Showing posts with label chelonians. Show all posts
Showing posts with label chelonians. Show all posts

Monday, June 20, 2011

Aripo Savanna & its Herpetofauna

Trinidad’s Aripo Savanna complex of tropical grasslands, palm islands, marsh forest and moriche palms with numerous slow moving streams, ponds, and puddles. On June 18-19 the Herp Group from the Trinidad and Tobago field Naturalists Club surveyed the herpetofauna. The weather cooperated to a degree with occasional showers and thunderstorms and blistering heat, which creates a very nice sauna-like effect. The TTFNC-HC will summarize the results elsewhere, but here are a few of the highlights.
Rare creatures are sometimes sited on the savanna. Here is Graham White looking 
for them while well camoflaged.

Aripo Savnna 1. The largest remaining remanant of the savanna.

Stevland Charles and Edmund Charles inspect Marsh Forest Vegetation
A Marsh Forest Pond
An Aripo Savanna Sundew.
Leptodactylus fuscus, the most commonly seen and heard amphibian on the Savanna,
The Scorpion Mud Turtle, Kinosternon scorpioides, a savanna and marsh
 forest inhabitat.
The Trinidad Wood Turtle, or Galup, Rhinoclemmys punctularia, another Marsh
Forest - Savanna chelonian.

Predator & prey. The Horse Whip Snake, Oxybelis aeneus and 
 its prey, the Streaked Lizard, Gonatodes vittatus.
A male Hypsiboans punctata (Hylidae) that was calling from this leaf.
The poorly known microhylid frog, Elachistocleis surinamensis
 is quite common  in the Marsh Forest and at the forest edge.


One of the day groups, with Mike Rutherford examining a turtle (middle).

Friday, March 18, 2011

Confusing Names and Relationships for the Pond Turtles



Turtles are an old and complex group of animals with a confusing taxonomic history. The turtles often called "freshwater turtles" of the family Emydidae have been particularly troublesome. As it happens morphology, mtDNA, and nuclear DNA produce conflicting results for their relationships. In a new paper, Uwe Fritz and colleagues review the situation and make a recommendation on what names should be applied to some of the North American pond turtles, formerly placed in the genus Clemmys.

Duméril (1806) established the genus Emys for virtually all freshwater turtles known at the time. His genus contained more than 90 species which are now known to be scattered in multiple families representing many distinct turtle lineages (Chelidae, Chelydridae, Dermatemydidae, Emydidae, Geoemydidae, Kinosternidae, Pelomedusidae, Platysternidae, Podocnemididae, and Testudinidae). Boulenger (1889) limited the genus Emys to two species, the European Pond Turtle, Emys orbicularis and the North American Blanding's Turtle, Emys blandingii. This arrangement remained until 1957 when Loveridge and Williams transferred the Blanding's Turtle to the genus Emydoidea. Blanding's Turtles have a unique skull, neck and thoracic rib morphology which more closely resembles the Chicken Turtle, Deirochelys reticularia. Some other species previously in Emys were moved to Ritgen's genus Clemmys established in 1828. From the early 19th century Clemmys contained Old and New World freshwater turtles that were considered unspecilaized species lacking distinct morphological. The exception was Louis Agassiz, in 1857 he considered each of the New World species assigned to Clemmys as a representative of a distinct genera (Actinemys marmorata, Calemys muhlenbergii, Glyptemys insculpta, Nanemys guttata). Sam McDowell's 1964 osteological study revising the ‘aquatic Testudinidae’,  Restricted Clemmys to the four Nearctic species Clemmys guttata, C. insculpta, C. marmorata and C. muhlenbergii, while the remaining Old World species were transferred to the genera Mauremys and Sacalia. McDowell (1964) realized Old World and New World freshwater turtles represent highly distinct groups and placed all Old World species plus the extraterritorial Neotropical genus Rhinoclemmys in the subfamily Batagurinae and the New World species plus the Palaearctic genus Emys in the Emydinae. These two subfamilies constituted, along with land tortoises (Testudininae), the family Testudinidae in McDowell’s (1964) classification. This arrangement is the one that is retained to the present, except each of these groups is now treated as a full family and the name Geoemydidae replaced Bataguridae because of name priority. McDowell recognized the close relationship of the four Nearctic Clemmys species, with the box turtles of the genus Terrapene and the Old World Pond Turtle, Emys orbicularis. He placed all of them in the ‘Emys complex’. However he did not include the Blanding's Turtle, Emydoidea blandingii. Instead he placed it with the distinct Chicken Turtle in the ‘Deirochelys complex’ (Emydoidea blandingii + Deirochelys reticularia). In 1974 Bramble pointed out, the morphology of structures associated with the plastral hinge of Emydoidea argues rather for a close relationship of Emydoidea with Emys and Terrapene, and not with Deirochelys. The plastral hinge of Emys, Emydoidea and Terrapene consists of ligamentous tissue that allows for almost complete closure of the shell, a trait  better developed in Terrapene. Based upon this, Gaffney and Meylan concluded that Emys, Emydoidea and Terrapene represent a monophyletic group within the subfamily Emydinae (as opposed to the subfamily Deirochelyinae within the family Emydidae). The three genera share not only a plastral hinge, but also a divided scapula, a unique character among living turtles. The morphological similarity of these structures of Emydoidea, Emys and Terrapene was unique enough that Bramble (1974) concluded the plastral hinge could not have evolved more than once. The four Clemmys species lacked not only the plastral hinge, but also all of the complicated morphological structures associated with this character, and were considered to have a basal phylogenetic position within Emydinae, an assumption already assumed by McDowell  and Bramble. Thus, Gaffney and Meylan placed all other emydid genera (Chrysemys, Deirochelys, Graptemys, Malaclemys, Pseudemys, Trachemys) in another subfamily (Deirochelyinae) within the Emydidae. By the mid 1990's mitochondrial DNA was revolutionizing how we looked at evolutionary relationships and Bickham and colleagues presented data that Clemmys is paraphyletic with respect to all other genera of the subfamily Emydinae (Emys, Emydoidea, Terrapene), and that the Spotted Turtle (guttata) was sister to all other emydines. Thus, the Wood Turtle, C. insculpta and the Bog Turtle, C. muhlenbergii, formed the sister group to a major clade divided into a subclade with the European Pond Turtle, Emys orbicularisEmydoidea blandingii, and  C. marmorata, and another subclade with all studied Terrapene species as its sister group. Bickham and colleagues used evidence from morphology, behavior and life history, to show the hinged taxa nested within Clemmys species. This prompted Burke et al. in 1996 to expanding the genus Emys to include all emydine species except C. insculpta and C. muhlenbergii.

To add to the confusion nuclear genomic data produced conflicting results, depending on which genes were used. The Spotted Turtle, Clemmys guttata, showed up as the sister to ((Emydoidea + Emys) + Actinemys) + Terrapene or as the sister to Actinemys marmorata and these two species together are the sister group of (Emydoidea + Emys). Box turtles, Terrapene then appear to be the sister to (Actinemys marmorata + Clemmys guttata) + (Emydoidea + Emys). The contradictory branching patterns depends upon the selected loci and suggest a lineage sorting problem. Ignoring the unclear phylogenetic position of Actinemys marmorata, one recently proposed classification scheme placed Actinemys marmorata, Emydoidea blandingii, Emys orbicularis, and Emys trinacris (the Sicilian Pond Turtlein one genus (Emys), while another classification scheme treats Actinemys, Emydoidea, and Emys as distinct genera. Fritz et al. consider the inclusion of Actinemys in the same taxon as Emydoidea + Emys as unacceptable under a phylogenetic classification framework because of evidence for the non-monophyly of this clade. The genra Actinemys, Emydoidea, and Emys are morphologically distinct, and their differences exceed the differences that typically occur among species of the same genus. Thus they recommend continued usage of the distinct genera Actinemys, Emydoidea and Emys. To find the full text of this paper follow the link below.

Citation

Thursday, March 3, 2011

Turtle Size & The Environment


Hatchling Chrysemys picta. JCM
Biologists from the UCLA Division of Life Sciences have reported the first quantitative evidence for an evolutionary link between habitat and body size in turtles and tortoises.

The study, whose lead author is a high school student volunteer in the laboratory of UCLA evolutionary biologist Michael Alfaro, is currently available online in Biology Letters, a journal of the Royal Society. It will appear in a print edition later this year.

Turtles and tortoises, also called chelonians, represent a diverse group of reptiles that have been present on Earth for more than 200 million years. The 330 species of present-day chelonians can be found dwelling on remote islands, traveling across vast expanses of ocean, and living in desert and freshwater habitats on every major continent.

Even more surprising than the wide variety of places animals call home is the vast disparity in their body sizes. The largest chelonians weigh over 1,000 pounds and are more than 6 feet in length, while the smallest weigh just a few ounces and would easily fit in the palm of your hand.

Combining statistical computer modeling with genetic data and the fossil record, Alfaro, an associate professor of ecology and evolutionary biology, and his colleagues demonstrated that different environments have specific optimal body sizes for their chelonian inhabitants.

These researchers act as "evolutionary detectives," piecing together how the tremendous diversity in living chelonians today evolved from a common ancestor that lived millions of years ago. DNA sequences from modern chelonians provide important clues for determining the evolutionary path followed by their progenitors, said co-author Graham Slater, a National Science Foundation–funded UCLA postdoctoral scholar in ecology and evolutionary biology.

The results show a surprisingly strong statistical correlation between habitat change and significant adjustments in body size. Chelonians living in marine or island habitats have an optimal body size several times larger than their cousins on the mainland, said first author Alexander Jaffe, a high school student at Harvard–Westlake School in North Hollywood, Calif. Marine turtles have the largest optimal shell length (about 4.5 feet), followed by island tortoises (approximately 2.5 feet), while freshwater and mainland chelonians are several times smaller (roughly 1 foot).

Evolutionary biologist have long assumed there is a connection between habitat and body size in chelonians, but it was not possible until recently to show quantitative evidence for the relationship, Alfaro said.

Chelonians have had a special place in the history of evolutionary biology due to the attention given them in the writings of Charles Darwin, Alfaro said.

Giant island tortoises found in the Galapagos and Seychelles provide a classic example of "island gigantism," a well-observed phenomenon in which an island-dwelling species evolves to be much larger than its mainland counterparts. Because they provide uniquely isolated habitats, islands are regarded as natural experiments in evolutionary biology, according to Alfaro.

"Our study was focused on testing whether there was any evolutionary signal in support of the idea that being on islands allowed the tortoises to evolve large size," he said.

While it is clear that habitat is an important signal in the chelonian evolutionary tree, the specific ecological conditions that trigger the change in body size are more difficult to determine, Alfaro said.

One of the oldest groups of reptiles, marine chelonians such as early sea turtles might have fallen prey to giant seafaring Mesozoic reptiles, a situation which would make larger size a distinct advantage, Jaffe said. Larger size also plays a key role in maintaining body temperature and allowing for migration across considerable distances.

In the case of the giant tortoises, a larger body size gives them the ability to survive long periods without food, which may be necessary due to prolonged droughts that can occur in island habitats. Large body size also may allow giant tortoises to "raft" across vast expanses of ocean while going weeks without food, a feat documented through observations of giant tortoises with barnacle growth found on the mainland, Alfaro said.

"What is exceptional about chelonians is that they are one of the most distinctive groups of vertebrates, arose early in the history of terrestrial vertebrates, and persisted for a long time," Alfaro said. "Chelonians are good examples of evolutionary survivors."

The main goals of Alfaro's research group include studying the evolution of vertebrates and their subsequent diversity in shape, size and structure. This involves developing methods to identify time periods and locations on the tree of vertebrate life in which unusual amounts of species diversification have occurred, Alfaro said.

An 'incredible opportunity'

Jaffe, a senior at Harvard–Westlake School, started volunteering in Alfaro's laboratory when he was 16, after e-mailing Alfaro about his interest in conducting research. Jaffe spent almost 30 hours a week in the lab for two full summers and was able to turn his results into a first-authored paper — a feat rarely accomplished by high school students.

"Being part of this research group has been an incredible opportunity for me," Jaffe said. "I can't say how grateful I am. Not only did I learn the tools of the trade, especially in the lab, but also what it is like to start off with an abstract question and address it through data collection and interpretation."

Jaffe hopes to study biological sciences and pursue further research in college.

"Alexander was ready to take intellectual ownership of a project," Alfaro said. "In addition to being a very conscientious young scientist, Alexander really showed an interest in the questions that we are asking and in getting the data to answer those questions."

This research was federally funded by the National Science Foundation.

For more on Alfaro's research, visit his website at http://pandorasboxfish.squarespace.com.

Jaffe, A. J., G. J. Slater,  and M. E. Alfaro. 2011.  The evolution of island gigantism and body size variation in tortoises and turtles. Biology Letters doi: 10.1098/rsbl.2010.1084