All of the American Pacific Coast population is regarded as subspecies fimbriatum
. This species is often found aggregating together. They often reproduce asexually by pedal laceration, and are said to be capable of binary fission as well. Sexual reproduction also occurs. This species will fire its acontia when placed in contact with Anthopleura elegantissima
, the green aggregating anemone. Animals on the border of a clone often develop up to 19 "catch tentacles", which generally occur close to the mouth. These tentacles, which are lerger and more opaque than the other tentacles, have special nematocysts and are unusually extensible (they can become up to 12 cm long or more). They probe the area around the anemone. While they do not respond to food, they do fire when they contact either A. elegantissima
or another clone of M. senile
. When it fires, the tip of the tentacle breaks off and sticks to the victim, which may retract and bend away. Tissue damage can generally later be seen in the stung area, and the attacked individual may even die. The acontia do not sting the skin but could definitely sting the eyes or tongue.
Sperm in this species have wedge-shaped heads. Eggs are pink and about 0.1 mm diameter. Fertilization is external, and release of sperm into the water triggers release of eggs from females nearby. In California peak spawning is in August and September.
Diet is mostly small zooplankton, though they may also eat small benthic polychaetes, fish, and squid. A substantial portion of their carbon comes from kelp. so this must also compose a substantial part of their diet. Ricketts et al., 1985 mention observing one digest a chiton in 15 minutes. It can survive in brackish water, such as in San Francisco Bay. Predators include the nudibranch Aeolidia papillosa (on small individuals), and the seastars Hippasteria spinosa and Dermasterias imbricata, which can eat even quite large individuals. Attacked individuals may detach and drift to a new location. In Alaska, young king crabs (Paralithodes camtshaticus) often nestle among these anemones.