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

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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)



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