IMHO that isn't entirely true...some of the rockpools around The Mount here in Tauranga heat up to around 28C nearly every day in summer and then cool back down at night, so the critters living in there are pretty hardy. I kept a local marine tank for about 7 months and had seahorses, pipefish, blennies, gobies, anenomes, crabs, shrimps and some schooling surface fish and they lived perfectly happily together with no chiller or temperature control.
Just my experience anyway........