Invertebrates in the News #5 (follow up)

Last week, I told you that Terry Gosliner was blogging about his field trip in the Philippines where he is looking for nudibranchs (sea slugs).

Today, there is a new post about his adventures.

Update: Here are his final posts that wrap-up his expedition.

Invertebrates in the News #6 - DNA reveals the complex life cycle of the stalked jellyfishes

If I say jellyfish, you probably picture a Frisbee-like animal with long tentacles that swims gracefully in an open ocean.

However, there is a group of jellyfishes (the Stauromedusae) that do things a little differently. First, they don't swim but live attached to the bottom with a stalk, hence their common name: the stalked jellyfishes.

Second, they are thought to have a "simple" life cycle. Most jellyfishes
go through several stages before they reach adulthood. The typical
jellyfish starts life as a small polyp (something that looks like a sea anemone) attached to the ocean floor. This polyp buds small medusae (jellyfishes) that grow until they reach sexual maturity. At this point, they release gametes into the ocean. After fertilization, the egg forms a planula larva which will settle on the bottom to give rise to a new polyp.

In stalked jellyfishes, things were assumed to be a little different, but simple: the planula attaches to the bottom and metamorphoses into a polyp (so far everything is normal). The top of the polyp metamorphoses into something that looks like a jellyfish while the bottom part stays the same and still looks like a polyp; hence a stalked jellyfish. The only problem with this story is that it is based on only a few observations based on few species, and it's hard to know if all the Stauromedusae do the same thing.

Stauromedusae (stalked jellyfish) - Haliclystus antarcticus

There is another problem: how can you tell if the two stages of the life cycle that look very different belong to the same species? In the best case, you can observe the full life cycle in the field or in the lab, but it is not always possible to do so. In other cases, you can only observe part of the life cycle.

In 1996, Gerhard Jarms & Henry Tieman published a paper where they described a new species of hydrozoan polyps that live on clam shells of the Antarctic that they named Microhydrula limpsicola. They kept the animals in the lab for 4 years, and only observed asexual reproduction by frustules (larvae produced asexually).

In a recent paper, Lucilia Miranda and her colleagues demonstrated that Microhydrula limpsicola is just a stage in the life history of the stalked jellyfish Haliclystus antarcticus. They were able to connect these two life stages by discovering that the DNA of the polyps (what was thought to be M. limpsicola) and the stalked jellyfish (H. antarcticus) were identical.


The life cycle of the stalked jellyfishes is thus much more complicated than we thought.

Furthermore, Microhydrula limpsicola was not the only one of its kind. There are two other species that were thought to belong to this family (the Microhydrulidae). It will now be interesting to determine if they are also stages of other species of stalked jellyfishes.


Links

• Miranda LS, Collins AG, Marques AC (2010) Molecules Clarify a Cnidarian Life Cycle – The “Hydrozoan” Microhydrula limopsicola Is an Early Life Stage of the Staurozoan Haliclystus antarcticus. PLoS ONE 5(4): e10182. doi:10.1371/journal.pone.0010182
• Jarms G & Tiemann H (1996) On a new hydropolyp without tentacles, Microhydrula limopsicola n. sp., epibiotic on bivalve shells from the Antarctic. Sci. Mar. 60(1): 109-115  http://www.icm.csic.es/scimar/index.php/secId/6/IdArt/2742/ 

Credits

• Photo of jellyfish (top) François Michonneau. Lizard Island, February 2009. CC 3.0
• Photo of H. antarcticus and drawing of life cycle by Miranda et al 2010. CC 3.0
• Photo of the close up of a stauromedusa by Minette Layne CC 2.0 

Invertebrates in the News #5

Currently, the New York Times is hosting a blog series entitled "Scientists at work, notes from the field".

Today, Terry Gosliner from the California Academy of Sciences writes about new species of nudibranchs he found in the Philippines.


You can read his blog posts here.

Images by Terry Gosliner

Invertebrates in the news #4 - Speciation in reef hermit crabs

The journal Science has a section entitled "Editor's choice" where they feature recent papers that caught their attention. This week, they chose to highlight a paper written by Machel and Gustav that looks at the patterns of speciation in the genus of hermit crab Calcinus.

Calcinus lineapropodus (photo by Gustav Paulay)

By combining information about the genealogical relationships of 56 species (almost all the species known in this genus as well as 9 undescribed species) and information about the color of the species and where they live, they were able to discover some interesting facts about the evolution of this group.

• Closely related species have similar shapes but they can have very different color patterns. This means that color patterns evolve rapidly and that they can be used to tease species apart. This also suggests that the hermit crabs themselves use these color patterns to recognize the members of their own species. So, the apparition of new color patterns could lead to new species. To illustrate this rapid evolution in color patterns, compare these closely related species that live most of the time on branching corals: Calcinus minutus (from Guam), Calcinus rosaceus (from Oman) and Calcinus nitidus (from Moorea).

Calcinus minutus from Guam (photo by Gustav Paulay), Calcinus rosaceus (photo by Machel Malay), Calcinus nitidus (photo by Gustav Paulay)

• Isolated islands and archipelagos such as Hawaii have several endemic species of Calcinus, which suggests that the formation of new species (speciation) happened on the edges of their geographical ranges.

• Most species of Calcinus are found in oceanic areas in particular in the Western Pacific and in Polynesia. This is different from what is known for other marine invertebrates. Indeed, in corals, fishes, and various groups of mollusks, most of the diversity is found in a more continental area called the "coral triangle" (from northern Australia to Indonesia and Papua New Guinea). To illustrate this difference, compare the 2 maps below. The first one shows the distribution of the diversity for the hermit crabs of the genus Calcinus whereas the map on the bottom is the same kind of map for the cowries. The unusual diversity pattern found in Calcinus highlights the importance of the ecological and historical processes characterizing each group of organism that have led to their current geographical distribution.

Distribution of the species richness of the genus Calcinus. Contours represent 4, 10, 13 and 17 species. (from Malay & Paulay 2010)

Distribution of the species richness of cowries. Orange to red colors represent high number of species (above 64), green to yellow colors represent intermediate number of species (between 40 to 64), light blue to dark blue represent low number of species (between 1 and 40). (from Paulay & Meyer 2006)

Links:

• Editor's choice. Science. 327 (26 March 2010): http://www.sciencemag.org/content/vol327/issue5973/twil.dtl#327/5973/1555-c
• Malay MC, Paulay G. 2010. Peripatric Speciation Drives Diversification and Distributional Pattern of Reef Hermit Crabs (Decapoda: Diogenidae: Calcinus). Evolution. 64-3:634-662. DOI: 10.1111/j.1558-5646.2009.00848.x
• Paulay G, Meyer C. Dispersal and divergence across the greatest ocean region : Do larvae matter ? Integrative and Comparative Biology. 2006;46(3):269-281. DOI: 10.1093/icb/icj027

Invertebrates in the news #3 - A new tree of life for the Arthropods

You may not have heard of the word "arthropod" but you are certainly familiar with at least some members of this group: the insects. While insects make arthropods the most diverse phylum, arthropods also contains many other groups. Arthropods can be tiny (fleas) or fairly large (lobsters). They live in a variety of habitats: from the polar waters (krill) to the top of the trees (beetles) through arid deserts (scorpions).

On top of this diversity, arthropods are also economically important. For instance, mosquitoes carry the parasite which causes malaria, bees pollinate many of the fruits we eat, and the fruit fly is a model organism for genetic and medical research. Understanding how different groups of arthropods are related helps understand their evolution.

Scientists have been trying for a long time to understand how the different groups of arthropods are related to each other using morphological characters. This is not an easy task because arthropods are an ancient group. They appeared some 550 million years ago and all the extant groups were formed at least 200 million years ago. This is plenty of time to accumulate morphological differences which may mask the true relationships among extant groups.

This week, the journal Nature published a study that used an unprecedented amount of information found in DNA to understand how the major groups of arthropods are related. The results elucidate some long-debated issues about the relationships among various groups of arthropods. I highlight here two main findings.

Simplified Arthropod phylogeny based on Regier et al (2010)

First, the authors confirm the results of previous DNA-based studies showing that the myriapods (the group which includes the centipedes and the millipedes) are not directly related to insects, and thus, that these two groups invaded land independently. It has been proposed that myriapods and insects were closely related because they both used special organs to breathe air. Furthermore, the myriapods are not directly related to the Chelicerata (spiders, scorpions, horseshoe crabs, mites, ticks) but belong to the Mandibulata (all the other arthropods).

A pycnogonid from Lizard Island, Australia

Second, the closest relatives to the insects are a group of rare arthropods that the authors grouped under the new name of Xenocarida ("strange shrimps"). This new group unites two classes of arthropods that have only been recently described. In particular, the Remipedia were described in the 1980's and are only known from a few places (the Bahamas, the Canary Islands, Mexico and Cuba) where they live in caves. This illustrates the issue of what is called "taxon sampling" when scientists try to infer the relationships among organisms. If the authors didn't include these groups, the conclusions of their study would have been different, and some other arthropod group would have been mistaken for being the closest relatives of insects. Furthermore, it also illustrates the importance of habitat conservation and field work to preserve and discover species that can help unraveling the tree of life.

Link to the study:

• Regier et al, 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature.

More blog articles about the study:

• The most ambitious arthropod phylogeny yet -- by Alex Wild at Myrmecos blog
• Blind Cousins to the Arthropod superstars -- by Carl Zimmer at The Loom

Invertebrates in the news #2 - David Liittschwager's images in National Geographic

For this second edition of invertebrates in the news, I chose to follow up on a previous post. In early December, Seabird reported on David Liittschwager's visit in Moorea. As announced at the time, the pictures he took during his visit are published in the February issue of National Geographic.

The photographs illustrate a feature article written by E. O. Wilson that emphasizes how important -- and yet little known -- are the small organisms that live in the soil, in the sea, around us. To unravel some of this diversity, David Liitschwager went to six different ecosystems around the globe, and carried with him a green metal-frame cube. At each location, he observed and took pictures of all the organisms he found in his one cubic foot cube. For each location, there is also a video showing how he proceeded.

If so many forms of life can fit into one cubic foot, it means that in order to fully appreciate the diversity of life one must also look closely for the smaller organisms.

Invertebrates in the news #1 - Bdelloid rotifers & Corynactis viridis

To keep you entertained about invertebrates when we are not in the field, I will write regularly about invertebrates that made the news recently. On the menu today: a summary of a story published today about rotifers and a video of a tiny sea anemone.

How do bdelloid rotifers do without sex?

There are less than 1% of all animal species that don't use sex at all to reproduce. And, for most of them it seems that they gave it up fairly recently. This suggests that, in the long run, species that do not use sex to reproduce end up extinct. By not reproducing sexually, species accumulate deleterious mutations, and cannot exchange mutations that could help them to adapt to changes in their environment -- such as new diseases.

Bdelloid rotifers are an exception, as it seems that they have been reproducing strictly asexually for tens of millions of years. A study published today in the journal Science by Wilson & Sherman sheds some light on the cause of this exception.

Bdelloid rotifers are microscopic animals that live in any kind of moist habitat all around the globe. In addition to their mode of reproduction, bdelloid rotifers are also exceptional in their ability to stay alive out of the water for up to 9 years (at any of their life stages). This ability might be one of the factors explaining why they do well without sex.

The authors of the study showed that staying out of the water for a few weeks reduced drastically the number of individuals killed by a pathogenic fungus. By staying dry for an extended period of time, they can get rid of the the fungus and reduce the selective pressure that these pathogens impose to these asexual rotifers.

More info on the ScienceNOW website and in the original paper.

Video of Corynactis viridis

Have a look at this video of this small sea anemone feeding on plankton. Note how it can change the shape of its tentacles and its mouth (in the center). This video has been filmed under fluorescent light and is played 1200 times faster.

'Corynactis viridis' from MORPHOLOGIC on Vimeo.