermata).

Bioluminescence in the ophiuroid Amphipholis squamata is produced by photocytes located within the spinal ganglia of arm spines. Ganglionic cells were dissociated (pronase digestion) and photocytes separated from other cell types by using a continuous density Percoll gradient. Aliquots from a stock suspension of photocytes in artificial sea water were stimulated to produce light by using KCl or acetylcholine and fixed for ultrastructural observation at different times of the luminous process. Preluminescent, luminescent, and postluminescent photocytes contained various intracytoplasmic structures, such as Golgi, flat and distended rough endoplasmic reticulum cisternae, bundles of fibrils, and up to six types of membrane-bounded vesicles. These structures either co-occurred or succeeded one another during the process of light production, indicating that they were most probably participating in the luminescence reaction. Two types of vesicles, sharing some ultrastructural features, probably represented the microsources of the photocytes. One type occurred almost exclusively in luminescent photocytes, and the other almost exclusively in postluminescent photocytes, suggesting that one may be transformed into the other. The latter type of vesicle contained densely packed fibro-tubular units, giving a characteristic paracrystalline appearance to postluminescent photocytes.

Symbioses in Amphipholis squamata (Echinodermata, Ophiuroidea, Amphiuridae): geographical variation of infestation and effect of symbionts on the host's light production.

Populations of the polychromatic and bioluminescent species Amphipholis squamata from eight locations were examined for internal and external symbionts. At three locations (two in the United Kingdom and one in Papua New Guinea), no symbionts were present, while four species were recovered from the remaining locations: Cancerilla tubulata and Parachordeumium amphiurae (copepods), Rhopalura ophiocomae (orthonectid) and an undescribed species of rhabdocoel turbellarian. No ophiuroid individual hosted more than one symbiont species, despite the presence of two or more within a population. Symbiont presence and prevalence varied with location, and with colour variety, but with no apparent pattern or trends. Light-production characteristics of the host were affected by the presence of all symbionts except C. tubulata. These effects, however, did not vary between colour varieties or between geographical locations, but were specific to the symbiont species: the presence of P. amphiurae resulted in enhanced intensity of light production, while that of R. ophiocomae and the turbellarian species resulted in reduced intensity. The kinetics of light production (time until maximum output) were altered only by the presence of the turbellarian. Changes in the light-production characteristics are discussed in relation to morphological, energetical and physiological effects of the symbioses.

Localization of S1- and S2-like immunoreactivity in the nervous system of the brittle star Amphipholis squamata (Delle Chiaje 1828).

The recent isolation and characterization of the SALMFanide neuropeptides S1 GFNSALMFamide; and S2 (SGPYSFNSGLTFamide) from the sea stars. Asterias rubens and Asterias forbesi have initiated numerous studies on their morphological localization and distribution within the phylum Echinodermata. It has been shown by immunocytochemistry and radioimmunoassay that these peptides are widely distributed in the nervous system of some asteroids, echinoids and ophiuroids. A physiological approach has also shown that S1 and S2 potentiate the luminescence of the small ophiuroid Amphipholis squamata. In the present study. S1- and S2-like immunoreactivity have been localized in A. squamata by immunocytochemistry on both wholemount preparation and histological sections. The results reveal a widespread neuronal distribution of S1-like immunoreactivity in the circumoral ring, radial nerve cord, and tube feet. S1-like immunoreactivity was found to be associated with axons and cell bodies in both the ectoneural and hyponeural components of the nervous. S2-like immunoreactivity was detected only in the ectoneural plenus of the circumoral ring and radial nerve cord.