Identifying species is not as easy as it seems.
Animals are generally distinguished from one another based on differences in their appearances. Generally, organisms that look the same and share similar characteristics are named the same species, and organisms that look different are classified as different species. Though seemingly simple, identifying species based on what they look like is quite difficult and can be a somewhat subjective exercise.
For instance, in nature, animals belonging to a single species may have a slightly different visual appearance based on the place where they live— an effect we call phenotypic plasticity. In contrast, some species may appear to look the same, but are not the same species. This can be a problem when there is a lack of easily identifiable features by which to distinguish between organisms—a phenomenon termed cryptic speciation. Such variation in an organism’s visual appearance makes taxonomy difficult, but this can often be clarified with the use of DNA information.
Sponges are among the simplest, most ancient animals. Their simple shape leaves them with few features to differentiate between species, so sponge identification is notoriously difficult as a result. Some recent research myself and my colleagues completed at Victoria University of Wellington examining the relationship of ‘golf-ball’ sponges in New Zealand highlights this problem.
‘Golf-ball’ sponges are a group of sponges that, as their name implies, look like golf balls. They are found worldwide in tropical, temperate and polar waters, with a particularly high number of species found in temperate Australasian waters. In our recent study, we revealed cryptic speciation in the golf-ball species called Tethya burtoni using both morphological and molecular characteristics. We found that more than one species are likely a part of what is currently called one species, Tethya burtoni.
Morphology as a tool to identify species
Sponges have few traits that can be used to identify them, making it difficult to identify species based on visual features alone, so sponge taxonomy is based widely on microscopic spicules. Spicules are the skeletal structures found in sponges that are made of either silica or calcium carbonate. Sponge species are often identifiable by a unique set and arrangement of spicules.
In our study, we examined spicule size, form and arrangement in the sponge from a sample of Tethya burtoni and found no distinguishable differences between specimens. While the use of spicules is commonly used to identify different species of sponges, studies show that within a single species, spicule composition may change based on the surrounding environmental conditions. Closely related species have also been found to contain very different skeletons, which means the use of spicules may not be a sufficient enough method to separate species. Instead, we can employ other methods like DNA analysis to further explain similarities and differences between species.
Genetics as a tool to identify species
Comparing the genetic characteristics of different organisms allows helps infer their evolutionary relationships. Organisms that have a similar genetic makeup are closely related, whereas genetic characteristics vary more between more distantly related organisms. In our study, we used multiple genetic markers to confirm our morphological findings that indicated there was only one species Tethya burtoni. However, surprisingly, our genetic analysis revealed two different genetic groups within Tethya burtoni, indicating a higher diversity than previously thought. It is likely that what is currently referred to as Tethya burtoni consists of two species that are cryptic species.
What misidentification of species means for conservation
Though our study only focuses on one golf-ball sponge in New Zealand, it demonstrates that in nature sometimes there is more to a species than meets the eye. Biological conservation is concerned with cataloguing and protecting nature’s diversity. A species occupies a niche and plays a role in the environment, and generally speaking a higher diversity of species will result in a more resilient and healthier ecosystem. The correct identification of a species is therefore critical for management and conservation efforts from the individual to the ecosystem level.
Sponges play many key roles in the marine environment, particularly because of their water filtering ability. Some recent research findings show that sponges may play a significant role in future oceans, as research suggests sponges may be less susceptible to climate change than corals. Understanding how sponges evolve and understanding their overall diversity will better inform scientists of how sponges may function in the future, which may help us better understand how future oceans may look.
This work was supported by Victoria University of Wellington and conducted by Megan Shaffer, Professor James Bell and Professor Simon Davy. We acknowledge Dr Manuel Maldonado, Dr Daniel Leduc, Associate Professor Michael Gardner and Dr Tessa Bradford for their assistance with this research.
Featured image: Golf Ball Sponge, Tethya sp. Point Cooke Marine Sanctuary, Victoria. Julian Finn, Museums Victoria CC BY