Building a complete picture of an organism

Scientists studying organisms living in the oceanic environment have to be detectives. They have to be able to build up data sets from information that is patchy and difficult to collect. They become adept at inferring information from comparisons with other deep sea and coastal organisms. Layer upon layer, each observation and each research cruise builds up pieces of the puzzle until scientists have enough material to begin to draw conclusions about aspects of an oceanic organism’s life history or how it fits into the bigger picture of the oceanic ecosystem.

In September 2006 a group of MAR-ECO cephalopod experts and other cephalopod researchers met at Bergen Museum for a workshop on the MAR-ECO cephalopod material. Read more about the workshop.

photograph taken by David Shale

Researcher Henk-Jan Hoving from the Netherlands was one of the scientists present. He was able to study the 46 Heteroteuthis dispar specimens in the collection and to propose some characteristics of the H. dispar reproductive strategies.

H. dispar are relatively small (up to around 7cm) pelagic deep water  cephalopods.They are believed to be a major component in the pelagic food web serving as prey for a number of top predators such as dolphins, tuna etc.

They are the most pelagic members of the bobtail squid family Sepiolidae, which contains the genus Heteroteuthis. Hoving states that many features of their reproductive strategies can be described as being adaptations to the oceanic lifestyle from more coastal or shallow water strategies; a process described as oceanisation.

samples of H. dispar collected during the MAR-ECO cruise of 2004 and curated at Bergen Museum

As a species evolves from being a coastal or shallow water species to being an oceanic species, it needs to adapt to the challenges posed by the different environment. Darkness, for one thing – below 200m there is effectively no sunlight. Many organisms have adapted to the darkness by using bioluminescence for communication, signaling, defense etc. H. dispar has photophores filled with a symbiotic bacteria that produce bioluminescence. Instead of releasing an ink-cloud when disturbed, H. dispar releases a luminescent cloud to confuse would-be predators.

As another adaptation to the dark, the bodies of hatchlings are very nearly transparent leading researchers to conclude that the eggs may be released into the water column rather that at the sea floor. However, the only H. dispar egg that has been collected was caught in a bottom trawl, suggesting that they might attach eggs to the bottom like other sepiolids! This underlines how little we know about this (and most) oceanic species and how much more study is needed!

Organisms in the oceanic environment tend to be more widely dispersed than they are in coastal or shallow waters and this affects their reproductive strategies. Data collected on H. dispar egg size frequencies suggested that growth and spawning are continuous in this species: an adaptation to the unstable environment in the oceanic environment with changing production conditions and patchy food distribution. Also their eggs are much smaller than those of other members of the same family.

Researchers from the cephalopod workshop - Henk-Jan Hoving is in the middle.