Deep-sea cephalopods    
Cephalopod Behavior & Ecology
   
 
 

What defenses do deep-sea squid

have against predators?

Northern Elephant Seal

All images copyright Stephanie L Bush

Midwater squid arm autotomy

Octopoteuthis deletron is a medium-sized (to 17cm mantle length) deep-sea species which possesses the typical eight arms of squid, but lacks feeding tentacles. Individuals possess a variety of bioluminescent photophores, many of which are ventrally directed and therefore presumed to counter-illuminate the animal. Each arm is tipped with a photophore and disturbed animals may splay their arms, emitting asynchronous blue bioluminescent light pulses from these photophoress. This presumably startles a potential predator. Octopoteuthis deletron also autotomize arms, just as a lizard drops its tail so that this non-essential appendage is attacked and the animal escapes.  Several questions are being adressed: When are arms autotomized? How common is arm autotomy? Can O. deletron re-grow its arms and associated photophores? How has such a defense evolved and been maintained in the low-nutrient, slow lifestyle of deep sea? 

 

Behaving in the dark

Shallow-water cephalopods are well-known for their body patterning, consisting of changes in skin coloration, skin texture, body posture, and body movement. These are used in visual communication with other cephalopods, potential prey, and predators. Low light presumably limits these fforms of visual communication, so scientists reasoned that body patterning in species living in environments such as the deep sea would have limited body patterning. I made the first inbservations of a deep-sea squid species to test the hypothesis that deep-sea cephalopods have limited coloration, posturing, and movement. Observations of Octopoteuthis deletron indicate this species is capable of many pbody atterns, comparable in number to shallow-water squid species (Bush et al., in press).

 

Deep-sea squid ink release

Cephalopod ink release is presumed to function as a visual defense. A dense, mucous-bound pseudomorph ('false-body') that mimics the releaser or a smokescreen/cloud that blocks the releaser from view may confuse a predator while the cephalopod escapes. Unfortunately the 'obvious' visual effectiveness of shallow-water cephalopod ink release means empircal tests regarding the effectiveness against natural cephalopod predators have not been performed. Recently I documented ink release by numerous deep-sea squids, including ink release forms that had not been previously described (Bush & Robison, 2007). Why deep-sea squids release ink in the deep sea is currently unknown. Light levels rapidly decrease with water depth, therefore by 1000 m in even the clearest oceanic water no surface-derived light remains - for most of the ocean, this occurs considerably shallower than 1000 m. Researchers have suggested that deterrent or attractant chemicals may play a a role in ink release. Whether deep-sea squid ink release has a visual function, chemical function , or both remains to be determined. However, it is probably an effective defense. If not, deep-sea squids would likely have lost the ability to produce and/or release ink over evolutionary time, which has occurred for many deep-sea octopuses.

Videos of mesopelagic squid ink releases

 

Deep-sea squid ink chemistry

Ink is used in different ways by different species; it is released in different forms, at different depths, even has subtle differences in color. One way that ink may function as a defense for deep-sea squid is that it has important chemical properties. Ink may contain molecules that are distasteful to predators and/or attractive to predators. If ink is distasteful, and a predator mistakes an ink pseudomorph for a squid, it may then concentrate hunting efforts on another prey type. If ink is attractive, it may distract a the predator while the squid escapes unnoticed. I have collected ink from 15 species of deep-sea cephalopod and several shallow-dwelling species to compare their chemical composition using liquid chromatography-mass spectroscopy. This method separates molecules within a mixture by type, then gives the molecular mass of each, allowing comparisons of molecules present in inks from different species and may be used to focus on molecules of interest.

 

Organismal reactions to deep-sea squid ink

If we see a deep-sea squid release ink in response to another animal or watch midwater animals react to squid ink, we could better understand why squids release ink. Unfortunately, observations of species interactions in the deep sea are rare. To determine midwater animal reactions to squid ink, I have to create my own ‘interactions’. With the help of MBARI ROV pilots and technicians, I designed an ‘Ink Ejector’ to release collected deep-sea squid ink in the water column. Encounters with squid and fish allow us to observe whether animals behave differently when ink is released.