Microplastics: Small Particles, Big Threat

You have probably heard that plastic pollution is becoming a big problem for the environment, and especially for the oceans. When a piece of plastic reaches the sea, the seawater and sunlight make it slowly fall apart into tiny plastic particles. These small particles are called microplastics and are even smaller than a ladybug—and sometimes not even visible. Scientists have found that many marine animals mistake these microplastics for food and eat them! But what about the animals in coral reefs? Many animals in reefs, including corals and giant clams, are fixed to the bottom of the sea and cannot move. So, they cannot escape from the microplastics that are literally “raining down” on them. Recently, we discovered that many of these coral reef animals are not only eating the microplastics, but the plastics can also stick on their bodies like flies on flypaper!

Diversity of Three Small Type’s Giant Clams and Their Associated Endosymbiotic Symbiodiniaceae at Hainan and Xisha Islands, South China Sea

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates, however, the diversity of the giant clams, as well as the diversity and distribution of Symbiodiniaceae in different Tridacnine species remain relatively poorly studied in the South China Sea. In this study, a total of 100 giant clams belonging to small type’s giant clams, Tridacna maxima, T. crocea, and T. noae, were collected from Hainan and Xisha Islands. Based on mtDNA cytochrome c oxidase subunit 1 gene (COI) and 16S rRNA fragments, T. maxima and T. crocea showed a closer phylogenetic relationship than T. noae. All the three species of giant clams hosted Symbiodiniaceae including genera Symbiodinium (formerly Clade A) and Cladocopium (formerly Clade C). Geographically, symbionts in Cladocopium are restricted to Xisha Islands, probably because Cladocopium prefers to inhabit in waters with higher mean temperatures. The endosymbiont specificity among the three giant clam species was also detected. T. noae and T. crocea are found to harbor Symbiodinium preferentially, compared with Cladocopium. These results could provide important information to understand various endosymbionts occurring in giant clams in the South China Sea.

First record of Tridacna crocea Lamarck, 1819 (Bivalvia, Cardiidae) from Patnanungan Island, Philippines

We present a new record and information on the distribution of the IUCN listed Tridacna crocea Lamarck, 1819 in the Philippines. The new record in Patnanungan Island extends the known distribution of this species by 80 km east of the nearest previously known occurrence. The collected specimens are found in shallow water at a depth of 3 m, exhibit a relatively small size, and showed the characteristic of completely burrowing its valves in coral substrates. DNA barcoding was also done, and the constructed phylogenetic tree demonstrated that the giant clams created a monophyletic group.  Tridacna crocea has a wide distribution and is relatively abundant throughout the Philippine reefs. We recommend updating the population status and stock assessment of giant clams in the country for local regulation and conservation management.

Giant clam growth in the Gulf of Aqaba is accelerated compared to fossil populations

The health of reef-building corals has declined due to climate change and pollution. However, less is known about whether giant clams, reef-dwelling bivalves with a photosymbiotic partnership similar to that found in reef-building corals, are also threatened by environmental degradation. To compare giant clam health against a prehistoric baseline, we collected fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red Sea. After calibrating daily/twice-daily growth lines from the outer shell layer, we determined that modern individuals of all three species ( Tridacna maxima , T. squamosa and T. squamosina ) grew faster than Holocene and Pleistocene specimens. Modern specimens also show median shell organic δ 15 N values 4.2‰ lower than fossil specimens, which we propose is most likely due to increased deposition of isotopically light nitrate aerosols in the modern era. Nitrate fertilization accelerates growth in cultured Tridacna , so nitrate aerosol deposition may contribute to faster growth in modern wild populations. Furthermore, colder winter temperatures and past summer monsoons may have depressed fossil giant clam growth. Giant clams can serve as sentinels of reef environmental change, both to determine their individual health and the health of the reefs they inhabit.

The Sparkling Tan: How Giant Clams Avoid Sunburns

As their name suggests, giant clams are among the biggest clams on earth, and they are very colorful animals that live in coral reefs. Giant clams get help from tiny organisms inside their mantles, the colorful part between their shells. These little helpers, tiny microalgae, can use sunlight and carbon dioxide to produce food, which they share with the clams. That is why these clams can grow so big! In return, the clams provide the microalgae with some nutrients. Although sunlight is very important for food production in these organisms, excessive sunlight can lead to a sunburn, like in humans. Therefore, the clams had to evolve their special sparkling tan, a natural sun protection that is a very effective way to protect themselves, and the microalgae inside their mantles, from too much sunlight and from getting a sunburn.