One of the reasons I chose mollusks as a topic is to illustrate how diverse life on our own planet is. The mollusks are an insanely heterogeneous phylum of creatures and they have been successful around the globe for the last 540 million years (at least). Even today in the ultracompetitive Holocene world, mollusks thrive just about everywhere. To illustrate this point I am showcasing the enigmatic deep water octopus Stauroteuthis, of which two species are currently known. These octopuses are only found more than 700 meters underwater in the Atlantic Ocean. Although they are most common around 2 kilometers beneath the surface the creatures have been spotted as far as 4 kilometers down. The Stauroteuthids are small benthopelagic octopuses (they are free-swimming but live in close proximity to the ocean floor).
The Tree of Life Web Project gives us the following overview of Stauroteuthid morphology:
Stauroteuthids are peculiar, gelatinous cirrates with a mantle opening that forms a complete tube around the funnel. They also have peculiar gills and internal shells and a large web that is nearly equally developed between all arms. When observed from submersibles, this octopod commonly has its arms and web formed into a bell-shape (bell-shape posture). Sometimes when the octopod is disturbed, it will inflate the web and draw the arms together at their tips to form a “balloon” with the arms and web (balloon posture). These postures are thought to be involved in feeding and/or defense.
That is nearly all we know about them—it is difficult to study creatures which live 2 kilometers underwater. However I have left out the most exciting fact: the Stauroteuthids are bioluminescent. Certain muscle cells around the suckers have been replaced with photophores which allow the Stauroteuthid octopuses to light up their eight legs like plane runways. The purpose of this luminescence is unknown but it is believed to be for predatory purposes (the lights are thought to direct prey to the octopus’ beak). Possibly the lights also help the octopuses to find and communicate with mates.