Cephalopods, the group of molluscs that includes octopuses, cuttlefish, squids, ammonites, nautiluses and belemnites, are a weird bunch. Not only are they strange when anatomically compared to their shelled relatives like bivalves, snails and chitons but their evolution, physiology and behaviour makes them almost as interesting as vertebrates (I’m kidding, they’re way more interesting).

Despite there only being around 700 living species of cephalopods, biologically, they have evolved an array of adaptations that modern science is still only just unpicking. Neurologically, they are head (and shoulders if they had them) above all other invertebrate animals, sometimes called honourary vertebrates for their cognitive ability and potential conciousness. They are famed for their ability to change colour, shape and size. Many of them are fast growing but short lived. They have adapted to live in the cold depths of the ocean, warm shallows and some species even “fly”. In terms of diversity, cephalopods include the egg case making argonauts, shelled nautiluses, venomous blue-ringed octopuses and enigmatic giants like the giant and colossal squid. Their anatomy has widely inspired art and design and research on their nervous system has lead to breakthroughs in our understanding of how the neurology of all organisms functions. However, before most of this was experimentally and observationally discovered, they were perhaps best known for their “almost unique” ability to squirt ink when harangued, creating a smokescreen before jetting off to safety.

In some of the earliest surviving natural history accounts, this curious behaviour is noted. Writing in 350 B.C.E, Greek philosopher Aristotle notes that the cuttlefish employs its dark liquid for the sake of concealment, although he supposes that octopus and squid only do so out of fear. Some 400 years later that other famed early natural historian, Pliny the Elder, theorises that cuttlefish have ink, or a black fluid, instead of blood. Fast forward to now and what do we know about how cephalopods evolved ink and inking? Surprisingly, there’s still a lot we don’t know about this well known behaviour.

What is ink?

From anatomical studies of living cephalopods, we know that ink is generated, stored and evacuated from a specialised structure, the ink sac which includes the ink gland. The ink sac feeds into the rectum, controlled by a sphincter and in some inking events mucus from another organ, the funnel organ is ejected with water and ink through the anus and the siphon to create a cloud of ink.

Diagram showing dissection of northern shortfin squid, Illex illecebrosus. Black arrow shows location on ink sac. Modified from Verrill 1880. Photograph: Mark Carnall

You’ll be relieved to know that “squid ink” used in food for colouring or as an additive, normally cuttlefish ink, is prepared directly from the ink sac and doesn’t include the mucus part. Chemically, the mucus hasn’t been characterised, there’s much we don’t know about it. Ink from a few species has been studied but the contents have been shown to vary depending on the extraction technique. Generally, cephalopod ink includes melanin, enzymes related to melanin production, catecholamines, peptidoglycans, free amino acids and metals (Derby 2014) . Cephalopod ink and ink sacs have been processed for a variety of human applications including anti-microbial, immune response enhancing, anti-retroviral and potential anticancer drugs as well as ink for writing and painting. The most studied component of ink is melanin. Melanin is a natural pigment found across life, it is the pigment in human skin, hair and eyes and it gives ink its characteristic black or dark brown colour.



Why ink?

Since Aristotle’s observations, studies in the laboratory and field have expanded our knowledge of cephalopod’s inking repertoire. In addition to the clouds of ink created to limit vision and provide an escape route cephalopods can create different effects by changing the amount of ink released, the direction and speed with their flexible funnels and presumably varying mixes of ink and mucus. In combination with changing colour, some cephalopods have been observed creating pseudomorphs of ink, ejections which are interpreted to resemble a cephalopod-like form to would-be predators to confuse them. Another form of longer thinner streams of ink are called ropes and are speculatively assumed to bear resemblance to stinging tentacles of jellyfish. Cuttlefish add ink to their eggs (Hanlon and Messenger 1996), presumably to help conceal them and the aptly named ‘fire-shooter squid’ Heteroteuthis dispar release luminous globs with their ink to create floating glowing blobs again presumed to create a distraction to predators (Bush and Robinson 2007).

Although some cephalopod inks have been studied chemically, there’s still a lot unknown about the bioactive function of ink when released in the wild. Experimentally, some ink has been shown to be unpalatable to fish (Wood et al. 2010) and observationally, ink can also function as an attractant to predators to give cephalopods a bit more time to escape. Mucus-rich ink is supposedly a dangerous or annoying substance that interferes with fish gills and some cephalopods react adversely to their own inkings in small containers or in the lab. The blue-ringed octopus Hapalochlaena lunulata has tetrodotoxin, the deadly toxin it also releases in a bite, in their ink but the concentrations and effect in inking are not known.

Model of Octopus inking Photograph: Mark Carnall

Who does and doesn’t ink?

Living species of the externally shelled nautiluses do not possess an ink sac. Of the “soft bodied” cephalopods, subclass Coleoidea, ink sacs are found in octopuses, squids and cuttlefish although it has been secondarily lost in some species. Notably, it is absent in the deep-sea octopus group Cirrina and the confusingly named octopus relative the vampire squid. In many groups it is reduced or vestigial including the ram’s horn squid and in some species of blue-ringed octopus. Surprisingly, considering how much we bang on here at Lost Worlds Revisited about preservation biases, ink sacs are found extensively in the fossil record, the earliest described by William Buckland in 1836. Fossils are particularly well described from the Carboniferous, Triassic, Jurassic and Cretaceous periods and have been found in the USA, England, Russia, Lebanon and Germany. Sites such as Lyme Regis in Dorset have particularly yielded number of Jurassic “squid” ink sacs and nodules (Doguzhaeva et al. 2004).

Although the extinct externally shelled cephalopods ammonoids have an extensive fossil record, their soft tissues are very poorly known and, like extinct and living nautiloids, they are largely presumed to not have possessed an ink sac. There is some inconclusive evidence that some ammonites may have possessed an ink sac, most recently tiny globules of possible ink remnants were described in Austrachyceras (Doguzhaeva et al. 2007).

Fossilised ink sacs are more conclusively known from the extinct “soft-bodied” Coleoidea cephalopods in groups Belemitida (including belemnites with bullet-like internal skeletons commonly found as fossils) and Phragmoteuthida as well as from squid, octopus and cuttlefish fossils. In fact the presence of an ink sac is a characteristic feature of this group. Ink is currently unknown from other extinct Coleoidea although this could be due to preservation bias or through secondary loss. Ink sacs have been found so well preserved in the fossil record that they were used in drawings as with one famous 1833 example from the Oxford University Museum of Natural History. The practice of grinding these fossilised ink sacs in order to produce ink has become something of a tradition with more recent examples of fossils drawn in their own ink from 2009 and 2016.

Fossil ink sac from Lyme Regis Photograph: Mark Carnall

The earliest ink sacs appear in the fossil record in the Carboniferous period around 330 million years ago in cephalopods such as Donovaniconus, Gordoniconus and Saundersites which show a mix of features from older and more modern groups and are placed in their own order, Donovaniconida (Doguzhaeva 2012). Some of this early evidence is preserved as microscopic globules but whole ink sacs do occur and resemble the same shape as found in modern cephalopods (Doguzhaeva et al. 2004, 2010).

Unfortunately, the physical and chemical changes to ink sacs as they decompose and fossilise normally means that the chemical signature of fossil ink sacs is not preserved, however, in 2012 one particular 160 million year old cephalopod ink sac made the headlines (well the science headlines) as it seemed to have escaped much modification before fossilisation and consequently provided a unique window into what the ink was composed of (Glass et al. 2012). Amazingly, even within the limitations of the analytic techniques at the time, it was found to contain the same form of melanin as found in modern cephalopods.

Evolution of ink?

Frustratingly, from their first appearance in the fossil record through to older fossilised examples, the presence and structure of cephalopod ink sacs doesn’t really shed any light on how and when cephalopods evolved this structure and presumably the associated inking behaviours along with it. Unexpectedly, given cephalopod ink’s strong association with how we characterise the group, there have not been many hypotheses put forward for how the ink sac evolved. One theory is that melanin, which is extremely efficient in dissipating UV radiation, was originally involved in protecting the eyes or skin of cephalopods from light damage (Derby 2014). Perhaps the excretion of excess melanin led to the development of a specific production chamber to generate it and BINGO! rectally situated bespoke ink sac (this is not how evolution works).

Unfortunately, this is one of those instances where the current fossil evidence and our tools and techniques for analysing them come up short. Irrespective of how the ink sac evolved cephalopods have possessed them for over 300 million years. As we’ve seen from the myriad of ways in which they use ink, with no doubt more ways to be discovered from observation, the production of cephalopod ink has been key to their success and survival in the ocean.

References



Bush, S.L. and Robison, B.H. 2007. Ink utilization by mesopelagic squid. Marine Biology. 152, 485–494.

Derby, C. D. 2014. Cephalopod Ink: Production, Chemistry, Functions and Applications. Marine Drugs, 12, 2700-2730.

Doguzhaeva, L.A., Mapes, R. H. and Mutvei, H. 2004. Occurrence of Ink in Palaeozoic and Mesozoic coleoids (Cephalopoda). Mitteilungen aus dem Geologisch-Palaontologischen Institut der Universitat Hamburg 88:145-155.

Doguzhaeva, L.A. 2012. The Original Composition of the Proostracum of an Early Sinemurian Belemnite from Belgium Deduced from Mode of Fossilisation and ultrastructure. Palaeontology, Vol. 55, Part 2, 249–260.

Doguzhaeva, L.A., Mapes, R. H. and Mutvei, H. 2010. Evolutionary patterns of Carboniferous coleoid cephalopods based on their diversity and morphological plasticity. In Tanabe et al. (eds.) Cephalopods - Present and Past. Tokai University Press, Tokyo, 171-180.



Doguzhaeva, L.A., Mapes, R. H. Summeseberger, H. and Mutvei, H. 2007. The Preservation of Body Tissues, Shell, and Mandibles in the Ceratitid Ammonoid Austratrachyceras (Late Triassic), Austria. In Landman, N.H. et al. (eds) Cephalopods Present and Past: New Insights and Fresh Perspectives. 221-238.

Glass, K. et al. 2012 Direct chemical evidence for eumelanin pigment from the Jurassic period. PNAS, Vol. 109, 26, 10218–10223.

Hanlon, R.T. and Messenger, J.B. 1996. Cephalopod Behaviour. Cambridge University Press.

Verrill, A.E. 1880. The Cephalpods of the North-eastern Coast of America Part II. The Smaller Cephalopods, Including the Squids and the Octopi, with other Allied Forms. From the Transactions of the Conneticut Academy of Sciences Volume V. 259-446.

Wood J.B., Maynard A., Lawlor A., Sawyer E.K., Simmons D., Pennoyer K.E. and Derby C.D. 2010. Caribbean reef squid, Sepioteuthis sepioidea, use ink as a defense against predatory French grunts, Haemulon flavolineatum. Journal of Experimental Marine Biology and Ecology. 338, 20–27

