|Galapagos marine iguana is a hideous-looking creature, of a dirty black color, stupid, and sluggish in its movements. (Darwin 1845) Nothing could be a more characteristic of the tortured, black coastline of the Galapagos than the marine iguana (Amblyrhynchus cristatus). The only sea-going lizard in the world, it is found throughout the islands wherever there are suitable shores. It may be found in densities of well over 3,000 per kilometer of coastline, though usually they are less abundant than this.
The Galapagos marine iguana feeds almost entirely on algae (seaweed) that it finds between the tide-lines or below the sea's surface. Whether above or below the sea, the food consists of small red or green algae. It does not seem to be very choosy as to which species of algae it eats, except that it avoids the large brown seaweed Blossevillea. This is an indigestible species for the Galapagos marine iguana. The algae that the Galapagos marine iguana feeds on are not apparent, leading to remark:
As well as feeding on seaweed, these iguanas occasionally feed on the faeces of their own species and those of sea lions and red crabs. To cope with the high salt intake in its diet, the Galapagos marine iguana must excrete salt. It has the most effective salt glands of any reptile. These are located above the eye and are connected via a duct to the nostrils. The salt is then forcibly expelled by frequent sneezing. This looks like spitting and results in the iguanas frequently having salt encrusted heads.
Most Galapagos marine iguanas feed once a day, but some of the larger ones will feed only every second or third day. The pattern of daily activity is largely determined by the temperature and the state of the tide. The larger males will usually wait until the middle of the day when the sun is at its hottest and they have had time to warm up before they will swim offshore and start diving to feed. The rest of the population leave the colonies a little before the tide has reached its lowest ebb and feed on the shoreline.
The Galapagos marine iguanas eat until the tide again starts to wash over them. It is remarkable that two parts of the same population have such different activity cycles. This may well be a result of the difficulties of offshore feeding for smaller individuals which cannot store much heat or swim well. They thus cannot make use of the more abundant offshore food resource. These activity patterns were elucidated from a population on Santa Cruz Island and may not be typical of all other parts of the archipelago.