Disease

Although numerous disease conditions are known in pearl oysters, in many cases, the causes, development and spread of diseases are not well understood. There are also major shortages in knowledge concerning the connections between stress, the environment and the oysters.  A diverse spectrum of infectious microbial and parasitic organisms, symbionts and links occur in pearl oysters. Organic and inorganic toxins are also known to compromise pearl oyster health. Furthermore, a number of important disease syndromes are known but again the cause is often obscure or multifactorial. The origin of disease in pearl oysters often occupies association of infectious and non-infectious factors making the diagnosis of disease often problematic.

The pearl oyster has limited capacity to respond to disease or to produce signs signaling a specific disease or condition, and the examination of living oysters presents difficulties. Constrained visual access inside the shell cavity restricts direct observation of the living animal. In the living animal viewed undisturbed in water, the observer is generally dependent on relatively subjective and variable criteria to evaluate the health of individual animals. Adductor muscular tone, evaluated on strength and speed of closure of the shell, and mantle withdrawal are two common criteria evaluated in response to external stimuli. Failure to lay down new shell may also suggest disease.

Clinical signs of disease in a population as a whole include deaths, empty shells, gaping shells, reduced growth, shell valve anomalies, mantle retraction, reduced adductor muscle strength and reduced rate of closure of the shell valves. Sub-clinical disease, in which no visible abnormalities are evident, may be manifest by sub-optimal shell growth, decreased body-weight gain, poor condition indices, poor pearl quality and quantity of pearl production.

Poorly nourished animals appear wasted, with transparent mantle tissues and watery, atrophic visceral masses. Mantle retraction may result in fouling of the nacreous surface of the shell normally covered by the expanded mantle, together with anomalous deposition of conchiolin and nacre by the retracted mantle, leading to “shell disease.” This process may result in a growth check, with the deposition of new shell on previously normal shell, giving rise to the name “double backs” for such oysters. Shells with such lesions are good indicators of a pre-existing upset to the animal or population.

Pearl oysters possess cell-mediated inflammatory and immune capacities brought on by cell responses within the hemolymph, and can produce swift and effective responses to injury or infection. These responses have, however, been poorly investigated when compared to other molluscan species such as the edible oysters (Southgate & Lucas 2008).

Predation

A variety of fishes, rays, octopus, predatory gastropods, crustaceans, turbellarians and echinoderms commonly prey on pearl oysters (Pit and Southgate 2003). Juveniles are particularly vulnerable to predation Predation usually declines with increasing oyster size, as is typically found in other bivalve molluscs and many other marine invertebrates.