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