We’ve been catching a lot of slickheads. These soft-bodied soggy fish are very common in waters greater than 1000 m deep, down to about 2.5 km deep. As it is pitch black at these depths, they have no need for light organs to hide their silhouette. They get their common name from the slimy look of their head. They lack a swim bladder and make themselves as light as possible by having weak bones and watery flesh. They are generally found in small to large schools near (but not on) the seafloor. This lifestyle is called “benthopelagic” meaning “swimming near the bottom”. There is little known about these fishes. The few diet studies found they feed on shrimps, jellyfish, free-swimming sea squirts (“salps”) and comb jellies (“ctenophores”).
Another slickhead story (Rouleina attrita)
Everybody groans when a catch of slickheads comes on board. This is because the entire catch gets covered in slimy sticky sheets of black skin. Some slickhead species feed by swallowing large mouthfuls of sediment and simply dissolve out the nutritious bits: tiny invertebrates and the fine organic rain that settles on the surface of the seafloor (“detritus”). One group of slickheads (members of the genus Rouleina) seem to have a similar diet, but they collect it in a bizarre way, up in the water column. Scientists have noticed that the loose skin on these fishes is always split neatly along the top and bottom edges of the animal. Other scientists in submersibles have observed these fish hovering with sheets of shredded mucous hanging out their mouths. One suggestion is that the sticky skin traps small animals and detritus. It’s like having your whole body covered in double-sided tape. The fish then just takes off its snotty coat to eat, for all the things that have become stuck to it. This may explain why we always get covered in gooey loose skin when we catch these fishes.
We have been finding the small empty shells of two special snails in some of our bottom trawls. Both animals do not live on the seafloor but as they’ve died their shells have sunk. One is a floater and the other is a flapper. The Violet Snail lives right on the surface of the sea by creating a raft of mucous bubbles full of gulped air. This gives them a small life raft to cling on to. They swim around with this raft and feed by mowing into the tentacles of jellyfish that also live on the surface. Animals that live right on the surface (such as this snail, some jellyfishes, some sea slugs and insects known as marine striders) are collectively known as “neuston”. Violet snails even make a raft of bubbles for their eggs before setting them adrift. This gives the young a chance to gulp air bubbles to make their own raft when they hatch. Otherwise they sink straight to the bottom.
The other animal is a small snail known as a “pteropod” (sounds like “terror-pod”, meaning “winged foot”), which lives in a turtle-like shell. It sticks its head and wing-like limbs through gaps in the shell. It lives in mid-water and is one of the many animals that make up the “scattering layer” (see diary 26 May). It looks like a small bat as it flaps up towards the surface every night to feed. When both these open-ocean creatures die millions of their shells sink down to the ocean floor. In some places the whole seafloor is covered in their dead shells like a pebble beach.
Also known as “bugs” or “flapjacks”, these lobsters live in mud and sand. Last night we caught three in the ratcatcher at 340 m. Their shovel-like nose is used to dig under the sand, both for hiding and to find their main food, shellfish. They have strong front legs for pulling apart clams as well as strong crushing jaws. These sorts of lobsters occur down to about 400 m deep. They bury in the seafloor during the day and emerge at night to feed. Some species are commercially harvested, their tails being marketed as “bug tails”. Like other lobsters, the females carry the developing eggs underneath their tail. When the eggs hatch, the young are transparent and float in the plankton until they find a nice place to settle down.
This name doesn’t do this fish justice. One researcher onboard suggested a name like the Starburst Anglerfish would be more appropriate. This is the female of this strange anglerfish. She has very long fin rays, and hairy tubes all over her head. These tubes are known as “neuromasts” and are extensions of the sensory structures found in the lateral line system of most fishes. They must help her detect her prey, soon captured in her large toothy mouth. Less than 20 specimens have ever been collected of this species, only six in the entire Pacific and Indian oceans (in an area two thirds of the earth’s circumference!).
To show how rare many of these deep-sea fishes are, this species is considered the “common” member of the family! Others are only known from a single specimen. Like other anglerfishes, males are very different. They are small and have simple fins. In this species, the male latches on to the female and doesn’t let go. Their skin fuses and he stays as a permanent pimple with eyes, drinking blood and making sperm.
There is some confusion over how cucumberfishes get their name. They are meant to smell like fresh cucumber, but some researchers suggest that it is because of their similarity with a freshwater fish that looks the same, goes under the same name and it smells like fresh cucumber. We sniffed last night and certainly didn’t recognise any vegetables. These fish get up to 40 cm long and live at depths of around 150-400 m. They probably sit on the seafloor and rest on the tips of their fin rays, ambushing small invertebrates and fish that they spot with their huge eyes. This species was described last year. It is turbulent times in cucumberfish taxonomy. In 2000, eight species were recognised in the world of which one was reported from Australian and New Zealand waters. Since then the can of worms has been opened, with new species turning up in every museum collection. Martin Gomon from Museum Victoria, who is on board, works on these fishes. It now appears that there are at least seven species in this region alone, separated on features such as fin shapes and sizes, colour patterns, counts (rays, spines and vertebrae) and aspects of the scales. Structures in the skin on the underside of the body may be light organs, but they have never been studied. The gut of these fishes is lined with pitch-black skin that may help hide any glowing prey in their stomach.
These small lobsters are one of the strangest lobster groups. Rick Weber of Te Papa helped explain these animals. They live in perpetual darkness, which has led to them being blind. They only have small spines where the eyes would otherwise be. The family name, Polychelidae, refers to the fact that the tips of 8, or all 10, legs have a claw (also known as a “pincher”). The front pair is always greatly elongated and fragile, like fine tweezers, and may be used to locate and capture food. These lobsters live on the seafloor from hundreds to thousands of metres deep. The body (or “carapace”) of blind lobsters is oval from above but flat on top and is often covered with fine mud. Perhaps this helps camouflage the lobsters when light-emitting predators are nearby. As in other lobsters, the young are raised under the tail of the female. When they hatch they join the deep-sea plankton before changing into a small spherical juvenile form. They look so different from the adults that in the 1800’s they were thought to be different type of crustacean in their own family. Three of these juveniles and many adults have been caught so far on this voyage.
Last night’s Sherman sled found a dense aggregation of a small brittle star new to science. Tim O’Hara a brittle star expert from Museum Victoria describes these sorts of animals as remnants from ancient seas. Around 200 million years ago they were one of the most abundant groups on the seafloor of the world’s oceans, before the bony fishes started doing so well. Now dense brittle star aggregations are nowhere near as common and only occur where pressure from fish predators is low and nutrients high. This sort of brittle star probably filter feeds by holding out its arms and trapping passing detritus and tiny animals. Our site was near the top of a ridge and presumably the deep-sea currents sweep over the ridge, increasing the food available to the brittle stars. This particular species appears to be new to science, but is closely related to a brittle star from off the eastern coast of Tasmania (Ophiacantha fidelis), which also occurs in huge numbers on patches of sediment on the continental slope (400-500 m). The density of these beasts can reach 500 per square meter and they have been estimated to take up half of all the available space on the seafloor. One of the main predators of the Tasmanian species is a sea star of the genus Mediaster. The same probably happens here as several bright orange specimens of Mediaster were also collected by the Sherman sled.
Barnacles are very strange creatures. They are highly modified crustaceans, related to the shrimps. They start their life swimming around in the plankton like a normal baby shrimp. When they get ready to settle down they glue their backs onto a rock or coral and build a little castle of hard plates around them, complete with front doors. This is where they spend the rest of their lives, lying back sweeping net-like legs through the water to trap small animals and plants.
Barnacles are best known from the intertidal zone but they also occur elsewhere, including the deep sea. Here they tend to be stalked forms related to the goose barnacles that hang off floating objects at the sea surface. They attach to rocks or corals by a rubbery neck. Some have hard plates (photo) while others are soft with little or no hard plates, such as the large one we found in deep water growing on the hard skeleton of a large sponge (photo). In some barnacles, the males are tiny and settle down on the edge of the female’s shell to become a microscopic sperm factory.