Reproduction for most marine invertebrates is a game of odds: females release their unfertilised eggs into vast oceans and rely on co-released peptides or protein pheromones to tempt sperm towards their eggs. A few species, however, such as the common octopus, have decided to adopt a more mammalian approach and use internal fertilization. It makes sense – surely, in the confined space of the oviduct, at least some sperm should reach the egg by chance without the need for additional attractants to induce chemotaxis (movement towards a signal). However, Anna Di Cosmo from the University of Napoli Federico II, Italy, thought otherwise. She explains that during mating, male octopuses will deposit sperm into the oviduct of the female, but females aren't always ready with an egg and so the sperm will bury themselves into the lining of the oviducal glands. When a mature egg is released, the waiting sperm needs a kick-start to get moving again and Di Cosmo suspected that a chemoattractant similar to those released by free-spawning animals might be involved (p. 2229).
Di Cosmo and her team caught several female octopuses off the coast of Naples and collected their mature eggs. The team then homogenized the eggs and, using a form of chromatography, separated the mixture into fractions of different proteins. Each fraction was then tested for its ability to coax sperm, collected from the oviducal glands, into moving through a fine mesh from one side to the other. One fraction in particular enticed sperm movement and the team identified the attractant as a small 11 kDa protein that they called octopus sperm-attractant peptide (Octo-SAP).
The team further characterized Octo-SAP's properties and showed that chemotaxis occurred in a concentration-dependent manner, with more sperm moving when Octo-SAP was concentrated. Using a microscope to film the tiny movements, the team also showed that the sperm moved up the concentration gradient towards areas of high Octo-SAP concentration. Together, the results suggest that the sperm were using the attractant to home in on what they thought was an egg. So, it seems that chemoattraction isn't just for free-spawning animals after all.