Is that a squid or a cuttlefish? A simple, and often asked question for divers, developed into this blog post. The title could as well be: “The exception of the rule. How to confuse divers.”
Short answer if you’re in a rush: we’re talking about the Bigfin reef squid. A squid it is, but the big fin around it’s mantle makes it look like a cuttlefish!
The question how to tell a squid from a cuttlefish is in our case not easy to answer. I love nature for that reason!
One general rule is that a squid is more slender that a cuttlefish, and squids move faster. It is kind of hard to tell when you can’t compare both though. Our squid comes in schools, and they explore the reef very slowly when not troubled by snorkelers.
The other feature was already mentioned. Cuttlefish have a fin around the entire mantle, while squids have shorter, and sometimes triangle shaped fins, like the giant squid (Architeuthis dux). Our squid unfortunately has a cuttlefish-like fin.
The insider-tipp is to look them in the eyes. Squids have a round pupil, while cuttlefish have these awesomely weird W-shaped pupils. Here they really got me. My photos are taken from a jetty, way to far away to see the pupils. Once I found a name, Sepioteuthis lessoniana (again without any guarantee for my photo ID!), I got confused big times. Type the name plus eyes into your search engine, and Google will show you Bigfin reef squid eyes with W-shaped pupils. Perfect confusion Mr. and Mrs. Squid!
We can note a long fin and W-shaped pupils, and still we call it squid. Why is that?
The answer lies inside, so it is invisible to the diver and snorkeler. Cuttlefish have a cuttlebone, a buoyancy control device made of calcium carbonate. Squid on the other hand have a pen, or gladius, made of chitin, a polysaccharide. For the scientist under you, cuttlefish prefer inorganic chemistry, squid organic.
Entomologist were well aware of the confusion that our squid creates. The name Sepioteuthis is a combination of sepia and theutis, meaning they named the Bigfin reef squid Cuttlefish-squid! Easy, isn’t it?
It was described in 1831, and 13 times more under different names in the following 97 years. They have a wide distribution range, and are variable in color. It may be the case that you can call them different species, depending on which attributes you call valid reason for separation. Possibly the DNA is different. That’s called a cryptic species complex.
The last part of my title has its origin in Egypt. Our cuttlefish-squid migrated to the Mediterranean Sea! This was possible due to Ferdinand de Lesseps, the French diplomat who was involved in the construction of the Suez Canel. 38 years after its description, Sepioteuthis lessoniana was given a way from the Red Sea to the Mediterranean Sea. It is one of over one thousand Lessepsian migrants since then.
Last one: this squid has the fastest growth rate of all larger marine invertebrates, what makes them one of four species of cephalopods to be produced in mariculture soon.