The mutualistic association with symbiotic microalgae (Symbiodiniaceae) is obligatory for the giant clams, but when did symbiosis begin during the larvae stages?

Background:

When it comes to symbiosis, many of the earlier studies investigated on how the juvenile/adult hosts (giant clams) interact with the symbiotic microalgae of family Symbiodiniaceae. Unlike most other bivalves, the giant clam shares a unique partnership with Symbiodiniaceae, whereby the former translocate inorganic nutrients to symbionts for photosynthesis, and in return, receive photosynthates as food/nutrition.

Symbiont acquisition in giant clams is horizontal transmission, which means that the larvae get their symbionts via oral ingestion during larval development (Fitt & Trench, 1981). Horizontal transmission is said to allow for the larvae to acquire a diversity of symbionts, some of which may be more compatible with different environmental conditions encountered after its dispersal from parents (Mies et al., 2017a).

But… How does symbiosis establish at the early larval stages? Which partner decides the symbiosis? Is this a mutualistic association from the point of inception?

Symbionts are taken up during the second larval stage (known as the veliger; see image of 5-days old), as the first larval stage (trochophore) is a non-feeding stage. Upon uptake, the symbionts remain in the digestive tract throughout the larval development. As the individual matures, the symbionts increase in numbers rapidly and lives within the specialised tubular system within the tissues (Norton et al., 1992).

Symbiosis GC
Acquisition of zooxanthellae by Tridacna squamosa: As early as 3 days old, the veliger can take up the tiny microalgae cells. Upon establishment in the gut, the cells proliferate and increase in density. As the host grows bigger, it builds up these zooxanthellal tubules throughout its tissues hosting high densities of microalgae cells. Photos from Neo Mei Lin.

My curiosity led me to these handful of studies that attempted to answer this question: Is this relationship of mutualistic nature already present during host larval development or becomes established at a later stage? 

Hereon, I share some of their key findings on who started it? 🙂

So… is the early larvae-symbiont a mutualistic association?

  • A single symbiosis-specific marker was used to experimentally confirm the mutualistic symbiosis between host larvae and symbionts – H+-ATPase. It is a proton pump that transports cations (positively charged) across the cell membrane, and is only expressed by Symbiodinium engaged in symbiosis (Mies et al., 2017a, 2017b, 2017c).
  • The gene for H+-ATPase has only been characterised for Symbiodinium microadriaticum (previously known as type A1).
  • Experimental studies by Mies et al. (2017b, 2017c) confirmed the expression of this gene in the veliger larvae of two giant clam species: Tridacna crocea and Tridacna maxima. Results suggest that symbiotic interactions are present.
  • However, it is not clear which partner triggers the expression of this symbiosis-specific gene, particularly because the process of establishing symbiosis is complex.
  • It is also important to note that the symbiotic microalgae taken up by hosts is in coccoid form (non-swimming form), compared to the free-living ones (swimming form); where the former may be tied to the specific expression of H+-ATPase (Mies et al., 2017b).
Screenshot 2019-05-23 at 6.39.44 PM
Steps involved in the establishment of symbiosis between microalgae and its host is a complex process (Extracted from Mies et al., 2017a).
  • Symbiosis appears to be important in enhancing growth of giant clam larvae. Studies shown that the presence of Symbiodiniaceae is necessary for their metamorphosis (Fitt & Trench, 1981), as well as for faster growth and development (Belda-Baillie et al., 1999; Gula & Adams, 2018).
  • Mies et al. (2017b) reported higher production of fatty acids by Symbiodinium associated with giant clam larvae, suggesting metabolite exchange.
  • The degree of mutualism between these partners may also depend on the specificity of this association and the ability of host to select for beneficial symbionts. Also, the performance of symbiont types depends on the host and environmental conditions, as different hosts may select different symbionts.

Conclusions?

Through identifying the biochemical pathway of symbiosis and the marker gene (H+-ATPase), the studies thus far confirmed the mutualistic symbiosis between host giant clam and symbiont relationship is set up during larval development.

This mutualistic partnership persists throughout the lifetime of giant clams, and the loss of symbionts could also mean death for the host clams. As the oceans become hotter, these giant clams are at risk of losing their symbiotic partners (and their source of free food!).

Further readings:

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