PB Fact Files #1: Ruditapes philippinarum

Ruditapes philippinarum (Manila Clam / Asari Clam)

Domain: Eukaryota

Kingdom: Animalia

Phylum: Mollusca

Class: Bivalvia

Order: Venerida

Superfamily: Veneroidea

Family: Veneridae

Genus: Ruditapes

Welcome to the Project BioLogical Fact Files series, where we do a deep dive into an organism / chemical / virus. We hope that this series of articles allows you to gain a deeper appreciation about the biological diversity all around us, from the molecular to the organismal level. In our inaugural edition, we look at the humble Ruditapes philippinarum, also known as the Manila Clam or Asari Clam. This humble organism plays vital roles in both aquaculture and the greater ecosystem. However, it is precisely because of its great value to humans that it has been introduced to nonnative habits and cause damage to these ecosystems as an invasive species.

Introduction to the Taxonomic System

With the sheer diversity of organisms out there in nature, scientists adopt a classification system for convenience and clarity. The science of classifying organisms is also known as taxonomy. The Linnaean System of Classification, initially developed by Carolus Linneaus based on common characteristics between organisms, consists of a hierarchy of groupings. They are as follows:

1. Domain (least specific)

2. Kingdom

3. Phylum

4. Class

5. Order

6. Family

7. Genus (most specific)

Occasionally, scientists employ the use of subgroups in order to further highlight smaller groups of organisms with a similar trait, as in the case of Ruditapes philippinarum (superfamily Veneroidea).

Habitat

Ruditapes philippinarum are located relatively close to the coastline, hence are found in areas which receive large amounts of light.

The clam is native to the coasts of the Indian, Philippines and the Pacific Oceans. Large populations of these clams can be found in East Asian countries (Japan, South Korea and China), with the species being particularly associated with Japan. However, it has since been introduced (both accidentally and intentionally) and established in other coastal ecosystems across the world, including those of the United States, Canada and Western Europe.

Taxonomy & Anatomy

Ruditapes philippinarum belongs to the class Bivalvia (bivalves). These organisms have laterally compressed soft bodies that are protected by a two-part bilaterally symmetrical hinged shell. Each part of the hinged shell is called a valve, giving rise to the name bivalve (two valves). The two valves are joined by an elastic hinge ligament. Bivalves are special amongst other molluscs in the fact that they lack a radula, which is a minutely-toothed tongue-like structure that is used by other molluscs for feeding. They also have a pair of lateral cilla (small hairlike projections) called ctenidia that can be used for filter feeding and breathing, and a greatly reduced head with no eyes. Other than clams, other shellfish like scallops, mussels and oysters also belong to the class Bivalvia.

Organisms in the class Bivalvia belong to the phylum Mollusca (molluscs), making them invertebrates. This phylum accounts for around 23% of marine organisms that have been named, which includes Ruditapes philippinarum. Common characteristics of molluscs include having a soft body made out of muscle, a mantle that can secrete calcium-containing substances like calcium carbonate (CaCO3), and paired nerve cords. Other organisms in this phylum include cephalopods, like octopuses, and snails.

Ruditapes philippinarum has a retractable “foot” muscle that can extend outside of the shell for the clam to move or burrow. Hence, Ruditapes philippinarum is also called a burrowing clam. The clam has two adductor muscles, one posterior at the back of the clam, and one anterior at the front of the clam. When contracted, these muscles serve to close the clam’s shell. These muscles are able to both quickly contract in order to quickly close the shell, and also remain contracted for a long time to ensure that the clam’s shell remains closed. The top part of the elastic hinge ligament is stretched while the bottom part of elastic hinge ligament is compressed when the shell is closed. Hence, when the adductor muscles relax, the elastic hinge ligament tends to revert to its original shape, springing the clam shell open.

Ruditapes philippinarum also has an umbro, where the secreted calcium carbonate of the mantle initially accumulates, located on the anterior (front) side of the clam.

Ruditapes philippinarum usually grow up to 4 to 5.7 cm wide, with large clams reaching up to just under 8 cm.

Transport of Substances

The respiration system of Ruditapes philippinarum involves the use of siphons. The mantle of the clams are fused at both ends to form two openings called siphons. One siphon, the inhalent (or incurrent) siphon, is used to suck in water, while the other siphon, the exhalent (or excurrent) siphon, is used to expel water. Both siphons are located on the posterior (back) side of the clam. This also implies that the clams have an open circulatory system that is exposed to the surrounding ocean. The ctenidia, or gills, of the clam take up oxygen and release carbon dioxide to the moving water in the clam via countercurrent exchange between the passing water and the hemolymph.

The circulatory system of Ruditapes philippinarum incorporates the use of sinuses and the relatively uncommon respiratory pigment hemocyanin. The heart of the clam consists of two auricles and one ventricle and pumps hemolymph through vessels to open spaces called sinuses10. Other than hemoglobin, the hemolymph of Ruditapes philippinarum also contains hemocyanin, which carries out a similar function to hemoglobin. Rather than having an iron ion in its centre, the hemocyanin protein has two copper and two oxygen atoms at its centre and is able to bind up to 96 oxygen atoms, which is significantly more than the 4 oxygen atoms hemoglobin can bind to.

Diet

Ruditapes philippinarum are filter feeders, meaning that they acquire nutrients by feeding on microorganisms like phytoplankton and detritus from the surrounding water. They do so by trapping substrate nutrients through mucus on the ctenidia.

Reproduction

Ruditapes philippinarum clams are mostly split into separate sexes and reproduce via external fertilisation. This involves the release of both male and female gametes into the surrounding ocean. Clams start forming gametes through gametogenesis in February, and most of the gametes mature in May. Spawning of these gametes also begins around this time and continues until September. Any gametes that mature in October and November are degraded and its constituent materials taken up by the clam.

While rare, some clams have been observed to produce both male and female gametes in scientific studies.

Ecological Role

As filter feeders, Ruditapes philippinarum are able to remove many small particles (sediments) from the surrounding water. This makes the water clearer by reducing its sediment loads. Clearer water allows more sunlight to reach submerged plants like seagrasses, enhancing photosynthesis and promoting growth. This results in more oxygen being produced, benefiting aquatic organisms in the long run. By removing excess nutrients from the water around them, the clams also prevent harmful algal blooms from occurring.

Human Uses

Ruditapes philippinarum is commercially harvested, and represents 25% of global mollusk production in 2014. As food, it is widely appreciated, particularly in its native region of East Asia.

Invasive Species

Ruditapes philippinarum is claimed to have hitchhiked along with oyster species to North America, where it is now widely prevalent along American coastlines. In Europe, the clam was intentionally introduced to expand the economy of the area. In these places, the clam has taken on a natural invasive character, undergoing a massive population spike and displacing native species.

The clam is causing particular concern in Europe as of recently, where it has mated and hybridised with a native European clam species, mixing the gene pools of both species together.

References

1. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/05%3A_Evolution/5.01%3A_Linnaean_Classification#:~:text=The%20Linnaean%20system%20of%20classification,just%20a%20few%20basic%20similarities.

2. https://www.inaturalist.org/observations/209680827

3. https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/manila-clam-palourde-japonaise-eng.html

4. https://oceanservice.noaa.gov/facts/bivalve.html#:~:text=Bivalve%20mollusks%20(e.g.%2C%20clams%2C,many%20different%20bivalve%20mollusk%20species.

5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/veneridae#:~:text=Their%20main%20characteristics%20are%20as,anterior)%2C%20hinged%20dorsally%20by%20an

6. https://pmc.ncbi.nlm.nih.gov/articles/PMC6743046/

7. https://www.thoughtco.com/facts-about-mollusks-4105744

8. https://www.museum.zoo.cam.ac.uk/collections-research/ddf-bivalve-mollusc-project/anatomy-bivalve#:~:text=The%20adductor%20muscles%20contain%20both,position%20between%20the%20hinge%20teeth.

9. https://invasions.si.edu/nemesis/species_summary/81477

10. https://www.youtube.com/watch?v=2PvkGt_Eq08

11. https://www.britannica.com/animal/mollusk/The-nervous-system-and-organs-of-sensation

12. https://shellfish.ifas.ufl.edu/environmental-benefits/#:~:text=By%20this%20very%20act%20of,nutrients%20from%20inshore%20coastal%20waters.

13. https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-mollusca

14. https://www.sciencedirect.com/science/article/abs/pii/S0044848605006617#:~:text=A%20study%20of%20the%20reproductive,May%20and%20continued%20until%20September.

15. https://www.sealifebase.ca/summary/Ruditapes-philippinarum.html

16. https://www.nature.com/articles/srep39745

Image Credits

1. https://invasions.si.edu/nemesis/species_summary/814778