Using genetic analyses, LMU geobiologists have shown that many sponge species in the Indo-Pacific are regionally unique.

The Indo-Pacific is the largest marine biogeographical region on Earth and a global center of marine biodiversity. Nevertheless, there are gaps remaining in our understanding of the diversity, distribution, and endemism of many animal groups in this vast area, especially in relation to sponges. This knowledge is crucial, however, to our ability to record and evaluate changes in biodiversity over the long term. By means of genetic analyses, a team led by LMU geobiologist Professor Gert Wörheide has now demonstrated that there are an unusually large number of endemic species of sponge that occur only in a certain region of the Indo-Pacific.

Sponges play a key role in marine ecosystems, particularly in coral reefs, where they perform important ecological functions. For example, they protect exposed areas from boring organisms through overgrowth, and they stabilize coral fragments in disturbed reefs and thus facilitate reef consolidation. Researching their diversity is difficult, as many species scarcely differ in external features and thus cannot be reliably identified in terms of morphology.

DNA barcodes for species identification

Genetic data in the form of so-called DNA barcodes permit more reliable identification. “For our study, we amalgamated and comprehensively evaluated the data from numerous DNA barcoding campaigns,” says Professor Dirk Erpenbeck, lead author of the study. The area investigated stretched from the Red Sea across the Indian Ocean to Polynesia. In this way, the researchers were able to analyze the largest molecular genetic dataset on coral reef sponges in the Indo-Pacific to date.

“Our findings demonstrate a very high degree of endemism,” says Erpenbeck. Although the researchers found faunistic overlaps between the Red Sea and the Persian Gulf, the populations in this area had little in common with other regions of the Western Indian Ocean. In addition, the researchers identified a clear biogeographical boundary between the Western Indian Ocean and the Central Indo-Pacific. The Polynesian sponge fauna also proved to be comparatively isolated.

Limited distribution

“It’s striking as well that the distribution of sponges differs clearly from that of other marine organism groups,” notes Erpenbeck. This is presumably related to their comparatively short free-swimming larval stage: Most sponges have short-lived planktonic larvae, which have to find a suitable place to settle within a few days. As such, it is difficult for them to disperse over large distances.

Moreover, suitable ‘stepping stones’ for sponge dispersal like continuous coral reef structures are often lacking, keeping the sponge populations geographically isolated. In the view of the researchers, historical geological separations – such as periods of low sea level – have also contributed to the long-term genetic differentiation and thus to the high endemism.

“Accordingly, our results are consistent with earlier studies which question the frequently postulated occurrence of cosmopolitan sponge species – that is, species with global distribution,” concludes Erpenbeck. “The pronounced regional endemism we found coincides with similar observations concerning other invertebrate marine animals and underscores the necessity of regionally differentiated biodiversity monitoring.”