The mechanisms of chemotaxis differ among species (see Metz 1978; Eisenbach 2004), and chemotactic molecules are different even in closely related species. In sea urchins, sperm motility is acquired only after the sperm are spawned. As long as sperm cells are in the testes, they cannot move, because their internal pH is kept low (about pH 7.2) by the high concentrations of CO2 in the gonad. However, once sperm are spawned into seawater, their pH is elevated to about 7.6, resulting in the activation of the dynein ATPase. The splitting of ATP provides the energy for the flagella to wave, and the sperm begin swimming vigorously (Christen et al. 1982).
However, the ability to move does not provide the sperm with a direction. In echinoderms, direction is provided by small chemotactic peptides called sperm-activating peptides (SAPs). One such SAP is resact, a 14-amino acid peptide that has been isolated from the egg jelly of the sea urchin Arbacia punctulata (Ward et al. 1985). Resact diffuses readily from the egg jelly into seawater and has a profound effect at very low concentrations when added to a suspension of Arbacia sperm. When a drop of seawater containing Arbacia sperm is placed on a microscope slide, the sperm generally swim in circles about 50 μm in diameter. Within seconds after a small amount of resact is injected, sperm migrate into the region of the injection and congregate there. As resact diffuses from the area of injection, more sperm are recruited into the growing cluster.
Resact is specific for A. punctulata and does not attract sperm of other urchin species. (An analogous compound, speract, has been isolated from the purple sea urchin, Strongylocentrotus purpuratus.) A. punctulata sperm have receptors in their cell membranes that bind resact (Ramarao and Garbers 1985; Bentley et al. 1986). When the extracellular side of the receptor binds resact, it activates latent guanylyl cyclase in the cytoplasmic side of the receptor (FIGURE 1). Active guanylyl cyclase causes the sperm cell to produce more cyclic GMP (cGMP), a compound that activates a calcium channel in the cell membrane of the sperm tail, allowing the influx of Ca2+ from the seawater into the tail (Nishigaki et al. 2000; Wood et al. 2005). These sperm-specific calcium channels are encoded by CatSper genes—the same genes that control the direction of sperm migration in mice and humans (Seifert et al. 2014). The increases in cGMP and Ca2+ activate both the mitochondrial ATP-generating apparatus and the dynein ATPase that stimulates flagellar movement in the sperm (Shimomura et al. 1986; Cook and Babcock 1993). In addition, the sperm sense the SAP gradient by curving their tails, interspersing straight swimming with a “turn” to sense the environment (Guerrero et al. 2010). The binding of a single resact molecule may be enough to provide direction for the sperm, which swim up a concentration gradient of this compound until they reach the egg (Kaupp et al. 2003; Kirkman-Brown et al. 2003). Thus, resact functions as a sperm-attracting peptide as well as a sperm-activating peptide. (In some organisms, the functions of sperm attraction and sperm activation are performed by different compounds.)