Symbionts
  This section on symbionts is divided into types of symbionts, symbiont entry into the larva, and role in nutrition located in subsections below, and HABITAT EFFECTS located in its own section.
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Research study 1
 
photograph of aggregating anemones Anthopleura elegantissima at low tide

Several west-coast sea anemones rely as much on photosynthesis as food capture to satisfy their nutrient and energy needs.  The photosynthesis is done by symbiotic plant cells contained within the digestive lining (gastrodermis) of the sea anemone and supplies the host with numerous photosynthetic products including amino acids, glycerol, and glucose.  So dense are the symbionts that they may impart an overall brownish or greenish colour to the sea anemone, depending upon the type of symbiont. Weis et al. 2002 Invert Biol 121: 190.


 

Symbiont-bearing Anthopleura elegantissima at low tide 0.4Xphotograph of great green anemones Anthopleura xanthogrammica on a rock wass

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Types of symbionts

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Research study 1
 

Sea anemones Anthopleura spp. contain two types of symbionts.  The first is a a single-celled dinoflagellate, thought to be Symbiodinium sp. and thus closely related to the species found in corals and giant clams (S. microadriaticum).  These photograph of zooxanthellae isolated from the tentacle of a sea anemone Anthopleura sp. courtesy Max Taylor, Univ of BCsymbionts, known as zooxanthellae, impart a mottled brownish colour to their hosts.  The second type of symbiont is a single-celled chlorophyte (green alga). These zoochlorellae symbionts impart a greenish colour to their hosts.  A description of the ultrastructure of the zoochlorellae in A. xanthogrammica can be found in O’Brien 1978 Trans Amer Micros Soc 97: 320 and an update on itsphotograph of new chlorphyte species Elliptochloris marina courtesy Letsch et al. 2009 J. Phycol 45: 1127phylogenetic status in the Chlorophyta in Lewis & Muller-Parker 2004 Biol Bull 207: 87. Photograph on Left courtesy Max Taylor, UBC.

NOTE  the symbiotic chlorophyte in both A. xanthogrammica and A. elegantissima has recently been identified through morphological and molecular means by a research consortium primarily based in Connecticut as a new species Elliptochloris marina.  Letsch et al. 2009 J. Phycol 45: 1127. Phograph on Right courtesy the authors.


Zooxanthellae isolated from a
tentacle of Anthopleura sp. 1000X


New species of chlorophyte Elliptochloris marina found
in both A. xanthogrammica and A. elegantissima

 

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Research study 2

 

graph showing effect of temperature on formation of cleavage furrows in zooxanthellae of a green sea anemone Anthopleura xanthogrammica High-shore individuals of Anthopleura xanthogrammica tend to have predominately zooxanthellae and are brownish in colour (zooxanthellae favour higher temperatures and higer light conditions).  In comparison, greener individuals have a greater proportion of zoochlorellae and, because this type of symbiont prefers lower temperatures and lower light conditions, tend to be found deeper in tidepools and on the lower parts of the shore.  graph showing effect of temperature on reducing zoochlorellae in the sea anemone Anthopleura xanthogrammicaStudies at the Bamfield Marine Sciences Centre, British Columbia show that a shift in water temperature from 13-20oC causes a progressive loss of mitotic activity (% cleavage furrows) in the zoochlorellae.

Coincidental with this is an expulsion of most of the symbionts within about 6wk. The effects are reversible even after 6wk if the anemone is returned to a cooler temperature (13oC), indicating that exposure to the higher temperature is not necessarily lethal to the zoochlorellae.  The authors are unable to differentiate whether the higher temperature affects the zoochlorellae with subsequent reaction by the host, or affects the host with subsequent reaction by the zoochlorellae.  O’Brian & Wyttenbach 1980 Trans Amer Micros Soc 99: 221.

NOTE  digestion of algal cells by the host is discounted by the author, as A. xanthogrammica
apparently does not digest its symbionts, as occurs in coral polyps

 

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Research study 3

 

Although long thought to be a single species of Symbiodinium in Anthopleura elegantissima on the west coast, recent molecular genetics studies suggest that there are, in fact, 2 species.  Specimens of A. elegantissima from Washington and Oregon apparently harbour a single dinoflagellate species, given the new name S. muscatinei (light green colour on map), while anemones from central and southern California harbour both S. muscatinei and the newly named S. californium (light brown colour on map). In the northern anemones, S. muscatinei may exist alone or together with a Chlorella-like green-algal symbiont (zoochlorellae).  The southern anemones apparently host only mixed populations of the 2 zooxanthellae species (Symbiodinium).  The authors speculate that the geographic distribution of these dinoflagellates may be correlated with the temperature cline created by latitude.  LaJeunesse & Trench 2000 Biol Bull 199: 126.

NOTE  restriction-length polymorphism analyses of large- and small-subunit ribosomal RNA genes

map of collecting areas for sea anemones Anthopleura elegantissima in a study of the genetics of the species
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Research study 4
 

map of California showing haplotype distributions of zooxanthellae Symbiodinium muscatinei in sea anemones Anthopleura elegantissimaRecent studies by researchers at Hopkins Marine Station, California reveal that Anthopleura elegantissima at 14 sites along the 1200km length of California coastline host not one, but 3 main genotypes of Symbiodinium muscatinei.  Breaks in the biogeographic distribution of the symbiont occur at Cape Mendocino and Monterey Bay (see map).  Note the degree of homogeneity of haplotypes in the northern and southern populations in comparison with those in central California. Sharp clines at each breakpoint suggest limited gene flow between adjacent regions. Although not much is known about dispersal distances of the larvae of A. elegantissima, the authors suggest that an average dispersal distance of 30km or less would explain the haplotype pattern found.  Since this is much less than what would be expected from these reasonably long-lived larvae, an interesting alternative explanation by the authors is that the larvae may aquire (or perhaps re-aquire) symbionts from the adults present at the location of settlement.  Sanders & Palumbi 2011 Biol Bull 220: 199.

NOTE  the symbionts are obtained from clipped tentacles.  Sequences obtained from the mitochondrial cytochrome b and chloroplast 23S ribosomal genes, and restriction-fragment-length polymorphism data obtained from the 18S nuclear ribosomal gene, are used to characterise the Symbiodinium populations

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  Symbiont entry into larva
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Research study 1
 
How do the symbionts gain entry? After about 4d in the plankton the planulae larva of a sea anemone begins to feed, and the symbionts are thought to be ingested at that time.  Recent studies suggest that one source for the larvae may be symbiont-rich mucus released from the guts of adults, produced in times of high-light stress or perhaps as a natural part of release of undigested food residues.  Presence of this material stimulates the larvae to feed vigorously. In Anthopleura elegantissima the symbionts are collected on trailing mucous threads, which are then drawn in and eaten. The symbionts are not digested, but are taken up in the gut, housed in the gut lining (gastrodermis) of the larva, and there begin to photosynthesise. Their presence may explain, in part, why sea-anemone larvae are free-living so much longer than other cnidarian larvae, such as jellyfish or hydroids, which lack symbionts. Schwarz et al. 2002 Mar Biol 140: 471; McCloskey et al. 1996 J Exp Mar Biol Ecol 195: 173.

photograph of a planula larva of an aggregating anemone Anthopleura elegantissima showing how it consumes zooxanthellae to create a stock for post-metamorphic existence
Planula larvae of A. elegantissima. The larva on the Left is consuming zooxanthellae and its gastrovascular cavity is full of them. The larva on the Right is 24h post-feeding and the zooxanthellae have been incorporated into the gastrodermal cells lining the gastrovascular cavity. The larvae are about 150um in length

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  Role in nutrition
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What role do the symbionts play in nutrition of sea anemones?  Researchers in everal studies have addressed this question using radiolabelled carbon compounds to track the movement of photosynthates, and the consensus is that while little or no symbiont-host translocation occurs with zoochlorellae, considerable translocation occurs with zooxanthellae.  Here is a short summary of each study listed in chronological order:
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Research study 1
  Los Angeles, California 1971: in vitro studies show that isolated zooxanthellae from A. elegantissima incubated with NaH14CO3 release glycerol-14C to the external medium as the major product.  Other 14C-labelled compounds released are alanine, glucose, fumaric acid, succinic acid, and other organic acids.  A substantial proportion (45-50%) of the end products of photosynthesis are released to the host animal.  No mention is made of contributions by zoochlorellae.  Trench 1971 Proc Roy Soc Lond B 17: 225; Trench 1971 Proc Roy Soc Lond B 177: 237.
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Research study 2
  San Juan Islands, Washington 1971: studies using whole specimens of A. elegantissima and tentacles only of A. xanthogrammica show that the zoochlorellae do not release soluble 14C-labelled photosynthetic products to the host, but that the zooxanthellae do.  What then are the zoochlorellae doing in the sea anemones?  The author conjectures that the zoochlorellae may be parasitic.  The anemone avoids possibly harmful “infections” of the parasites by regulating the numbers of zoochlorellae to a “compatible” level.  Muscatine 1971 Pac Sci 25: 13.
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Research study 3
 

Bamfield, British Columbia 1980: in vivo translocation of radiolabeled carbon from fixation of 14CO2 by zoochlorellae symbionts in A. xanthogrammica is only about 4% (at 15oC).  In comparison, translocation from zooxanthellae to the epidermis in A. elegantissima is estimated to be 18-31% of total carbon fixed, or 50% to epidermis and gastrodermis combined.  As to why the host anemone does not digest its symbionts, the authors note that acid-phosphatase reaction product (a component of digestion-breakdown process) is absent within the vacuoles containing the symbionts.  In some way the presence of the alga inhibits the cellular digestive processes of the host.  O’Brien 1980 J Exp Zool 211: 343.

NOTE  most of the A. xanthogrammica specimens used in the study have zoochlorellae, and zooxanthellae are absent.  Thus, they are possibly from a lower position on the shore

photograph of zooxanthellae from an aggregating anemone Anthopleura elegantissima courtesy Max Taylor, UBCSymbiodinium
Symbiodinium sp. (zooxanthella) from A. elegantissima 5000X. Photo courtesy Max Taylor, UBC
 
Research study 4
 

Santa Barbara, California 1981: a comparison of energy metabolism in A. elegantissima under conditions of light/dark and starved/fed reveal that anemones with symbionts in the light are able to  maintain carbohydrate metabolism for at least 1mo when starved.  Lipid levels in these individuals do not differ significantly from fed controls and are 40-60% higher than in starved aposymbiotic individuals after 1mo.  The authors suggest that the source of carbohydrate is probably photosynthates translocated by the symbionts.  Fitt & Pardy 1981 Mar Biol 61: 199.

NOTE  the researchers use respiratory quotients (RQs),or the ratio of CO2/O2, as indices of metabolic conditions.  RQ of 1=carbohydrate metabolism, RQ of 0.7=lipid, and RQ of 0.8-0.9=protein

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Research study 5
  Santa Barbara, California 1982: estimates of relative contribution of photosynthetically fixed carbon translocated from the zooxanthellae to A. elegantissima is 13% for well-fed anemones and 45% for 2-wk starved anemones or ones fresh from the field.  Fitt et al. 1982 J Exp Mar Biol Ecol 61: 213.
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Research study 6
  Bodega Bay Harbor, California 1984: freshly collected and continuously immersed specimens of Anthopleura elegantissima receive 34-42% of their repiratory carbon requirement from their symbionts.  This decreases to 17% when the animals are exposed to air for 15h during daytime spring low tides, primarily owing to shading of the symbionts when the anemone retracts its tentacles and contracts its marginal sphincter muscles.  Shick & Dykens 1984 Biol Bull 166: 608.
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Research study 7
  San Juan Islands, Washingon 1996: a zooxanthella in A. elegantissima has twice the volume and carbon content of a zoochlorella, but zooxanthellae occur in lower density than zoochlorellae.  Zoochlorellae grow 8 times faster than zooxanthellae, but the latter respire more and have twice the net photosynthesis of zoochlorellae.  These last features combine to allow zooxanthellae potentially to translocate 5 times more carbon to the host than zoochlorellae. Verde & McCloskey 1996 J Exp Mar Biol Ecol 195: 187.
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Research study 8
 

San Juan Islands, Washington 1996: studies on A. elegantissima show that normally the internal population of symbionts remains fairly stable at about 106 cells . mg host protein-1.  How a constant density is maintained is not well understood, but expulsion of symbionts is one possibility.  From time to time under natural field conditions an anemone may expel a bolus of mucus, undigested food matter, and symbionts.  Light appears to be one factor involved.  If anemones are held in the lab for 24d under 3 experimental light conditions, significantly more symbionts are expelled under the higher irradiance level, and significantly more of these are zoochlorellae.  Moreover, more actively dividing cells are expelled.  Thus, with more light, more algal cells divide, population numbers increase, and more are expelled by the host.  McCloskey et al. 1996 J Exp Mar Biol Ecol 195: 173.

NOTE  light conditions are 100% (natural window light), 50%, and 13%.  Equal numbers of zooxanthellate (brown colour in graph) and zoochlorellate (green) anemones are used

graph showing efect of light on expulsion of symbionts from aggregating anemones Anthopleura elegantissima
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Research study 9
 

Shannon Point, Washington 1999: studies on Anthopleura elegantissima at the Shannon Point Marine Center, Washington using 14C show that about 30% of the symbiont-produced photosynthates are transferred to the host from both zooxanthellae and zoochlorellae (at 13oC), but with overall less carbon being translocated from zoochlorellae than from zooxanthellae. However, owing to the 2 to 3-fold greater density of zoochlorellae than zooxanthellae, the amounts are equivalent.  If the experiments are run at a higher temperature of 20oC, though, zoochlorellate anemones receive 3-fold less carbon because of significant reduction of zoochlorellae numbers.  Engebretson & Muller-Parker 1999 Biol Bull 197: 72.

NOTE  the substrate is not identified

graph showing rate of translocation of photosynthates from zooxanthellae and zoochlorellae symbionts in aggregating anemones Anthopleura elegantissima
 
 
Research study 10
 

Shannon Point, Washington 2008futher studies on Anthopleura elegantissima at the Shannon Point Marine Center, Washington confirm the greater density of zoochlorellae over zooxanthellae (4-fold) during summer and winter, but greater productivity of zooxanthellae over zoochlorellae (by 2.5-fold) in summer.  Estimates based on stable isotope composition (13C and 15N)  suggest that zooxanthellate anemones receive more nutrition from their symbionts than do zoochlorellate anemones.  The isotopic data, moreover, indicate substantial reliance on external food sources.  Bergschneider & Muller-Parker 2008 Biol Bull 215: 73.

 
Research study 11
 

photograph of sea anemones in a tidepool Anthopleura xanthogrammicaShannon Point, Washington 2012sea anemones Anthopleura xanthogrammica in the Olympic peninsula, Washington as elsewhere occur in tidepools and surge channels. At low intertidal levels, tentacles of individuals in both habitats contain primarily zoochlorellae. In comparison, at high intertidal levels, individuals in tidepools contain primarily zoochlorellae, while those in surge channels contain primarily zooxanthellae - a result likely of greater intolerance of zoochlorellae to higher temperatures in the shallow surge channels. Stable-isotope analysis reveals that a sea anemone feeding on sea mussels Mytilus californianus in the light obtains only about 34% of its dietary carbon from its mussel food, in comparison with about 66% from its zoochlorellae. Levine & Muller-Parker 2012 Mar Ecol Progr Ser 453: 79.

NOTE the authors indicate that this green-algal species is now designated Elliptochloris marina, recently confirmed as being present in both A. xanthogrammica and A. elegantissima. Letsch et al. 2009 J Phycol 45 (5): 1127.

Brownish-coloured Anthopleura xanthogrammica in a tidepool,
likely hosting predominantly zooxanthellae symbionts 0.3X

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