Tag Archives: cnidaria

On the Hunt for Tiny Polyps

Two weeks ago I had the chance to go field-sampling on the research vessel Hans Brattström. The sampling this time was focused on a broad range of marine invertebrates ranging from Hydrozoans, Bryozoans, Polychaetes, Phoronids and Brachiopods. I was especially on the hunt for polyps of the family Hydractiniidae (Cnidaria: Hydrozoa) that grow preferably on shells of molluscs or hermit-crabs. I was happy to look for new specimens for NorHydro and my master’s project, especially since opportunities to go field-sampling have been rare due to the covid-19 restrictions. The area of Bergen has been sampled quite well for the NorHydro project, but I was especially looking for rare species or species that haven’t been sampled before.

The first sampling for NorHydro this season – and with great conditions! Picture Credit: Lara Beckmann

To collect hydractiniids, we took bottom samples using a triangular dredge and a grab sampler. When the dredge gets back on board, the sample gets sorted on a large table on deck. Then the detailed search begins, and every stone and cranny gets inspected. The polyps I was looking for can be tiny, ranging from less than 1 mm up to 8 mm. The substrates that they grow on vary in size and shape, it can be crabs, molluscs but also algae or stones, often not larger than a few centimeters. So it isn’t an easy task to find the polyps in a freshly collected sample. Luckily I found several conspicuous hermit crabs and also one snail that I took back to the museum. At first, I didn’t see the polyps – only under the microscope in the museum laboratory I was able to see that hydractiniid colonies were growing on the shells.

Video: A polyp colony of the species Podocoryna areolata (Family Hydractiniidae). The polyps were growing on the shell of a living mollusc, probably of the species Steromphala cineraria. Video Credit: Lara Beckmann

One colony of the species Podocoryna areolata was growing on the shell of a living mollusk. The mollusk provides a nice substrate because the movements of the snail provide the polyps with more opportunities to encounter food. Also, the colony is protected by the small wrinkles of the shells surface where the polyps can hide. The polyps of this species are super difficult to measure, but most are smaller than 0.5 mm. When disturbed, the polyps shrink to small blobs even smaller than this. When relaxed, they can extend a bit longer in size. Especially the tentacles reach out to get hold of any potential food that swims by, such as small crustaceans. This species releases medusae, which can frequently be found in the plankton in this area.

A single polyp of the same colony of Podocoryna areolata. Picture Credit: Lara Beckmann

On shells inhabited by hermit crabs of the species Pagurus bernhardus, I found several colonies of a yet unidentified species of the genus Podocoryna. This species is very commonly found as polyp almost along the entire Norwegian coast. I’m still studying the specimen to figure out the correct identification. Since there is a lot of confusion in the hydractiniid taxonomy, I need to combine genetic information and morphology to overcome the existing problems in their identification and naming. The colony was reproductive and medusa buds were growing on it. Interestingly the medusa of this species is rarely found in the plankton.

Polyps of the genus Podocoryna. On the right are parts of the grasping claws visible belonging to the hermit crab Pagurus bernhardus. Picture Credit: Lara Beckmann

All over the colony were medusa buds. These are growing medusae, which will be released in the water when they are mature. The medusae can do what the colony itself can’t: releasing eggs and sperm and thus reproduce sexually. Picture Credit: Lara Beckmann

Besides the polyps, I found several other organisms living with the colonies on the shells including Crustaceans, Nudibranchia, Foraminiferans and other hydroids. The shells provide a home for a diverse range of marine life and it resembles a tiny forest. But it is not all peace and harmony in there, the smallest amphipods were quickly munched by the Podocoryna polyps. Those, in turn, get eaten by nudibranchs, that crawl on the colonies and some species feed specifically on hydroid polyps.

Video: An amphipod that lives on top of the hermit crab shell, walking through the colony of Podocoryna polyps. Video Credit: Lara Beckmann


I didn’t find any more hydrozoan species that were interesting for NorHydro during the sampling trip (at least not while scanning with the bare eye). But, I want to show one more very common species around Bergen –Ectopleura larynx– just because it is such a nice-looking hydrozoan. It even was reproductive and released its larvae right into my petri-dish. The small bulbs that grow between the polyp tentacles contain the larvae, which are called actinula. They break free and swim around, swinging their tiny tentacles until they will settle on a piece of algae for example, and grow to a large colony again.

The species Ectopleura larynx is a common species at the Norwegian coast. On the left the released larvae, called actinula. On the right a polyp that usually grows in a large colonies with up to a hundred polyps. Picture Credit: Lara Beckmann


You want to learn more about hydrozoans and why it is important to study them? Read more about it in my blog article for Ecology for the Masses: link.

Also, keep up with the activities of NorHydro here in the blog, on the project’s facebook page  and in Twitter with the hashtag #NorHydro.

Research Internship – Francesco

In the last part of 2019 Francesco Golin collaborated with us as an intern in project NorHydro. Francesco is a student at the University of Algarve, where he is enrolled in the International Master of Science in Marine Biological Resources (IMBRSea). We asked him about his internship and this is what he told us:

During the 2019 autumn semester I joined Luis Martell and Aino Hosia in project NorHydro as a research intern. My research contribution was aimed at finding how many species of the hydrozoan genus Euphysa are present in Norwegian waters, and how to define them morphologically and genetically. Euphysa is a common genus with 22 accepted species, but many of them are not easy to tell apart from each other, which is why we decided to implement an integrative approach for species delimitation including morphological and molecular analyses.

Some of the species of Euphysa occurring in Norway. From left to right: Euphysa aurata, Euphysa flammea, and Euphysa sp

Working on board during the cruise

My first mission as an intern was collecting some samples of Euphysa and other gelatinous organisms. Luckily, the opportunity to do so presented itself during the student cruise associated to BIO325, a course in which I participated as part of my studies at UiB.

During this cruise I used a light table to spot the tiny jellyfishes brought on board by the Multinet, then I placed them on a Petri dish and took pictures of them with a camera attached to a stereomicroscope, before transferring them to an Eppendorf tube filled with ethanol.

All these elements (the pictures of each organism, the associated sampling data, and the samples themselves) are needed for species delimitation of hydromedusae. The pictures are used to compare the morphology of different individuals and to identify important diagnostic characters (unfortunately, ethanol-fixed jellyfish are not useful for morphological analysis), while the ethanol-preserved samples are used to obtain DNA sequences.

The light table used to spot the gelatinous zooplankton

Some siphonophore parts are very transparent, and thus they are some of the most difficult animals to spot in plankton samples.

The hydrozoan Aglantha digitale (left) was very abundant in all my samples. Other cnidarians, such as this anthozoan larva (right) were also present.

My second mission consisted on gathering the original descriptions of the different species of Euphysa. This information is necessary if we want to understand what makes each species different, and will come handy when analyzing the individuals and their pictures collected on the field. Talking about species boundaries, I had the opportunity to attend a course on “Molecular Species Delimitation” offered by the University Museum. In this course I learned how to perform the analysis of DNA sequences for species delimitation, using some common software (MEGA and R) for this purpose. These are important tools that will allow us to assess the diversity of Euphysa in Norway, and together with the morphological analyses these data will help us determine if new species have to be described.

Now the semester has ended and my internship is over. Nevertheless, I hope my help was meaningful, as I want to continue being a part of this research project in the future. I will keep myself updated with the changes in the taxonomy of Euphysa, so I’m sure I will be able to join NorHydro again when I’ll come back to Bergen!


Guest researcher: Eric

Eric, from the Federal University of ABC, visited the University Museum in November. We asked him about his time in Bergen examining some of the least common species of siphonophores in the collections and this is what he told us:

My name is Eric Nishiyama, and I am a PhD student from Brazil. The main focus of my research is the taxonomy and systematics of siphonophores, a peculiar group of hydrozoans (Cnidaria, Medusozoa) notorious for their colonial organization, being composed of several units called zooids. Each zooid has a specific function within the colony (such as locomotion, defense or reproduction) and cannot survive on its own.

Fig_1. I had the opportunity to examine both ethanol- and formalin-fixed material from the museum. For morphological analyses, specimens preserved in formalin are preferable because ethanol-fixed individuals are usually severely deformed due to shrinkage.

Understanding how zooids evolved could provide major insights on the evolution of coloniality, which is why I am looking at the morphology of the different types of zooids. In this sense, siphonophore specimens available at museum collections provide valuable information for visiting researchers such as myself.

During my short stay at the University Museum of Bergen in November, I was able to examine a few siphonophore samples deposited at the museum’s collections. By examining the specimens under a stereomicroscope, and using photography and image processing tools, I was able to gather a lot of information on the morphology of several species.

Fig_2. Documenting the morphology of the nectophores of Rudjakovia plicata (left) and Marrus or-thocanna (right) was particularly interesting because these species are not commonly found in museum collections.

Fig_3. Other ‘unusual’ siphonophores that I was able to examine were Crystallophyes amygdalina (left) and Heteropyramis maculata (right).

Fig_4. Some large nectophores of Clausophyes preserved in formalin.

The data obtained will allow me to score morphological characters for a phylogenetic analysis of the whole group, and hopefully will help me revise the group’s taxonomy.

– Eric

Door # 11: Animal rocks and flower animals

The phylum Cnidaria is a diverse group of animals united by their ability to synthesize a complex type of ‘stinging’ cells called cnidocytes, which they use to hunt for their prey. The more than 13 000 species of cnidarians come in many shapes and colors, from the familiar jellyfish and corals, to the less famous myxozoans, hydroids, and siphonophores (read some more about those here). Because cnidarians live and thrive in marine and freshwater environments all around the world, humans have become familiar with them since ancient times: they have been feared for their sting, worn as jewelry, or simply admired for their beauty.

Sea nettles (genus Chrysaora) and ‘terrestrial’ nettles (genus Urtica) belong to very different groups of organisms, but share their name because of their stinging abilities. In some languages, like Norwegian and Swedish, cnidarians are called “nettle animals” (nesledyr and nässeldjur, respectively). Photo: Luis Martell (left), Nannie Persson (right).

Despite this familiarity, the true nature of cnidarians long remained unclear to naturalists and non-professionals alike. Perhaps to a greater extent than any other large phylum in the animal kingdom, people have historically failed to recognize cnidarians as animals, or even as living beings. Early civilizations had some knowledge about corals, sea-anemones and large jellyfish, all of which were encountered frequently along the coasts, but although fishermen and sailors knew about the existence of coral reefs (the massive bodies of coral represented major hazards for navigation), the animals themselves were probably seen only as pieces of rock. Some of the sessile species of cnidarians with a hard skeleton were considered minerals until the second half of the 17th century, when the use of magnifying lenses and the invention of the microscope allowed scientists to realize that the stony coral fragments washed up on the shore were actually made up of small flower-like organisms.

With their tentacles surrounding a central disc, sea anemones (in the image a specimen of Aiptasia) look somewhat like submarine flowers. Their plant namesakes (for example the wood anemone Anemone nemorosa) are strictly terrestrial. Photo: Joan J. Soto-Àngel (left), Nannie Persson (right).

Historically, the most persistent confusion regarding the cnidarians has been with plants and algae. For more than 1 500 years, the immobile sea anemones, sea fans, and hydroids were thought to be strange marine flowers and were consequently studied by botanists, not zoologists. They grow attached to the substrate and many species die if detached, which left early naturalists in doubt as to whether they were plants or animals. Thus, the category ‘zoophytes’ (from Ancient Greek ζῷον, zoon, ‘animal’ and φυτόν, phytón, ‘plant’) was created for them. It was only in the first half of the eighteenth century when this view started to change, thanks to the observations of J. A. Peyssonnel and the work of botanist Bernard de Jussieu, who together managed to convince their colleagues about the animal nature of the zoophytes.

The ‘sea tomato’ (Actinia equina) is a common cnidarian along the Atlantic coasts of Europe. It may look like a tomato when it is not covered by water, but is not related to its vegetal look-alike. Photo: Nannie Persson

The flowers of submerged marine plants (like this Cymodocea nodosa) are usually not as colorful or conspicuous as sea anemones and corals. Photo: Joan J. Soto Àngel

Today we know more about these organisms and there are no longer doubts about their affiliation to the animal kingdom, although we can still see evidence of their botanical past in the names of several cnidarian groups. The word Cnidaria comes from the Greek word κνίδη (knídē, meaning ‘nettle’, referring to the plant genus Urtica), and was inspired by the stinging power of the plants. One of the largest groups of cnidarians, the Anthozoa (which includes the flower-looking sea anemones and corals) is aptly named with a word deriving from the ancient Greek roots for flower (antho-) and animal (-zoa). Because there are still many open questions in cnidarian biology, initiatives that chart the diversity of cnidarians (like the successful project HYPNO and the upcoming project NORHYDRO) are necessary to get to know more about the particularities of these interesting animals!

-Luis Martell and Nannie Persson 


Jussieu, B. de, 1742. Examen de quelques productions marines qui ont été mises au nombre des plantes, et qui sont l’ouvrage d’une sorte d’insecte de mer. Mem. Acad. Roy. Sci. Paris, 1742, 392.

Edwards, C. 1972. The history and state of the study of medusa and hydroids. Proc. R. S. E. (B). 73, 25: 247-257.