Challenges

Cuttlefish Summer Project

Common cuttlefish (Image:Wikimedia Commons)

What are cuttlefish?

Cuttlefish are related to squids and octopuses – a group of molluscs known as cephalopods. They have a well-developed head, large eyes and mouths with beak-like jaws. They have eight arms plus two feeding tentacles around the mouth and a fin that runs around their body. Cuttlefish are extremely variable in colour, but are usually blackish-brown, mottled or striped.

Common cuttlefish are the largest found in UK seas and are fierce predators . They make light work of crabs, fish and even small cuttlefish! They can live in water up to 200 metres deep but come to shallow waters to breed in spring. Their eggs are dyed black with cuttlefish ink, giving them the name ‘sea grapes’.

Common cuttlefish eggs (Image from one of our Wild Beach sessions)

Cuttlefish can quickly change colour and texture to merge into the background and escape detection. This ability is also used to distract predators or attract mates. During spring and summer, males engage in spectacular displays to attract females, passing pulses of colour rapidly along their bodies.

About the summer project for Nature Bureau

The common cuttlefish is the most important commercial species in the northeast Atlantic, and the English Channel population accounts for most of the species abundance. Before the year 2000, this population reproduced from February to July at 2 years old. After reproduction, all cuttlefish quickly die, so the entire life cycle is 2 years. In recent decades, because of climate change and ocean warming, a large proportion of the Channel cuttlefish population (10-30%) have begun to reproduce and die at just I year old.

After death, the body decomposes or is eaten by scavengers and the cuttlebone, which is lighter than water, floats up to be washed ashore, sometimes far away from spawning sites. These cuttlebones do not remain on the shore for long periods of time, as seabirds eat them searching for calcium. Therefore, when found, a cuttlebone is likely to belong to a 1 or 2 year old Channel spawner.
We can use the cuttlebones to age the cephalopods and determine whether they are reproducing (and therefore dying) at a younger age due to global warming.

Aims

  1. To estimate actual proportion of cuttlefish that halved their lifecycle due to climate change.
  2. To get an idea of seasonal changes in the relative abundance of 1 year old spawners.
  3. To provide in situ materials to verify the size – related natural mortality of cuttlefish for developing a 2-stage model.

Instructions

A common cuttlefish cuttlebone should measure 160-180 mm in length (Image: MarLIN)

Cuttlebones
1. Common cuttlefish (Sepia officinalis)
2. Pink cuttlefish (Sepia orbygniana)
3. Elegant cuttlefish (Sepia elegans)

  1. Find an intact cuttlebone on the shore.
  2. Be certain that it is the cuttlebone of a common cuttlefish (the photo above will help you decide). Aside for the common cuttlefish (Sepia officinalis), cuttlebones from the pink cuttlefish (Sepia orbygniana) and the elegant cuttlefish (Sepia elegans) may be found, but they are much smaller, rare and should be ignored for this study.
  3. Once you have located a common cuttlefish bone, pick it up and measure its length with a ruler, and record the measurement as well as where you found the cuttlebone (e.g. the name of the beach/town).
  4. Then take a clear photo of the underside of the cuttlebone (the flat bottom as opposed to the rounded top), making sure it is in bright light and that the entire cuttlebone is visible in the photograph. The photograph will then be used by Athena to determine the age of the cuttlefish at death.
  5. Break the cuttlebone in two so that it is not measured again by somebody else (Celas staff are also carrying out this work).
  6. Email the measurements and their corresponding pictures to Athena (athena@naturebureau.co.uk), making it clear which measurement applies to which picture.

Fun facts about cuttlefish

  • They can change colour in 200 milliseconds, as fast as a human can blink. 
  • They have W-shaped pupils which are thought to help control the intensity of light entering the eye.
  • They have three hearts, two of which pump blood to its gills, while the third circulates blood around the body
  • They have blue blood, thanks to the copper-rich protein it contains known as haemocyanin, which transports oxygen around the body. Our blood is red due to its iron-rich haemaglobin, which does the same job.
  • They have a doughnut-shaped brain, boasting the largest brain-to-body ratio of all invertebrates.
  • They can squirt clouds of ink to create a smokescreen and escape predators.
  • Cuttlefish might dream. Recently, researchers filming European cuttlefish in the lab observed short periods during their sleep-like states where individuals would erratically change colour and body pattern, and make irregular eye and arm movements. This strange behaviour, which was not repeated when awake or during inactive periods of sleep wasn’t triggered by any external stimuli and was very different from normal cuttlefish activity. 

From BBC Wildlife magazine