Tag Archives: plankton

Beautiful butterflies of the sea

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Pteropods (or ‘sea butterflies’) are snails that are part of the zooplankton. As part of my PhD I am especially interested in shelled pteropods, the thecosomes (euthecosomes and pseudothecosomes; here I focus on euthecosomes). Also shell-less pteropods exist: gymnosomes. Pteropods have modified parts of their soft tissue to resemble paired swimming wings, using them to migrate vertically in the water column, coming towards the ocean surface at night. This is why we sample overnight. Most genera are epipelagic and are represented in our samples during the cruise, but a minority of species lives in deeper waters.

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Cuvierina cancapae (photographed by Alice Burridge)

Euthecosomes have amazingly diverse shell morphologies and ontogenies (development of an organism). Limacinidae (Limacina, Heliconoides, Thielea) have characteristic shells that coil sinistrally (anti-clockwise direction) and are generally small in size (0.5 – 4 mm), one exception is the deep water species Thielea helicoides (up to 15 mm). Shells of Cavoliniidae have all kinds of shapes and are generally bigger than Limacinidae shells. Styliola, Creseis and Hyalocylis have pointy shells throughout their lives, with Hyalocylis striata having remarkable ribs on its fragile shell. Also Clio species have pointy shells, but they have a much broader aperture. Cuvierina species have large bottle-shaped shells (up to 10 mm). Cavolinia, Diacavolinia and Diacria have roundish, complex shell morphologies, with some Diacria species having long spines (such as Diacria trispinosa). Diacria danae and Cuvierina species shed their pointy juvenile shells before reaching their adult morphologies.

Apart from sampling, counting and preserving pteropods, I have also tried to run some experiments with them, but they are difficult to keep alive for very long. During the incubation the water had to be cleared from mucus regularly. Pteropods feed by building mucus webs in which they entangle food. These webs can be several cm wide, much bigger than the animals themselves. Although they were hard to culture, it was fascinating to observe the different ways they swim through the water column. For example, Creseis virgula swims with its curved tip pointing in its horizontal swimming direction, while the bulky and roundish Cavolinia uncinata and Diacavolinia clumsily and seemingly less efficiently try to swim along. Cuvierina cancapae is, when unharmed from the sampling procedure, a very active swimmer, its bottle-shaped shell tilting back and forth. Their active swimming does not seem to be the most energy efficient in the universe of zooplankton.

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Cavolinia tridentata (photographed by Alice Burridge)

My most interesting catch during this cruise was a beautiful and big Cavolinia tridentata, probably a one-time catch. It took a while, but when it came alive in its jar it was too active to image using the microscope camera, and too big anyway (15 mm), so I tried to image it directly through the lens of the microscope. Quite some other scientists came over to gaze at this amazing creature. You could even hear it swim.

It only took us a few days to get through the tropics and now we are in the subtropics, which we are about to leave by October 23rd already. Still, every time the net comes up it is a surprise what’s in there. The transition to a new oceanographic province is noticeable in the changing species composition. This shows that biogeographic barriers in the ocean do exist but they are not hard boundaries. As such, the tropical zone is a barrier for some species, but a niche for others. Some pteropod species have returned in our samples after having been absent in the tropical zone, occurring in both the northern and the southern subtropics. I wonder what I will find even further down south, apart from more turbulent waters.

The ocean is filled with undescribed species

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Pleuromamma_2_copyrightAnd here is one of them. This beautiful reddish copepod is also in the genus Pleuromamma, and does not yet have a scientific name. I show this for those of you who don’t yet know that the ocean is a mysterious and relatively uncharacterized place, with lots of secrets left to uncover. Despite a decade of discovery in the international Census of Marine Life, there are still many marine species left to be discovered and described. For some faunal groups, the majority of the species present may be new to science. We tend to think of marine zooplankton as being relatively well studied, in comparison to the deep sea. However, even for surface zooplankton, I suspect that the majority of species may be undescribed in high diversity ocean regions, such as the subtropical gyres. Learning about this diversity is critically important to understanding how these communities function, and the role different species play in marine food webs.

Pleuromamma_1_copyrightThis beautiful Pleuromamma copepod is distinct from both P. abdominalis and P. quadrungulata, with which it co-occurs here in the equatorial region. It has long been noted that there is variation in the presence and degree of development of spines on the antennules of Pleuromamma copepods. In the photo, you can see two animals, oriented head to head, with the top animal lacking the strong spines on the antennules that are very visible in the animal below. It turns out that this variation is informative, and can be used to distinguish good species. Stay tuned for an upcoming publication from Junya Hirai (University of Tokyo) on the genetic lineages within the ubiquitous species P. abdominalis. I hope to find the time to establish a name for this organism, along with the many other undescribed lineages within this nominal species. Establishing a new name for a species is an opportunity to honor your predecessors and acknowledge the heroes that inspired your scientific journey. I hope to name at least one of these species P. frosti, in honor of Bruce Frost.

Keep in mind that even though we are now out in the middle of the Atlantic Ocean, you don’t have to go very far to find new species. You may even have some right in your backyard. For example, I know there are some undescribed copepods in the coral rubble right along the beach at Diamond Head, at one of the most popular surfing spots on Oahu (my backyard at home in Honolulu). There are probably also a myriad of small, but important, and unknown species lurking in the habitats around your home. Find them and be amazed!

How to catch plankton?

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Well… remember that plankton can range in size from things that are very, very small, like bacteria, to things that are really quite large, like giant jellyfishes? How we catch plankton depends quite a bit on what type of plankton we would like to catch. It is also important to know that things that are small tend to be very abundant in the ocean, and organisms become more rare as they increase in size. This pattern means that we can usually sample small volumes of water to study small plankton, and we need to sample very large volumes of water to find large plankton. If we focus just on the animals that are in the plankton (and not the smaller things), we usually sample them using a few different ways:

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Bongo nets

Using plankton nets. People have been collecting planktonic animals in nets for hundreds of years, and this still remains the most common way of sampling. There are lots of different types of nets, some very simple and some very complicated. Some have very fine mesh and are good for collecting small animals (e.g. copepod nauplii, see posts by Michelle), and some have very coarse mesh and are good for collecting things like shrimp, or small fishes, that really are good swimmers. We have to tow the net quite a bit faster to collect animals that are good swimmers, because they can feel the net coming and and are quite good at escaping!

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Cod end with plankton in a bucket

During the daytime, when there is sunlight on the ocean, they may also see the net coming, and be able to avoid getting caught. We are using two different types of nets on this cruise, and we are towing them in two different ways: (1) vertical tows of a 60 um mesh net to collect copepod nauplii (baby copepods), and (2) oblique tows of a bongo net (see picture), to filter larger volumes of seawater and collect larger animals in the upper ~ 350 m of the ocean. Oblique just means that we are towing the net at an angle (45 degrees), so the net is moving both along in the ocean horizontally as we are pulling it up towards the surface. We can filter more seawater this way, and collect animals that are relatively large and relatively rare. Once we get our nets back in, we rinse them and collect the plankton in our buckets (see picture of the collecting end on the net, which we call ‘cod end’). On this cruise we carefully split the catch of each net using a Folsom Plankton splitter (see picture of me in the lab splitting a sample).Erica Lab Plankton Splitter_cropped_1576small

Another way to catch plankton is by using the CTD Niskin rosette. Using the CTD rosette, we are able to collect and study plankton from particular depths in the ocean. This is important, because the ocean changes dramatically across depth, very much more so than across the surface of the sea (horizontally). Animals often like to live only at certain depths, and are very good at maintaining themselves where they would like to be in the water column. Most animal plankton are too rare to sample with a CTD rosette. We are using it to sample copepod nauplii, which are often very much more abundant than the adults.

There are yet other methods, which we don’t use on this cruise. For instance, while scuba-diving. Many planktonic animals are gelatinous, and are therefore very fragile and hard to collect without damaging them. Jellyfishes are one good example, but there are many other types of gelatinous plankton that you might not be familiar with, like ctenophores, or siphonophores (ever been stung by a portuguese man-o-war? that’s a siphonophore). In order to really see these animals as complete organisms, you need to collect them by hand.

Gooey yellow-brown plankton

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Yesterday was my first day of sampling on the cruise and it was a success. While we waited for the CTD rosette to come to the surface with my first set of samples, we decided to do the first naupliar-net tow. Naup_net_cropped_1557small

When the net came back to the surface, it didn’t seem like we had captured much material. But then we took a look at the collection reservoir, the “cod end”, which was clogged with thick, gooey yellow-brown plankton. It was so thick that I ended up having a really hard time separating out my nauplii from the rest of the larger plankton in the tow! We got our first look at live plankton on the cruise, and were able to get photos of some of these tiny critters under the microscope. After the net tow, the CTD came up to the surface and I went to work concentrating and collecting nauplii from the Niskin bottle samples. This took a long time, because I concentrate 10 Liters of seawater onto a single filter for each sample, and then repeat this for 4 depth features, with duplicate 10 Liter volumes for each feature.

There was some doubt as to whether 10 Liters would be enough seawater to get the necessary number of nauplii for my analyses. So early this morning I took a few minutes to look at how many nauplii I collected in these samples. Actually this is quite difficult to do on a ship. It requires a strong stomach and intense focus because it can be nauseating to sit at the microscope trying to count the animals in a small petri dish as they swish back and forth under the microscope due to the motion of the ship. Luckily I am quite good at it. As I had hoped, I found that I will have enough critters to do the barcoding analyses I planned, with roughly 40-90 nauplii in each 10 Liter sample.

Michelle_CTD_cropped_1556smallWith Day 1 of nauplii sampling complete, I look forward to seeing how these samples change as we move South, approaching the subtropics.

Copepod nauplii

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My research is all about copepod nauplii. These are the early developmental stages of copepods, which can be extremely abundant in the water column because a single adult copepod can lay numerous eggs per day. This abundance means they could be important consumers of small algae in the ocean, or as prey for larger organisms like fish larvae or other invertebrate predators. Nauplius_cropped

But, because copepod nauplii are nearly impossible to identify to species under a microscope, and are small enough that they are at the boundary between being able to sample with whole seawater and having to concentrate larger volumes of water, they are a challenge to study, and thus we don’t know very much about them. One of my primary goals on this voyage is to use targeted sampling with a CTD Niskin rosette to look at whether nauplii are concentrated at particular features within the water column. A CTD Niskin rosette is a common oceanographic tool that allows us to see how physical and chemical parameters of the water column like seawater temperature, salinity, or pH vary with depth or location in the ocean, and holds a ring of sampling bottles that we can close at particular depths and bring to the surface to analyze. To date there has been little description of naupliar depth distributions in the open ocean, and very little is known regarding species-specific distribution patterns. I am targeting my sampling on several features of the upper water column. For example, I am sampling in the upper region of the water column that is well-mixed by wind stress on the surface ocean, called the “mixed layer”, and also at the “deep chlorophyll maximum”, or a layer that includes the highest chlorophyll concentrations in the water column (within phytoplankton cells). MIchelle_naups

A second goal is to look at whether particular species or developmental stages of nauplii concentrate within these water column features using DNA barcoding methods to identify nauplii to species. In addition to the CTD samples, I am also collecting nauplii with a very fine mesh net, with a pore size of 0.06 mm in the upper 200 meters of the water column. These samples will allow me to have more nauplii to work with, to compare to my CTD collections (for nauplii) and to our sampling of the adult copepod populations (with a bongo net).