Meeting Abstract
Glass sponges (Class Hexactinellida) are one of the most abundant filter feeders in the deep ocean. We present our hypothesis that their syncytial body construction arose to cope with the food-poor conditions in the deep sea. We also propose that once syncytial tissue arose in the glass sponge a coordination system using action potentials became possible. Glass sponges have managed to survive in the deep sea because they have low energetic costs. They have little tissue to maintain – the syncytial tissue is reduced to a thin veneer on a scaffolding of spicules. Thin syncytial tissue allows them to take advantage of passive flow for feeding, using ambient currents to draw more water through their body with no additional energy expenditure. However, becoming syncytial meant the loss of the contractile ability common to all other sponges, which is needed to clear canals of irritants or clogging. Since they could not contract effectively, syncytial sponges needed a way to avoid clogging altogether. What arose was an elaboration of what was likely an ancestral calcium-based action potential that each individual cell could do. This action potential, when propagated across the continuous membrane of the syncytium, could trigger an arrest of flagellar beating across the whole animal and thereby cease water flow through the canals of the aquiferous system. We propose that the evolution of action potentials as a coordination system occurred indirectly as glass sponges became specialized to live in the deep sea. Glass sponges present an interesting case of how an ‘alternative’ to nervous systems has evolved.
