Juvenile fluted giant clam

In the past year, I have received a number of requests and initiated discussions regarding giant clam species identification. This is not surprising as the number of described species continues to expand since Joseph Rosewater’s (1965) seminal paper!

The initial list comprised Hippopus hippopus, Tridacna gigas, T. derasa, T. squamosa, T. maxima and T. crocea. In 1982, Hippopus porcellanus was described by Rosewater (1982) from the Sulu Archipelago (Philippines), and in 1991, a new Tridacna species – T. rosewateri was described by Sirenko & Scarlato (1991) from the Indian Ocean. Subsequently, three species were rediscovered from various regions: T. mbalavuana (Lucas et al. 1991) in Fiji and Tonga, T. squamosina (Richter et al. 2008) in the Red Sea, and T. noae (Su et al. 2014) in the Indo-Pacific region. In 2016, a new species was described from the Indian Ocean – T. lorenzi (Monsecour, 2016)! This brings the total species standing to 12 for the family of giant clams!

So how can we tell the species apart? Here’s a quick guide to first identify a giant clam and its basic anatomy, and then identify species. Although I have been working on giant clams for a while, I have yet to encounter some of the species in the wild! Here, I will focus on discussing the more common and widespread species within their geographic range.

Basic Giant Clam Anatomy

Schematic diagram_Ellis 1999.jpg
Schematic diagram of a giant clam (top view). Taken from Ellis 2000.

Giant clams, as its name suggest, are indeed large clams! Size is an unmistakable feature of the family: rhe smallest of all species (Tridacna crocea) grows up to 15 cm, while the largest (Tridacna gigas) grows up to 130 cm! Another feature is the unusually enlarged mantle tissue, which extends beyond the shell margins for most species (exceptions include the Hippopus spp. and T. mbalavuana), and this is absent in other bivalve species. The most eye-catching feature is the vibrant and colourful mantle tissues that comes in a variety of colours: green, blue, purple, brown, yellow, etc…

Now that we have established the basic features of a giant clam, let’s take a look at the individual species!

Hippopus hippopus (Linnaeus, 1758)

Common names: Horse’s hoof clam; Strawberry clam

Mostly free-living (unattached to substratum) as adults, individuals may grow up to 40 cm long. Shells of H. hippopus are thick with strong radial ribbing on surface (sometimes absent due to abrasion/erosion with time) and display reddish blotches in irregular bands. Mantle tissues of Hippopus species do not extend beyond shell margins, and usually exhibit green, yellow-brown or grey mottled patterns. A distinguishing feature between H. hippopus and H. porcellanus (no image provided) is that the former species lack tentacles on the incurrent siphon.

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Tridacna gigas (Linnaeus, 1758)

Common name: True giant clam

The only truly gigantic giant clam species, T. gigas is most easily identified by its size, and distinctive elongate and triangular projections on the upper shell margins (when viewed from top view). Free-living as adults. Shells are smooth with no ribbing; mantles exhibit dull-brown and olive green and the mantle edge bears iridescent blue-green circles (hyaline organs or ‘eyes’). The only Tridacna species whose incurrent siphon bears no tentacles. Another feature of the species is the white ‘stretch marks’ that occurs throughout the central axis, which are areas with no zooxanthellae cells.

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Tridacna derasa (Roding, 1798)

Common name: Smooth giant clam

The second largest species grows up to 60 cm long. Free-living as adults. Shell valves are also smooth with no ribbing; mantles exhibit brilliant colours of green, blue, purple with ‘tiger’ striped patterns. Incurrent siphon bears guard tentacles.

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Tridacna mbalavuana Ladd, 1934

Common name: Devil’s giant clam

Species resembles closely to T. derasa, except for the following features: presence of shell ribbing, its mantle does not extend beyond shell margins, rugose (uneven) mantle surface, and highly prominent guard tentacles on incurrent siphon. Free-living as adults, individuals may grow up to 50 cm. Species is also mostly found inhabiting relatively deep waters (>20 m), which is different from the other shallow-water species.

Devil’s giant clam, Tridacna mbalavuana. Credits to ReefBuilder.com

Tridacna squamosa Lamarck, 1819

Common name: Fluted giant clam

A medium-sized species, free-living individuals may grow up to 40 cm long. Juvenile individuals observed are usually byssally attached onto the substratum. Shells are well-sculptured and identified by its large flutes/scutes; sometimes valves are coloured (yellow and orange-pink). Mantle tissues exhibit various mottled patterns in combinations of yellow, orange, blue, green and brown, and the incurrent siphon bear distinct tentacles.

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Tridacna maxima (Roding, 1798)

Common name: Small giant clam

Although widely known as the small giant clam, individuals may grow up to 35 cm long. Tridacna maxima is one of the three giant clam species with a burrowing habit – juvenile individuals are usually fully embedded in the reef substrata but older individuals will outgrow their burrows and eventually partially embedded only. Identified by its close-set scutes on shell valves, the neat row of tightly packed ‘eyes’ along the mantle edge, and brilliantly coloured and mottled mantle patterns. Some individuals also show obvious protuberances on the surface of mantles.

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Tridacna noae (Roding, 1798)

Common name: Noah’s giant clam

One of the newly rediscovered species, it was once mistaken as T. maxima due to similar shell morphology and partial burrowing behaviour. When compared using molecular approaches, they found distinct molecular differences between the two species. A closer examination had identified other features of T. noae: mantle patterns exhibit ‘tear-drop’ ovals enclosed with white outline (sometimes other colour outlines), and the irregular rows of ‘eyes’ along the mantle edge. Unfortunately, mantle morphology is highly variable within this species, and individuals found in Western Australia looked quite different from the Indo-Pacific region. The most reliable identification is through means of molecular genetics.

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Tridacna crocea Lamarck, 1819

Common names: Boring giant clam; Burrowing giant clam

The smallest giant clam species, individuals grow up to 15 cm long. Juvenile individuals may easily be mistaken with T. maxima and T. noae juveniles, but distinguished by the distribution and arrangement of ‘eyes’ on the mantle. Fully embedded throughout its lifetime; mantle tissues exhibit a wide variety of colours and patterns.

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Do note that these descriptions would apply to the generic cohort of giant clam species, but there have been exceptions! In most instances, a combination of shell and mantle morphology, and behavioural ecology would suffice in providing the right species identification. 🙂 Have fun identifying and differentiating the giant clams!

P.S: I’m very happy to announce that our collaborative research paper examining the global distribution and status of giant clams has been fully accepted!! 😀 Will keep everyone updated when it is fully published – the paper also has additional descriptions on species ecology and habits that are also useful for field identification. Cheers!

Disclaimer: All photographs in this post belongs to author ML Neo unless otherwise stated and acknowledged accordingly.

Reference List:

Rosewater (1965) The family Tridacnidae in the Indo-Pacific. Indo-Pacific Mollusca 1: 347-396.

Rosewater (1982) A new species of Hippopus (Bivalvia: Tridacnidae). The Nautilus 96: 3-6.

Lucas et al. (1991) Tridacna tevoroa Lucas, Ledua and Braley: A recently described species of giant clam (Bivalvia: Tridacnidae) from Fiji and Tonga. Nautilus 105: 92-103.

Sirenko & Scarlato (1991) Tridacna rosewateri sp. n. A new species of giant clam from Indian Ocean. La Conchiglia 22: 4-9.

Richter et al. (2008) Collapse of a new living species of giant clam in the Red Sea. Current Biology 18: 1349-1354.

Su et al. (2014) Tridacna noae (Roding, 1798) – a valid giant clam species separated from T. maxima (Roding, 1798) by morphological and genetic data. Raffles Bulletin of Zoology 62: 124-135.

Monsecour (2016) A new species of giant clam (Bivalvia: Cardiidae) from the Western Indian Ocean. Conchylia 46: 69-77.