So what is the truth about feeding tridacnids? How do tridacnid clams acquire nutrients?
In this thread I will do my best to lay everything out for you. I will post quotes and links from, and to research in the hopes you will have a better understanding of how these animals work.
Its an old myth that clams under X inches need to filter feed.
Tridacnid clams are not dependent on filter feeding phytoplankton no matter what size they are. They are primarily photosynthetic and can sustain themselves on the food provided by their zooxanthellae, through light alone. Clams will extract dissolved nitrogen and phosphorus from the water and pass this onto there zooxanthellae and then the zoox gives the clam sugars as food. They do not need to filter particulate or extract nutrients through the gills. They can absorb nutrients directly through their mantles (Hypertrophied Siphonal Epidermis).
From INTRACELLULAR DIGESTION OF SYMBIONTIC ZOOXANTHELLAE BY HOST AMOEBOCYTES IN GIANT CLAMS (BIVALVIA:TRIDACNIDAE), WITH A NOTE ON THE NUTRITIONAL ROLE OF THE HYPERTROPHIED SIPHONAL EPIDERMIS
"The evolution of hypertrophied siphons in tridacnids has allowed both greater exposure to solar radiation for proliferation of their algal populations (Yonge,1936 and 1953) and the development of an extensive microvillous surface, which appears to possess a prodigious capacity for assimilation of both fluid and particulate matter from the surrounding seawater. The ultrastructure of the siphons of Tridacna crocea has been described by Kawaguti (1966), who mentions the presence of microvilli covering the siphonal epidermis in passing, but attaches no functional significance to these membraneous structures.
Uptake of nutrient material from seawater by microvillous epidermal cells has been clearly established in echinoderms, molluscs, and pogonophores (Fontaine and Chia, 1968 ; Little and Gupta, 1968 ; Pasteels, 1968 ; Southward and Southward, 1968) . Hence, a similar phenomenon in tridacnids is not novel except possibly in a functional sense. For example, absorption of particulate material by the siphonal surface in tridacnids might contribute directly to the nutrition of the siphon's epidermal cells, but does it necessarily follow that the same function is present in the uptake of fluids?
Zooxanthellae have nutrient salt requirements which might be gratified by means of uptake from the adjacent seawater by the epidermal cells of the hypertrophied siphons. Yonge ( 1936) has found phosphorus metabolism in Tridacna crocea strikingly different from that of the tropical bivalve Spondylus in that, not only does T. crocea remove significant amounts of phosphorus from its environment, but, unlike Spondylus, it also retains the phosphorus excretion products of its own protein catabolism. Yonge attributes these metabolic differences to the demanding nutrient salt requirements of the tridacnids zooxanthellae. This idea of a strong physiological dependence on phosphorus by zooxanthellae gains additional support from the more contemporary findings of McLaughlin and Zahl( 1966) who observe that the population structure of axenically cultured zooxanthellae suffers deleterious effects when grown in culture medium which is phosphate depleted.
What, then, might be the pathway of phosphorus removal from seawater? In the case of some non-tridacnid bivalves, the majority of nutrient salts are simply drunk and later absorbed through the gut walls (Allen, 1970; Fretter, 1953).
However, in comparison to the greater portion of the Bivalvia, the tridacnid alimentary tract is categorically small in relation to its total biomass. This aspect, in addition to the apparent extra phosphorus requirements of its symbiont algae, suggests that most salt uptake must enter from another site. At present, the microvillous epidermis of the hypertrophied siphons is the only tridacnid tissue in external contact which appears to be clearly capable of fulfilling this critical role. Further, in terms of conservation of metabolic energy, the siphonal epidermis would likely be the most direct path to the zooxanthellae for the transport of nutrient salts."
Clams will filter phyto (and bacteria and zooplankton) but when they do this all they are doing is extracting the same N & P and passing it to the zoox.
One of the arguments for feeding clams is that clams mantles are not fully developed untill they are 3" or 4" in length. This is completely false. From the paper. The Zooxanthellae Tubular System in the Giant Clam
“The entire branched tubular system associated with the zooxanthellae communicates with the stomach via a single opening, which is visible in clams that are only a few weeks old (22) and which would appear to explain
the initial entry of zooxanthellae into the mantle “
Clams mantles are fully developed and full of zoox within weeks of metamorphosis. The above link and quote is from a study on the ZTS and in it it states “During a study of the anatomy and histology of giant clams, numerous Tridacna gigas, from a few millimeters to 35 cm in shell length, were dissected.” And not once does it mention a different stage of development of the ZTS between the clams of “a few millimeters to 35 cm”.
From this paper Establishment of the photosymbiosis in the early ontogeny of three giant clams
“Zooxanthellae were exclusively found in the stomach of the juveniles just after the metamorphosis from veligers. Differentiation of the zooxanthellal tube was recognized when zooxanthellae in the juvenile clam appeared in a line. We thought that this was the sign of the establishment of symbiosis. The zooxanthellal tube, in which zooxanthellae were packed, mostly appeared in the juveniles of about 2 weeks after fertilization . At this stage, shell length of juvenile clams was about 200 μm. The zooxanthellal tube extended from the stomach toward the edge of the mantle (Fig. 3), and then the tube further extended along the mantle edge (arrows in Fig. 3c). In the earliest case of our observation, the zooxanthellal tube appeared in most of the juvenile clams on day 10 and all the juveniles of all three species had established the symbiosis by day 20.”
So this paper states that with in 20 days the ZTS is developed and the symbiosis is established.
Another one is that clams mantles are not large enough to house enough zoox to support the clam untill its 4", false again. The size of a clams mantle is proportionate to the size of the clam through out its life.
Another argument some people have for feeding phyto is that clams have a fully functioning digestive system and that if they didn't need to filter feed they wouldn't have this.
So lets look at this. Clams gills are multifunctional, they are use for respiration and capturing particulate matter. They can't get rid of the gills or they wouldn't be able to breath , clams also constantly replenish there zoox, they use their gills to do this.
From this paper The Zooxanthellal Tubular System in the Giant Clam
“It also reveals that the giant clam-zooxanthellae symbiosis is actually like other known invertebrate-algal symbioses, being intimately associated with the digestive system”.
A clams stomach is connected to it's zooxanthellae tubular system (where the zoox live) the stomach passes new zoox from the gills to the ZTS , processes the sugars the zoox make (to feed the clam) and pass old, dead and unviable zoox to the anus.
Even though the digestive system isn't needed for filtering phyto, it is still used as a basic function of the clam.
If you want to feed your clams phyto, go ahead but don't think that they will die if you don't. As long as you have strong lighting and N & P (fish pee and poo) in the water the clam will do just fine !
Here's a few more snippets from research papers to back up what i say.
From klumpp and lucas 94
It is now established that photosynthates fixed by symbiotic zooxanthellae are able to provide sufficient energy to cover at least the metabolic needs of Tridacna gigas (Fisher et al. 1985, Mingoa 1988, Klumpp et al. 1992), T squamosa (Trench et al. 1981), T. derasa and T. tevoroa (Klumpp & Lucas 1994
From Contributions of phototrophic and heterotrophic nutrition to the metabolic and growth requirements of four species of giant clam (Tridacnidae)
"The absolute amounts of carbon trans located daily by the zooxanthellae to the host (TP in Table 4) follow similar patterns of variation with size and species of clam described for P, That IS, in the smaller size categories (0 1 to 10 g tissue weight) Trldacna gigas has a considerable nutritional advantage over the other 3 species, gaining 2 to 20 times more energy in the form of photosynthates TP was similar in the 3 species which attain 100 g In all 4 species and size categories of clam TP was well in excess of host respiratory needs (RH in Table 4) Calculation of the percent contribution of zooxanthellae to the host's daily carbon requirements for routine respiration (l e CZAR = (TPIRH)lOO)a, s given in Table 4 shows that symbiotic algae were capable of providing 2 to 4 times more carbon than required by the host for respiration CZAR increased with clam size in all species, except in H hippopus, which had a comparatively high and more constant CZAR of -340% The lowest CZAR value was 186 % in the smallest T squamosa"
This study actually indicates that clams may need to acquire additional nutrients through filter feeding as they grow larger. However their zoox through photosynthesis can still provide them with at least 2x there Carbon Energy needs. This go against the notion that very small clams need to be feed phyto.
This next study was done to determine how clams acquired there zoox and what they did with them. Two sets of clams were used, one was given zoox the other was not. They were both kept in micro filtered water and not allowed to receive any particulate matter. The only particulate that one set received was its initial dose of symbiotic zoox. These are very tiny clams, the kind everyone says cant live through photosynthesis alone. They did just fine.
http://www.jstor.org/pss/1540800
Fatherree 2006
"let's take a look at some CZAR and CZARG values for some small to clear up any possible confusion. the smallest clams offered for sale to hobbyists are usually in the 2.5 range, but far more "small clams" are in the 3.8 to 5cm range. keep this in mind when you see the CZAR and CZARG numbers going up.
Mingoa (1988) found that 1.75cm gigas specimens (smaller than what you can buy) had an average CZAR values of only 92% under bright sunlight. close, but not quite enough C/E from the zooxanthellae for basic maintenance. however that was in 1988 and Mingoa, using unpublished data from Griffiths, had chosen a translocation value of 32%. so you can see the same thing happening for these little clams. change the translocation value to 95% and the CZAR values will triple to 273%.
In addition, Fischer et al. (1985) reported a CZAR value for gigas (using a transference value of 95%) of 149% for 1cm specimen, 259% for a 1.15cm specimen, and 318% for a 1.55cm specimen. all smaller then what you can buy. then, Klumpp&Lucas (1994) found CZAR to be as high as 178% for 2.2cm derasa and 2cm tevoroas, with CZARG values of 140% for both, while data from Klumpp&Griffiths (1994) shows a CZAR of 265% and CZARG of 191% for 4.2cm gigas, 233% and 206% for 2.4cm crocea, 186% and 118% for 4.2cm squamosa, and 300% for 4cm hippopus"
So according to that they are getting C/E from photosynthesis just fine.
People often bring up an article that was written by Dr. Shimek commissioned by DT's. How much money do you think DT's would have paid him if the conclusion was that clams are not dependent on phyto? No offense to Dr. Shimek, but i dont think this was his best work. The references used are old and out dated. Some of the claims made in it are completely false and show either sloppy research by the author, or selective research to come to a desired conclusion.
Whats the magic # in that article? 4" I think. A crocea at its fastest growth rate, grows about 3/4" per year. So it would take a crocea at least 5.33 years to get to 4". Gigas has an average growth rate of about 3" per year, and at its fastest growth almost 4.5" per year. So it can achieve the magic 4" in one year. Why more then 5 years for crocea and barely 1 year for gigas to fully develop there mantles or house enough zoox to support the clam? It doesn't make any sense. If its going to take clams so long to be able to use zoox for photosynthesis why do the start collecting them between 2 and 4 weeks after fertilization while there still pediveligers? Only to wait 1 to 5 years to see the benefit.
Lets look at just crocea for a minute. I think everyone would agree that crocea is considered to be the most light demanding of all the clams. They are most commonly found in very shallow water of just a few feet. They can be sporadically found down to about 20', no more. And we all know that clams are broadcast spawners. There eggs and sperm are at the mercy of the currents for up to a month then settle out. They have no control where they settle and I'm sure that many more larva settle deeper then 20' then that do. If they are so dependent on filtering phyto how come there aren't a bunch of small croceas under 4" at 30', 40', 50' deep? There's plenty of phyto down there for them. They should be able to do just fine down there filtering away untill the magic 4" comes along and then they would just die.
I want to give special credit to James Fatherree for this write up. His book was a real eye opener for me and some of the comments I made above were sparked from personal communications with James.
