Colours

Distinguishing features

This species has an angular, heavy shell with numerous radial ribs. The animal is attached to the benthos by the byssal threads when small, however it does not embed in the substratum. Large specimens are not attached. The mantle does not extend out over the edge of the shell, and mantle colour is relatively subdued, being either greenish or light brown around Lizard Island, with fine white lines. Distinguish from other species of Tridacna by the large number of radial ribs on the shell, and the non-extensible mantle.

Size

Synonyms

Distribution

Distribution and habitat preferences

Found in most habitats, but most abundant in shallow, sheltered lagoonal and reef flat habitats.

Found in most shallow habitats around Lizard Island, and extends down to about 4m.

Behaviour

Like other Tridacnid clams, this species is a hermaphrodite, with the male testes forming first at about 4 years of age, followed by the female ovaries a year or so later. Tridacnid clams spawn in the warm summer months between October and February, usually during neap tides when water movement is minimal. This allows enough time for adequate mixing of the gametes. H. hippopus has been found to have a longer spawning season than some of the smaller species of Tridacna. Spermatozoa are usually released first, with eggs only released if the animal detects the presence of sperm from a conspecific clam in the water column. Hence, reproduction is most successful when the animals are located in natural clusters. Eggs are only 100µm (1/10th of a mm) in diameter, and several million are released by each clam during a spawning event. Fertilisation is external, and a trochophore larvae hatches after about 12 hours. A bivalved veliger larvae develops after 48 hours, with a shell length of 160µm. This stage feeds on plankton and continues to swim in the water column, although it periodically ventures down to assess the conditions on the benthos. The veliger larvae settle permanently to the substratum at about 9 days of age and about 200µm, and the juvenile clams may reach 20-40mm shell length after their first year. Growth accelerates after this point, however mortality in the first few years is very high, with predation by fishes and crabs being a major factor.

Tridacnid clams obtain over 95% of their food requirements from the sugars produced by the symbiotic dinoflagellate algae (zooxanthellae) that inhabit the outer mantle tissue. Juvenile clams acquire the zooxanthellae directly from the water column at the time of settlement, and the algae live in tubules that are tertiary extensions of the gut and extend throughout the mantle tissue. Like hard corals, Tridacnid clams may bleach and lose most of their zooxanthellae after high temperature stress. Recent research shows that tridacnid clams may have several different strains of symbionts present simultaneously in the mantle tissue, a situation not generally seen in hermatypic hard corals.

Web resources

References