A Visit Under the Sea with a Master Mimic
Looking back on my previous seasons of stories, I realized that I had yet to include any stories on cephalopods (the name is Greek for ‘head foot’). This is unfortunate because they are a truly fascinating group and include octopi, squid, nautilus, and cuttlefish. They are all mollusks, which include large well-known groups such as bivalves (e.g. clams and oysters) and gastropods (snails and slugs). However, the muscular foot present in other mollusks has developed into a set of flexible arms or tentacles in cephalopods. The hard shell found in other mollusks is also absent with the exception of the six species of nautilus, which have remained relatively unchanged over millions of years and still possess the outer shell of more ancient extinct cephalopods.
The group is also well known for their intelligence. Their nervous system is incredibly complex for an invertebrate and they have a high brain-to-body-mass ratio along with highly developed senses including vision, gravity, and chemical sensory structures, although the majority cannot distinguish color. Some squids can even detect sounds. They have been known to escape their aquariums in captivity and enter others to feed on crabs before returning. Because of this intelligence, cephalopods have some animal research protections that are usually reserved for vertebrate species.
All cephalopods possess chromatophores, special cells that contain pigments and reflect light. They can use these to achieve impressive feats of camouflage, as shown in the video below on cuttlefish. These cells and structures give cephalopods an unequaled talent for mimicry and today we’ll be looking at one of their mimicry champions: the mimic octopus, Thaumoctopus mimicus. This species is relatively small, no more than 60 cm long from mantle to tentacle tip and about as wide as a pencil when splayed out. They can be found throughout the Indo-Pacific and prefer sandy bottoms less than 15 m deep, as these allow the best opportunity for it to blend in with its surroundings.
The examples from the three previous stories this season all mimicked one dangerous species or group. What sets the mimic octopus apart is the sheer number of model species that it can imitate. Its flexible body allows it to assume a variety of shapes and its ability to change the coloration of its skin means that it doesn’t have to stick to mimicking a single pattern.
The mimic octopus will tailor its mimicry to the predator that it encounters, often appearing as a venomous species or one that is a predator of the species it is attempting to avoid. These include venomous lionfish, genus Pterois, sea snakes, and jellyfish. They also will imitate flounder when moving and sessile (non-moving) organisms like sponges and tunicates when stationary.
In total, the mimic octopus has been observed imitating up to 15 species of local marine organisms. Many of these belong to widely divergent and not closely related groups. No other species is currently known to possess a mimicry repertoire like that of the mimic octopus. Most mimics only have a single species (or multiple species of similar appearance) as a model. The fact that Thaumoctopus mimicus is able to mimic so many, and choose the most appropriate form for the situation, is a testament to both its intelligence and the versatility of the cephalopod pigment and shape imitation. It also drastically increases the survival chances of any individual, despite the relatively open nature of its sandy bottom habitat.
Next week, we’ll be looking at another well-known mimic/model pair. I have chosen to wait to introduce them because of interesting developments in its mimicry classification. If you like my stories, please like, subscribe, and share them. If you have an idea for a topic for a future season, let me know and if I use it, I’ll give you a shout out in my introductory post.