Half of all the giant clam species are at risk of global extinction, with the largest,
Tridacna gigas, assessed as Critically Endangered, in an update published in October. The most recent IUCN Red List update now includes all the species of giant clams, but also reflects their precarious existence with staggering population declines due to overfishing.
Giant clams, once a popular resident in shallow-water tropical reefs, have suffered plunging population declines due to decades of unchecked harvesting of their meat and impressively massive shells. The threatened state of Tridacna gigas is well-known as this species is already protected by several conservation measures. The current status of all twelve giant clams was not understood until the recent update to the IUCN Red List for endangered species published at the end of October.
The IUCN Red List of Threatened Species is the world’s most comprehensive information source on global extinction risk. It is an important tool in the protection and conservation of individual species and global biodiversity by providing objective information and data that communicates threats, informs policy, influences resource allocation and shapes decision-making of conservation efforts worldwide. The most recent update to the list includes all twelve giant clam species. In addition to Tridacna gigas listed as Critically Endangered, three clam species were assessed as Endangered, two as Vulnerable, four as Least Concern, and two as Data Deficient. These Red List assessments come at a critical time for the ocean giants, which are currently the focus of new efforts for the protection under the US Endangered Species Act.
“The state of our giant clam species is alarming. It reflects a long history of exploitation that really hammered these long-lived animals,” said Prof Julia Sigwart, an expert on mollusc evolution and conservation at the Senckenberg Research Institute and Museum Frankfurt, Germany. “We hope the data included in these Red List assessments can fuel the urgency needed to take action to protect them.”
Tridacna gigas is not only the largest of all the giant clams, it is the largest living bivalve mollusc. This species can weigh over 200 kilograms and live for over 100 years. For decades, the population of this giant species has seen an alarming decline. Even though it has protection from multiple conservation efforts, its risk of global extinction increased by two levels since it was last assessed as Vulnerable in 1996. With an estimated 84% decline in population, Tridacna gigas is now assessed as Critically Endangered, a category of organisms considered to be facing extremely high risk of extinction in the wild.
Dr. Anne Helene Tandberg leads the Marine Invertebrate Red List Authority team, also in Senckenberg. She noted: “The Red List is an authoritative risk assessment of the potential for a species to go extinct. It is important that this information gets used to support conservation planning and action.”
Not all giant clams are giants. The smallest of the group is Tridacna crocea, measuring in at around 15 cm, and makes a home by drilling a hole into corals across the Indian and Pacific oceans. Unlike some of its larger cousins, Tridacnae crocea is still widespread, and was assessed as Least Concern because there is no strong evidence for major recent declines. This a good reminder that the majority of animals included on the IUCN Red List of Threatened Species are not, in fact, listed in any threatened category. Ongoing data collection for all species is important to understand population trends and changes over time.
As part of ongoing efforts by the Senckenberg Ocean Species Alliance to increase the number of assessed marine invertebrates on the IUCN Red List, the new Red List update included thirty-four assessments of marine invertebrates. In addition to the giant clams, twenty-two cold-water coral species were assessed. Despite being widespread and poorly understood, one deep sea coral is globally Vulnerable: the well-known deep-sea reef builder, Desmophyllum pertusum. Eight other coral species were assessed as Near Threatened, ten were Least Concern, and three were Data Deficient.
Dr Tandberg explained: “Right now, the assessment process for a species takes about a year from assessment to publication on the IUCN Red List, but for many species it is much, much longer. We want to serve as a resource – both to the IUCN Red List and to researchers looking to assess their species, to help streamline and speed up this process, ultimately making sure these important animals are better represented in conservation efforts.”