Symbionts
 

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  Habitat effects
  This section on symbionts is divided into habitat effects considered here, and TYPES OF SYMBIONTS, SYMBIONT ENTRY INTO THE LARVA, and ROLE IN NUTRITION located elsewhere.
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Research study 1
 

photo schematic showing fates of symbionts in translocated sea anemones Anthopleura xanthogrammica

High-shore individuals of Anthopleura xanthogrammica tend to have mostly zooxanthellae and are brownish in colour, while low-shore and deep-tidepool individuals tend to have zoochlorellae and are more greenish in colour. If high- and low-shore specimens are reciprocally translocated, the ones moved from lower to higher tidepools predictably exhibit a shift to zooxanthellae, while the ones moved from higher to lower tidepools maintain their predominance of zooxanthellae.  The reason for lack of change in the second translocation is unexplained.  Bates 2000 J Exp Mar Biol Ecol 249: 249

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Research study 2
 

photograph of bleached sea anemones Anthopleura xanthogrammicaSpecimens of Anthopleura spp. that live in poorly lit environments such as caves will mostly or entirely lack symbionts, and will appear bleached.  In the case of A. xanthogrammica, such specimens are less healthy than ones containing symbionts – they grow more slowly and/or reproduce less.  In contrast, specimens of A. elegantissima seem to live just as well with or without symbionts.

 

 

 

Bleached sea anemones Anthopleura xanthogrammica in a
cave on the west coast of Vancouver Island, British Columbia

 

 

 

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Research study 3
 

Studies on symbionts in aggregating anemones Anthopleura elegantissima in caves on Tatoosh Island, Washington show that within a few meters distance from the cave entrance, numbers of light-loving brown symbionts (zooxanthellae shown as brown bars in the accompanying histogram) in anemones drop to zero, while numbers of the light-avoiding green symbionts (zoochlorellae) increase enormously.  However, at a distance greater than 15m into the cave, anemones lack symbionts and are bleached (indicated by open circles in the graph).  In marked contrast to the situation at the cave entrance, in the depths of the cave light and temperature conditions are essentially unvarying. The vertical arrows indicate the points along the light gradient corresponding to 90 and 99% light attenuation compared with full ambient light outside of the caves. Secord & Muller-Parker 2005 Limnol Oceanogr 50: 272.

NOTE  however, owing to the much smaller size of the zoochlorellae as compared with zooxanthellae, volumes of symbionts are about the same at the entrance and  about 5-6m into the cave

histogram showing types of symbionts present in aggregating anemones Anthopleura elegantissima with increasing distance into a cave
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Research study 4
 

In view of the importance of light to the vitality of the zooxanthellae symbionts and, thus, to the health and nutrition of the host sea anemone, does an anemone adjust its position for optimal illumination of its symbionts?  For aggregating anemones Anthopleura elegantissima in the Monterey Bay area of California, the answer is 'yes'.  Studies show that Anthopleura elegantissima with zooxanthellae living in lighted habitats such as on open-rock surfaces will expand their tentacles in moderate light, and contract them in intense light (and in darkness), thus regulating the amount of light to which their zooxanthellae are exposed. graph showing effect of dark on expansion of anemones Anthopleura elegantissima In comparison, anemones living in dark habitats such as caves will not regularly expand or contract their tentacles under changes in light. Interestingly, even if an individual has recently lost its symbionts it may still expand and contract with changes in light, suggesting that conditioned responses may be involved. In one laboratory experiment (data shown on left), zooxanthellate anemones are about 60% expanded in lighted conditions, but soon close up when placed in the dark. graph showing effect of light on crawling behaviour of anemones Anthopleura elegantissima

In another experiment, anemones taken from 3 different habitats are placed randomly in a seawater tray which is half shaded and half lighted. Within 2wk, 95% of zooxanthellate anemones taken from a bright, sunny habitat have moved into the light, while a second group of zooxanthellate anemones taken from a shady habitat (about 9% light intensity as compared with nearby sunny areas) have mostly (75%) moved into the shaded part of the seawater table (see graph on Right). A third group of anemones, ones without zooxanthellae, show no significant light response and their distribution on the seawater table remains random. This is true even with individuals that previously harboured the symbionts and that are genetically identical clone-mates of the phototactic individuals.  Thus, for the most part, individuals with zooxanthellae display phototaxes. The first group of anemones is presumably responding to increased flow of photosynthates from their symbionts as they move into the lighted area of the tray, but the behaviour of the second groups is less easy to interpret. The subject is deserving of further research. Pearse 1974 Biol Bull 147: 630; Pearse 1974 Biol Bull 147: 641.

NOTE specimens in this area have only zooxanthellae, and no zoochlorellae

NOTE  extended periods in the dark will reduce zooxanthellae numbers in A. elegantissima, although it may take over half a year for them to be eliminated in this way.  A quicker way to rid an anemone of symbionts is to maintain it in warm seawater for a few days (e.g., 30-32oC for 48h, from ambient seawater of 14oC).  This causes the anemone to egest its zooxanthellae in mucus-wrapped pellets until they are all gone

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Research study 5
 

A later study at Hopkins Marine Station, Pacific Grove, California confirms that individuals of Anthopleura elegantissima containing zooxanthellae tend move out of the shade into the light.  This suggests that anemones can detect oxygen differences at least as small as those established by their symbionts when illuminated unequally.  Furthermore, in the field, anemones with symbionts space closer to one another than ones without symbionts, while in the lab individuals space further away when oxygen concentrations are experimentally lowered.  Increased spacing leads to better circulation of water and, thus, to increased oxygen availability and to increased surface area of each anemone in contact with seawater (rather than in contact with clonemates).  All of these observations suggest that oxygen availability may play a bigger role in intraclonal spacing dynamics than previously thought.  In view of this, the author suggests that, rather than being a phototactic response, the tendency of zooxanthellate anemones to seek out light might be better considered an oxygen-tactic response.  Fredericks 1976 Mar Biol 38: 25.

Anthopleura elegantissima, presumably bearing symbionts, photographed at high tide 0.2X

 
Research study 6
 

graph showing effect of shell cover on chlorophyll content in high- and low-level anemones Anthopleura elegantissimaSea anemones Anthopleura elegantissima protect their symbionts both behaviorally and biochemically.  Behavioral responses to bright sunlight include drawing in the symbiont-laden tentacles and contracting the oral disc, thus shading the symbionts, and attaching gravel and other debris to the body column.  Biochemical responses to bright sunlight, which defend against oxygen toxicity produced by hyperactivity of the symbionts, include production of disproportionately high levels of the enzymes superoxide dismutase (SOD) and catalase, most notably in the body column, the part that is mainly exposed to sunlight when the tentacles are retracted.  Studies on shade-adapted or aposymbiotic A. elegantissima at the Bodega Marine Laboratory, California show that SOD and catalase activities increase by 590% and 100%, respectively, after 7d exposure to sunlight. 

Interestingly, anemones from high and low shore positions cleaned of debris and placed in sunlight for 12h, attach gravel to their bodies in direct proportion to the amount of chlorphyll they contain (see graph). Note also in the graph that the high-shore individuals attach significantly more gravel than low-shore individuals.  That the gravel is effective as a sun-block is shown by the fact that anemones deprived of their gravel cover lose significantly more chlorophyll after exposure to sunlight than ones with gravel cover intact. An additional finding is that chlorophyll content of the anemones fluctuates seasonally, in inverse relationship with solar radiation, thus also action on control on oxygen production. The variation does not owe to change in number of algal cells; photograph of sea anemones Anthopleur elegantissima bearing shell debrisrather, to change in chlorophyll content.  Dykens & Shick 1984 Biol Bull 167: 683.

NOTE  the authors explain that while the precise causes of oxygen toxicity are not well known, production of 3 highly reactive chemicals superoxide radical, hydroxyl radical, and hydrogen peroxide is implicated. The enzymes SOD and catalase act to remove the first and last of these, thus minimizing the formation of hydroxyl radicals.

NOTE  another function of gravel coverings on A. elegantissima appears to be in reduction of water loss. See PHYSIOLOGICAL ECOLOGY: EVAPORATIVE WATER LOSS


Several A. elegantissima with shell covering 1X

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Research study 7
 

A study on the effects of temperature and light on symbionts of  “green” and “brown” Anthopleura elegantissima at Shannon Point Marine Center, Washington additionally shows that zoochlorellae are negatively affected by warm temperatures (20oC), but that they are unaffected by low light levels.  In comparison, densities of zooxanthellae increase over time under conditions of low light, but are unaffected by warm temperature. Interestingly, in “mixed” anemones (those containing both types of symbionts), responses to temperature and light parallel those of zooxanthellae and zoochlorellae in brown and green anemones, respectively.  The implication of the results is that population densities of each type of symbiont may vary independently with changes in temperature and light.  Possession of both types by an anemone, then, could be a bet-hedging strategy that ensures functioning symbionts under a range of environmental conditions.   Saunders & Muller-Parker 1997 J Exp Mar Biol Ecol 211: 213.

NOTE  defined by the authors as anemones whose tentacles contain >90% zoochlorellae.  “Brown” anemones contain >90% zooxanthellae.  Anemones containing mixtures of symbionts are termed “mixed”

NOTE  the authors perform ANOVAs on these data but fail to present any statistics. With the considerable overlap in the data, especially for the "brown" set, one is inclined to focus on trends evident from Day 20-25

graphs showing effects of temperature and light on symbionts of aggregating anemones Anthopleura elegantissima
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Research study 8
 

map showing collection sites used in study of symbiont distributions in sea anemones Anthopleura spp. along the west coast of North AmericaGeographic and microhabitat variation in affecting distributions of symbionts in Anthopleura elegantissima and A. xanthogrammica are investigated by researchers at Friday Harbor Marine Laboratories, Washington.  Collections of specimens from 9 locations spanning 2500km from northern Washington to Baja California reveal that zooxanthellae predominate in: 1) high intertidal habitats, 2) more southerly locations, and 3) A. elegantissima.  In contrast, zoochlorellae predominate in: 1) low intertidal habitats, 2) more northerly locations, and 3) A. xanthogrammica.  The authors note that their data are consistent with other published information on photosynthetic efficiency of these symbionts under different conditions of light and temperature in the laboratory. Secord & Augustine 2000 Invert Biol 119: 139.

 

 
Research study 9
 

"Buried" anemones Anthopleura artemisia are reported to lack symbionts (= azooxanthellate), but this may vary with location.  If their overall colour appears greenish or brownish, then they are likely to contain them (= zooxanthellate). Weis et al 2002 Invert Biol 121: 190.

 

Left: azooxanthellate A. artemisia 0.8X
Right: zooxanthellate A. artemisia 0.7X

photograph of an azooxanthellate burying anemone Anthopleura artemisia photograph of a zooxanthellate burying anemone Anthopleura artemisia
 
Research study 10
 

histogram showing effect of temperature on photosynthetic rates of sea-anemone symbiones Symbiodinium muscatineiOther studies on symbionts Symbiodinium in Anthopleura elegantissima show that there are, in fact, 2 species: one, S. muscatinei, has a broad latitudinal distribution extending at least from northern Washington to southern California; the other, S. californium, occurs only in southern populations of A. elegantissima.  Studies on thermal responses of A. muscatinei by researchers at Shannon Point Marine Center, Washington reveals, as known from other studies, that it has high thermal tolerances.  Photosynthetic rates for S. muscatinei are more-or-less constant over a range of incubation temperatures in the laboratory from 12-24oC, but decline significantly when the temperature reaches 26oC. The high part of this temperature range exceeds mean summertime seawater temperatures in both Puget Sound and southern California, and likely explains how S. muscatinei is able to coexist with its congener S. californium in the south.  Muller-Parker et al. 2007 J Phycol 43: 25.

NOTE  LEARN  ABOUT SEA ANEMONES: SYMBIONTS: TYPES OF SYMBIONTS

NOTE  a series of papers by Verde & McCloskey based on research on the photobiology of symbionts of Anthopleura elegantissima done at the Rosario Beach Marine Laboratory of Walla Walla University at Anacortes, Washington.  As this work focuses mainly on the physiology of the symbionts, rather than on the host, it is not described further in the ODYSSEY.  The papers are Verde & McCloskey 2001 138: 477; Verde & McCloskey 2002 141: 225; and Verde & McCloskey 2007 152: 775

 
Research study 11
 

histogram comparing photosynthetic activities of residues of sea anemones Anthopleura after being eaten by leather stars DermasteriasSea anemones Anthopleura elegantissima are commonly eaten by leather stars Dermasterias imbricata and the question arises as to the fate of the algal symbionts during the feeding process. An investigation at the Shannon Point Marine Center, Washington reveals 2 types of solid waste products released by the sea star during consumption and digestion.  The first are pellets discarded by the cardiac stomach during extraoral digestion, while the second are fecal pellets. Analysis of the pellets reveals that both types of symbionts are present in each and are still photosynthetic, but to different degrees.  Zooxanthellae from feces and pellets have photosynthetic rates of 85 and 13%, respectively, of rates of zooxanthellae freshly isolated from A. elegantissima (see histogram).  Comparable rates for zoochlorellae are 20 and 45% of rates of freshly isolated zoochlorellae, respectively.  Percentages of dividing cells are not significantly altered by the feeding process.  The fates of the algal symbionts in nature are not known, but as the feces are mostly soluble and break up easily after being released, the algal photograph of leather star Dermasterias imbricata showing cardiac stomach evertedcomponents probably disperse readily.  The “extra-oral” pellets are a different matter, as they are bound in mucus coatings, and may or may not readily release their contained algal cells.  However, in view of the generally good health of the symbionts, the authors suggest that they may have the potential to recolonise other sea anemones.  The study is the first to examine fates of algal symbionts following consumption by an echinoderm.  Bachman & Muller-Parker 2007 Mar Biol 150: 369.

NOTE  the sea anemones used are either the brown (zooxanthellate) variety or the green (zoochlorellate) variety, each having just the single type of symbiont

NOTE  the authors do not provide even rough estimates of quantities of algal cells in either feces or pellets.  This would be interesting to know


Oral view of leather star Dermasterias imbricata
showing cardiac stomach everted 0.5X

 
Research study 12
 

The extent to which different habitat conditions will affect relative abundances of zoochlorellae and zoooxanhellae symbionts in sea anemones is examined at the Bamfield Marine Sciences Centre, British Columbia.  The researchers select 3 open-coast rocky-shore sites with similar-sized polyps of both Anthopleura elegantissima and A. xanthogrammica located less than 5cm distant from one another in 4 types of microhabitats: 1) tidepools, 2) air-exposed crevices, 3) undersides of ledges, and 4) along light gradients in caves.  At each site, tentacle or tentacle and body-wall samples are taken and counted for proportions of the 2 types of symbionts.  Additonally, growth rates of zooxanthellae are estimated in selected tentacle samples for both species.

Results show, as in other studies, that zoochlorellae are rare in A. elegantissima, but common in A. xanthogrammica, even in low-irradiance habitats.  In cave habitats, A. xanthogrammica at the cave entrance host the green symbionts zoochlorellae, with a sharp transition occurring deeper in the cave to a symbiont-free condition.  In comparison, nearby A. elegantissima at the cave entrance host zooxanthellae, with even a few symbiont-free (white) individuals being present, and  the transition deeper in the cave to symbiont-free individuals is more gradual that in A. xanthogrammica.  Mitotic indices vary inconsistently in the 2 species but with some association with light levels.  The authors conclude that host-specific differences exist in symbiont distributions and mitotic indices in the 2 anemone species, even when habitat conditions are similar Bates et al. 2010 Biol Bull 218: 237.

NOTE   growth rates are estimated from mitotic indices of zooxanthellae in homogenised preparations of tentacles of each species, that is, from counts of cells with and without division furrows

NOTE  given that genetically identical individuals of A. elegantissima can be readily obtained from clones, one wonders if this feature could not be exploited in a study of this kind, using translocations of clone-mates to different habitats, then measuring symbiont complements and mitotic indices

 

photograph of bleached great green anemones Anthopleura xanthogrammica living in a cave
Anthopleura xanthogrammica living in a cave 0.2X

photograph of bleached aggregating anemones Anthopleura elegantissima living in a cave
A. elegantissima living in a cave with sponge Halichondria sp. 0.3X
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