The diurnal rhythm and the processes of feeding and digestion in Tridacna crocea (BivaMa: Tridacnidae)

Abstract

Tridacna crocea Lamarck 1819 possesses a diurnal rhythm of activity. During the daylight hours the hypertrophied siphonal tissues, containing zooxanthellae, are expanded and phasic adductions occur that pump fresh sea water into and filtered water and pseudofaeces out of the mantle cavity. During the night the shell valves become more apposed, the mantle folds are withdrawn and few phasic adductions occur. Such a pattern of activity is similar to that described for other sublittoral bivalves. With this cycle of activity and inactivity is correlated the process of extra‐cellular digestion of filtered food in the stomach. This is accomplished by the alternate, but incomplete, dissolution and subsequent reformation of the crystalline style. The style partially dissolves at night and reforms during the day. The digestive diverticula undergo a sequence of cytological changes related to the intra‐cellular digestion of filtered food, again related to the cycle of night and day and thus to feeding. During the night there is a transport and intra‐cellular digestion of zooxanthellae by amoebocytes from the mantle to the haemocoel surrounding the visceral mass. During the day digested zooxanthellae are transported to the digestive diverticula where they are discharged (together with the waste products of the intra‐cellular digestion of filtered food) into the tubule lumina and thus back into the stomach to be eventually voided via the anus. Recognizable zooxanthellae are also transported to the kidney, where they are taken up by the cells of the distal limb to form cellular globular concretions; these too are probably also eventually voided. The discharge of digested zooxanthellae via the digestive diverticula has resulted in modifications to the processes of tubule breakdown and reformation involving the digestive cells only. These unique modifications are described and compared with the structural changes seen in the digestive diverticula of other bivalves. It is concluded that the unusual functioning of the digestive diverticula is a direct consequence of the utilization by Tridacna of a secondary source of food without the concomitant evolution of special organs of digestion, as for example there has been in the Teredinidae feeding on filtered food and wood. Specific comparisons are made between the Tridacnidae and the Teredinidae and suggestions are made as to how zooxanthellae came to be used as food in these unusual bivalves.

Microplastic removal by Red Sea giant clam (Tridacna maxima)

“…First of all, the transfer of the excess of carbon from the symbiotic zooxanthellae to the host's tissues, which seems to satisfy the main nutritional requirements (Trench et al, 1981;Fisher et al, 1985). Furthermore, they possess a digestive system typical of heterotrophic filter-feeding bivalves (Morton, 1978;Reid et al, 1984). Thanks to these autotrophic and heterotrophic nutritional sources, which act synergistically, giant clams can act as trophic opportunists in oligotrophic environments, such as tropical coral reefs (Yonge, 1982;Heslinga and Fitt, 1987).…”

The behaviour of giant clams (Bivalvia: Cardiidae: Tridacninae)

“…The feeding behaviour in giant clams has a marked circadian rhythm (Fankboner 1971 ; Morton 1978 ; Reid et al 1984 ; Fankboner and Reid 1990 ); solar and lunar cycles which determine food availability in turn influence the diurnal feeding activity. At night, they withdraw their mantles and close their valves either half-way or fully, remaining quiescent till dawn (Gwyther and Munro 1981 ; Heslinga et al 1984 ).…”

“…SCUBA divers often observe that giant clams are able to sense their presence and respond to shadows by closing their valves to varying degrees (Rosewater 1966 ; Hickman and Gruffydd 1971 ; Morton 1978 ). Adults possess numerous (>3,000 for a 900 mm T. gigas ) pinhole eyes along their mantle margins, constituting the visual mechanism to mediate defensive withdrawal responses (Fankboner 1981 ).…”

“…Vibrations travelling through the water were thus not the stimulus necessary to elicit mantle retractions typical of shadow responses. However, direct mechanical stimulation of the mantle can elicit valve adduction in adult giant clams, often generating a forceful jet of seawater from the exhalant siphon (McMichael 1974 ; Morton 1978 ). Numerous observations of such squirting reflexes have been reported by SCUBA divers and scientists; for example, McMichael ( 1974 , p. 254) noted that it was ‘an occupational hazard when measuring clams to be regularly squirted in the face’.…”

Cautious clams? Energetic state modifies risk assessment in giant clams

“…), puffer fish ( Tetradon stellatus ), and eagle rays ( Aetobatis narinari ) (Chambers, 2007). Giant clams can detect predator stimuli through at least two mechanisms: visual (Fankboner, 1981; Wilkens, 1986) and mechanical (bites or grazes from predators) (McMichael, 1974; Morton, 1978; Soo and Todd, 2014). Clams possess several hundred pinhole eyes on their mantle that are capable of detecting light or shade (Fankboner, 1981).…”