Sponges are very simple colonial animals with a spongy appearance and there are numerous different growth forms. Some are encrusting (breadcrumb sponge), others are branching. Sponges use tiny openings to draw water inside themselves and filter out suspended food, such as plankton. The water is pumped through their structure and exits through large openings in their surface called oscula. Sponges are very slow-growing and fragile and they are easily damaged or destroyed by objects hitting them; such as lobster pots, divers' fins and fishing line.