Can anyone tell me the difference between cuttlefish and squid? For me, in some cases, it is extremely difficult, particularly the following genera: Sepiadarium, Euprymna, Heteroteuthis, Rossia or Sepioloidea, for example.
I'll post something shortly. These little bumble-bee-like squid are difficult to tell apart, unless you have a good look at certain key characters and character states, particularly the funnel- and mantle-locking cartilages, presence or absence of a gladius (one of the interesting things about Sepioloidea is that it lacks a shell), tentacular club sucker detail, and the extent of (or lack of) fusion of the dorsal surface of the mantle and head.
Here are a couple of pics of Sepioloidea (I see that your avatar is of Sepioloidea lineolata, the type species of the genus. The New Zealand species, one described (Sepioloidea pacifica) and two undescribed (new species) are dissimilar to lineolata in that they lack the 'eyebrows' (superocular cirrus) and vivid colour pattern. They have a distinctive funnel and mantle-locking apparatus, and also lack a gladius (shell/pen). I was supposed to describe these things years ago, but got distracted .... sigh. Maybe 2006 will be the year that I pull my finger out.
The fins on the whole animal pic are folded down either side the mantle, but they are (if flattened out) rather substantial in size, extending almost the length of the mantle. The other images are of the hectocotylus in each of the three species (a quick way to differentiate males at least; both males and females can be differentiated on tentacle club sucker morphology and size.