Cirrate octopods: The dumbo elephants of the deep sea.
Cirrate octopods, or as they are more commonly known, “Dumbo octopods” belong to the cirrna suborder of deep sea octopuses which are found at depths that range between 300 to 7000 metres. They gain the name Dumbo from the two fins that are attached to their mantles (heads), giving them a striking resemblance to Disney’s Dumbo the elephant. Cirrate octopods have other distinctive features such as; semi-gelatinous bell shaped bodies, small internal shells which acts as an attachment point for their robust fin muscles and a well developed umbrella-like webbing in between their tentacles (Figure 1).
So, there is know question that these octopods look unique, but does this uniqueness also apply to their behaviours?
How many cirrate octopods are there?
Before the behaviours of cirrate octopods are discussed a distinction needs to be made. Cirrate octopods aren’t a group of identical species, but are instead a collection of multiple species. In fact, 45 different species, which belong to 8 different genera have been described – with new discoveries being made every year. Cirrates can even be more broadly categorised into four distinct families, the Cirroteuthidae, Cirroctopodidae, Grimpoteuthidae and Opisthoteuthidae.
This is all very well and good I hear you ask, but how did the cirrates gain their more scientifically recognised name. Well, like most things in science the name came from the octopods most distinguishable feature – their paired finger like projections called cirri. These paired cirri span the entire length of each tentacle and are used in prey capture by forming feeding currents (Figure 2).
Just like Disney’s Dumbo the elephant, cirrate octopods are able to create lift by flapping their ear-like fins. Cirrate octopods have been observed to beat their ear-like fins 4 to 30 times a minute in perfectly timed symmetry (Figure 3). These fins aren’t simply flabby extensions of the mantle that flail around, they contain robust muscle fibres that are wrapped around a central cartilaginous core, which attaches to the internal shell providing support.
The fin swimming behaviour shown in the video below is the main locomotion (movement) behaviour used by cirrates and is thought to be especially important in juveniles as they have proportionally bigger fins (Figure 3). Fin locomotion is believed to be less energetically expensive when compared to jet propulsion, which is used by other shallow water octopods. In fact, jet propulsion has been ditched entirely by cirrate octopods as a method of transport.
This is because the decrease in predator prey detection distances that is associated with the low light levels of the deep sea favours slower less energetically expensive movements. However, a type of indirect jet propulsion caused by timed breathing motions of the mantle and fin flaps result in water being pumped in a similar way to jet propulsion. But this has been described as a by product of the main fin flapping locomotion.
Figure 3: As you can see from the above video the fins on the mantle undergo a precise yet graceful symmetrical flapping motion. Source.
Cirrate octopods have a rather novel if not a bizarre way of dealing with the presence of predators, called ballooning. Ballooning is characterised by the transformation of the octopus into a pumpkin shaped ball – this transformation takes around 15 seconds (Figure 4). As the tentacles are thrust outward the sections of the umbrella-like webbing begin to fill and expand with water, which enhances the animals size and volume.
This expansion of the umbrella-like webbing doesn’t occur all at once, as each section seems to expand independently, because some sections remain empty of water. This increase in size from a flat position to a more rounded position is thought to confuse and scare away any would be predators that bump into this confusing tactile (touch) based cue. Known predators of the cirrates are the patagonian toothfish (Dissostichus eleginoides), Cuvier’s beaked whales (Ziphius cavirostris) and blue sharks (Prionace glauca).
This defence mechanism isn’t observed in any other octopods, because they rely on chromatophores which work on visual cues. However cirrates lack chromatophores due to the lack of reflective light, and so are more dependant on the use of touch related cues for predator defence. The ballooning response not only dazes would be predators, but can also make for a relatively quick getaway. This is because peristaltic waves, which are a series of muscle contractions travel along the webbing, pumps water away from the octopus – propelling it safely away from a would be attacker.
A less peculiar and more ingenious behaviour that’s reliant on tactile cues used by cirrates to defend themselves against predators, is web inversion. The octopods pulls its arms and associated webbing over its mantle, which creates a protective barrier of flesh (Figure 5). However this leaves the oral side exposed, but this can be protected by resting on the sea bed.
Bioluminescence is yet another ingenious, but rarely observed behaviour in octopods regardless of depth. As only 2 out of the 46 total genera of octopods have species that are confirmed to be bioluminescent, and its specifically only found in breeding females.
Stauroteuthis syrtensis is the only known cirrate octopod to show bioluminescent behaviour. They are able to produce blue-green bioluminescence at a wave length of 470 nm from their modified sucker-like photophores via the chemical break down of luciferin. These photophores (light producing organ) span the entire length of the arms and can emit light for up to 5 minutes, but this light intensity has been shown to decreases with time. Individual photophores flash asynchronously producing a twinkling effect, which is believed to be used as a light lure to aid in prey capture.
So are cirrate (Dumbo) octopods unique?
The short answer, yes.
They can be considered unique in terms of their appearance, as they posses visually striking external features that other octopods lack. These are; finger-like cirri, photophore suckers and ear-like fins that give cirrates their distinctive Dumbo appearance. Cirrate octopods also possess undeniably unique behaviours that aren’t observed in any other octopods such as; fin flapping, ballooning and web inversion. Cirrates also have bioluminescent abilities which are seldom seen in other octopods.
So, the Dumbo octopod physical appearance not only looks unique, but allows them to posses a broad range of unique and novel behaviours adapted for deep sea living.