Tiny Fishes from the Deep – Rebecca Asch

Petri dish full of mesopelagic fishes caught in the MOCNESS.  Organisms marked with yellow stars are Cyclothone; fish marked with red stars are myctophids, and; the fish marked with green stars is a rather beat up hatchetfish.  Note that not all Cyclothone, myctophids, and hatchetfishes shown in this picture are marked.

Those of you who are regular readers of this blog will know that one of the key foci of this research cruise is to develop a greater understanding of how the biological pump functions.  The biological pump refers to a set of processes in which atmospheric CO2 enters the ocean and is taken up by phytoplankton during photosynthesis and then converted to organic matter.  If this organic matter is transported to depth quick enough, it will be “sequestered” there, meaning that it will stay at depth and won’t return to the surface ocean where the organic carbon can be remineralized and converted back into CO2, which can in turn contribute to global warming. 

It is hypothesized that certain fishes may contribute to the biological pump.  More specifically, the fishes that may affect this process are mesopelagic fishes that generally live at depths between 200-1,000 m.  Many of these fishes participate in diel vertical migration (DVM), in which they hang out at deeper depths during daylight hours but then travel to surface waters at night to feed on organisms found there.  By eating at the surface, but respiring and pooping at depth, these fishes may serve as a vector for carrying organic carbon to deeper into the water column. 

During this cruise, I’m conducting research to explore this hypothesis.  We are collecting mesopelagic fishes at both and night with a MOCNESS net.  MOCNESS stands for Multiple Opening/Closing Net and Environmental Sensing System.  The MOCNESS can collect zooplankton and fishes from 10 discrete depths during a single deployment, allowing us to better understand the depth range inhabited by particular organisms.  After the cruise, I plan to identify the fishes collected to species, dissect them removing their stomachs, and then measure the total organic carbon in their gut contents.  By doing this, we will be able to see how much of the food eaten by these fishes at the surface makes its way down to depth.  This will provide us with some initial information about these fish’s potential contribution to the biological pump.

Some of the mesopelagic fishes that we are sampling have a fairly cosmopolitan distribution and are found across wide swathes of the Pacific Ocean.  However, one notable pattern that we see at Station ALOHA is that the mesopelagic fishes found here are tinier than those in many other regions.  This is noteworthy because most mesopelagic fishes are not very big to begin with.  Typically, sizes are 2-10 cm, whereas on this cruise we are capturing fish at the smallest end of this range.  This reflects the fact that the area around Station ALOHA has relatively little nutrients.  When nutrients are scarce, smaller sized phytoplankton tend to be more competitive.  These small phytoplankton are eaten by small grazers, which are in turn consumed by small fishes.  As a result, the whole ecosystem is miniaturized.

Below are some “fun facts” about common fishes found in the mesopelagic realm of the ocean:

Cyclothone spp. These fishes, whose common name is the bristlemouths, are the most abundant type of vertebrate on planet Earth.  They have an elongate body and a large mouth relative to their body size.  Having large mouths is fairly common mesopelagic and deep-sea fishes.  This is because such fishes live in environments with relatively little food.  By having a large mouth, these fishes can capture food of a variety of sizes, allowing them to eat whatever sized meal they encounter. 

Another characteristic of Cyclothone that is shared among other mesopelagic fishes is that their body is covered with photophores.  Photopores are light producing cells.  Photophores can help fishes camouflage with their environment by mimicking downwelling light from the sun.  This is why Cyclothone and many other mesopelagic fishes have their photophores concentrated on their belly.  That way they won’t be easily seen by predators looking up at them from below.  Among some mesopelagic organisms, photophores can also be used to detect and capture prey and may be involved in attracting mates. 

Talking about mating, an interesting aspect of the life cycle of some fishes in the genus Cyclothone is that they are protandrous hermaphrodites.  That is a technical way of saying that these fishes start off life as males but, as they grow, they will switch sexes and become females.  This life history characteristic is common among many fishes, including clownfishes, which are the fish that Finding Nemo is modeled off of.

Myctophids. Myctophids are the second most common group of fishes that we have been sampling during this research cruise.  Their common name is the lanternfish, because, like Cyclothone, myctophids also have photophores that can produce bioluminescent light.  Myctophids are an incredibly diverse family of fishes, with ~250 species found worldwide.  Most myctophid species engage in DVM.  As a result, they possess a variety of adaptations to help them migrate between the mesopelagic ocean and the surface.  These adaptations include having a more muscular body than many other deep-sea fishes.  Their swim bladder is filled with fat to increase their buoyancy and reduce the energetic cost of migrating to the surface.  Lastly, they have adapted to tolerate the broad range of temperatures that they encounter along their migration route.  Although myctophids do not grow to be very big in size, there are fisheries that catch them in some parts of the world.

Hatchetfishes. We have caught a few hatchetfishes during this cruise, but they only seem to be captured in a few depths fished by the MOCNESS net.  This may reflect the fact that most hatchet fishes do not engage in DVM, so they may be more limited to inhabit a narrower range of depths.  Hatchetfishes have a particularly distinct body shape.  Their body is compressed side-to-side, but their body is rather deep when looked at dorsoventrally.  Their sides tend to be silvery and covered with photophores.  Their silvery color helps them blend in with their environment, making them harder for predators to detect.  Most hatchetfishes have a short gut, which is indicative of a carnivorous diet.  They are also characterized by upward pointing eyes and an upward pointing mouth, which allow them to better hunt organisms found above them in the water column.  Some fishes in the hatchetfish family have two sets of retina and lens in each eye, allowing to both look upward and sideways without moving their head. 

So far we have only deployed the MOCNESS net during the day.  We have a bunch of nighttime deployments coming up, so we will likely see new things in the coming days.  Stay tuned!

Me with the MOCNESS net in all of its glory.