Effects of particle aggregation and disaggregation on their inherent optical properties.

In many environments a large portion of particulate material is contained in aggregated particles; however, there is no validated framework to describe how aggregates in the ocean scatter light. Here we present the results of two experiments aiming to expose the role that aggregation plays in determining particle light scattering properties, especially in sediment-dominated coastal waters. First, in situ measurements of particle size distribution (PSD) and beam-attenuation were made with two laser particle sizing instruments (one equipped with a pump to subject the sample to aggregate-breaking shear), and measurements from the two treatments were compared. Second, clays were aggregated in the laboratory using salt, and observed over time by multiple instruments in order to examine the effects of aggregation and settling on spectral beam-attenuation and backscattering. Results indicate: (1) mass normalized attenuation and backscattering are only weakly sensitive to size changes due to aggregation in contrast to theory based on solid particles, (2) the spectral slope of beam-attenuation is indicative of changes in PSD but is complicated by instrument acceptance angle, and (3) the spectral shape of backscattering did not provide as clear a relationship with PSD as spectral beam attenuation, as is predicted by theory for solid spheres.

Infrastructure in the electric sense: admittance data from shark hydrogels.

Elasmobranchs (sharks, skates, and rays) possess an electrosensory system with an infrastructure of canals connecting the electrosensors to the environment. The electrosensors and canals are filled with a uniform hydrogel, but the gel's function has not yet been determined. We present electrical admittance spectra collected from the hydrogel from 0.05 to 100 kHz, covering the effective range of the electrosensors. We have taken samples of this gel, postmortem, from Triaenodon obesus and Carcharodon carcharias; for purposes of comparison, we have synthesized a series of collagen-based hydrogel samples. The shark hydrogels demonstrate suppressed admittance when compared to both seawater and collagen gels. In particular, collagen hydrogels with equivalent ion concentrations are roughly 2.5 times more polarizable than the shark samples. We conclude that the shark hydrogels strongly localize ionic species, and we discuss the implications for the related roles of the gel and the canals in the electric sense. The gel-filled canals appear better suited to fostering voltage differences along their length than to providing direct electrical contact to the seawater environment.

Thermoelectricity in natural and synthetic hydrogels.

We describe a technique for measuring a Seebeck effect in gels and present data for three systems. Notably distinct signals are obtained for gel originating in the electrosensitive organs of marine sharks, synthetic collagen-based gel, and as a control, seawater, the gels' solvent. Only the gel of sharks shows a reversible thermoelectric signal. The difference between gel samples and seawater simply confirms that gels suppress mass transport. The difference between synthetic gel and the gel of sharks shows that the charged polymers of the shark gel restrict mass transport much more successfully than the polymers of the collagen gel, and we submit that this sort of ion localization is key to the emergence of thermoelectricity in a gelatinous substance. We compare the properties of the natural gel to those of established thermoelectrics.