Red tide in New England waters is somewhat of a misnomer as it has no discernable red color.  However, the term is used to describe Harmful Alagal Blooms which occur worldwide and which can render shellfish which filter these algae to become toxic to warm blooded animals – such as humans.  Although not perfectly understood, we do know that the most common Red Tide occurs locally in Nauset Marsh during the spring months although not every year.  The algae which is the problem suddenly grows very quickly and since shellfish are filter feeders they consume it, and concentrate a toxic chemical in their tissue.  It does not affect the shellfish, but if a warm blooded animal were to eat the shellfish it has the potential to disrupt the central nervous system functions of heartbeat and breathing.  This phenomenon has evidently been happening for many years and was understood by the Native Americans so there is no clear link to pollution or other environmental degradation.  Testing of the shellfish is done on a weekly basis to insure the safety of harvesters, and the protocol for reopening an area affected by Red Tide is very conservative.  No known instances of Red Tide poisoning have occurred in Eastham although closures have been frequent in the past decade.  The 2005 season was marked with the widest extent and most intense levels of Red Tide throughout the Commonwealth since records have been kept.