*4.1. Sea Urchin Models*

The test samples in these experiments were polysaccharides of three CRG types. They were dissolved in sea water at 50 ◦C, to the level of the initial concentrations, from 0.5 to 1.0 mg mL−1. Ech dissolved in 50% EtOH was used at the initial concentrations of 0.5 and 1 mg mL−1. Adult sea urchins *Strongylocentrotus intermedius* (collected in the Troitsa Bay (Peter the Great Bay, the Sea of Japan) during August–September 2017, at a depth of 5–10 m) were stored in an aquarium with a closed-filter system, at a seawater temperature of 20 ◦C and salinity of 32 ± 0.5‰. Pooling male and female gametes and eggs, fertilizations were performed according to the standard procedures described in References [37,38]. The quality of the isolated sperms and eggs was checked with fertilization, prior to experiment. The fertilization membrane was formed within 1–2 min, after insemination, in at least 95–99% of the eggs, under normal conditions.

#### *4.2. Sea Urchins Sperm Cell Toxicity Test (SUSCT Test)*

We used the standard bioassay record of the SUSCT test for the analysis of the obtained preparations [23,24]. Sperm from sea urchins (15 × <sup>10</sup><sup>6</sup> cell mL−1), prior to the experiment, were sustained in seawater for 30 min, with various concentrations of the substances. Next, the spermatozoa were added to the suspension of eggs (2.5 × 103 cell mL−1). The final sperm:egg ratios were about 300 and 150:1. After 15 min, the percentage of fertilized eggs were counted on a Motic AE 21 inverted microscope (Xiamen, China). The experiments were carried out in triplicate using a 12-well plate. The effect of substance toxicity was assessed, visually, according to the number of unfertilized eggs in the four fields of view, in each experiment. The number of eggs fertilized by the sperms, after incubation in the seawater, without substances (control), was assumed to be 95–100%.
