Behaviour

Feeding: 

Like all gastropods this herbivore feeds using its radula, made up of approximately 150 teeth to graze on turf or fleshy algae. In an unpublished experiment conducted on Heron Island, 2011 it was found that T. niloticus prefers filamentous, soft, turf algae forms rather than fleshy coralline algae. This was indicated by significant increases in feeding rates when provided with filamentous algae rather than Laurencia sp.,  a coralline algae.  The experiment correlated with other experiments (Helsinga, 1981) in seeing that this species is nocturnal, with feeding rates increasing significantly at night time compared to day. The photo below shows the feeding experiement set- up on Heron island. 

  According to Nash (1993) T. niloticus     extracts its nutrients mostly from bottom deposits consisting of organic and inorganic materials but supplements these with nutrients derived from vegetable matter. 

When FAO examined the digestive tract of the snail, they found remains of foraminifer, diatoms, sponge, hydoids, crustaceans, and molluscs as well as small amounts of algae and large amounts of bottom deposits like sand. 

A study (Thilaga et al, 2010) revealed where different elements of digestion occur in T. niloticus. A dominance of carbohydrases in the foregut region of the animal indicates this is the primary site of carbohydrate digestion, which is consistent with many other herbivores. The midgut also contains digestive enzymes and is a site of digestion, while the high bacteria count in the hindgut show that this is where absorption occurs. The bacterial gut microflora aid this organism in assuring normal enzyme function as well as acting as digestive enzymes if only insufficient amounts are present. 


Reproduction:

T. niloticus are dioecious meaning that there are separate sexes of the animal, but there are not obvious morphological distinctions between the two. Microscopic observation of the gonads shows that the male's are pale brown to creamy white in colour, while the female's are dark green (FAO, 1999). 

Reproduction occurs when they release their gametes (eggs and sperm) into the water column for external fertilization (Chambers, 2007). This release is called spawning and occurs throughout the year at low latitudes, although only in the summer months at higher latitudes (FAO, 1999). In the Great Barrier Reef region spawning occurs throughout the year (Pakoa et al, 2008). This tends to be at night, a few days before or after a new or full moon (FAO, 1999). The males tend to spawn first, with the females joining in 10 minutes to an hour after them (Chambers, 2007). 

The video embeded below shows trochus spawning in a tank. 



The eggs released by T. niloticus are leichtotrophic, meaning they are yolky which the larvae can feed off (FAO, 1999). These eggs tend to be about 220- 240μm in size, which is a little smaller than a dust mite. Females usually release between 100, 000 - 1 million eggs (Heslinga& Helman 1981, Nash 1993) but can release up to 2 million (Nash, 1985).

Life Cycle:

The life cycle of T. niloticus begins after fertilisation. Approximately 12 hours after the eggs hatch into trochophore larvae. Trochophore larvae use several bands of cilia to swim freely, which is very important for dispersal of the species. This trochophore stage is then followed by the veliger stage, which further enhances dispersal capabilities. After 3- 5 days the veliger will settle on suitable habitat in the reef flat, usually among the coral rubble (Nash 1985). They will then begin feeding on turf algae and small microorganisms.

T. niloticus live for between 15- 20 years, and become sexually mature after two years, when they are approximately 6cm long across the base (Chambers, 2007). They continue growing as they age, and are usually around 15cm when they die (Chambers, 2007).

This link provides a basic diagram of the T. niloticus life cycle, from the FAO website.

Dispersal:

The short larval stage of T. niloticus limits its dispersal capabilities (Bertram, 1998). In the planktonic trochophore and veliger larval stages the organism can be transported with currents. However as this stage only lasts 3- 5 days T. niloticus populations are isolated and unlikely to spread to other reefs (Bertram, 1998).