scholarly journals The Control of Carbon Translocation in a  Sea Anemone-Dinoflagellate Symbiosis  from New Zealand

2021 ◽  
Author(s):  
◽  
Tiffany Bock
Keyword(s):  
Algal Cell ◽  
Sea Anemone ◽  
Host Tissue ◽  
Tissue Homogenate ◽  
Host Feeding ◽  
Rocky Shores ◽  
Sea Anemones ◽  
Fixed Carbon ◽  
Photosynthetic Rates ◽  
Carbon Translocation

<p>Anthopleura aureoradiata, a common sea anemone of New Zealand's intertidal mudflats and rocky shores, hosts symbiotic dinoflagellates of the genus Symbiodinium. This study investigated the control of photosynthetic carbon translocation in this symbiosis, and in particular the presence and operation of socalled 'host release factor' (HRF). Evidence for HRF exists in a number other algalinvertebrate symbioses, where tissue extracts of the host stimulate carbon release by isolated algal symbionts. However, its identity remains elusive and it has never been studied before in A. aureoradiata. Translocation of photosynthetically-fixed carbon in the intact symbiosis and in the presence of host tissue extract was measured using a 14C label. Zooxanthellae in the intact symbiosis released around 40% of their photosynthetically-fixed carbon to the anemone. Isolated zooxanthellae, however, translocated only 8%, even less than the amount of photosynthate liberated by zooxanthellae in FSW alone (11%). Photosynthetic rates per algal cell were similar in the intact symbiosis and both host homogenate and FSW incubations, meaning that the total amount of photosynthetically-fixed carbon released (in pg C/cell/h) by the zooxanthellae in these different situations reflected the %translocation values. Given the failure of homologous zooxanthellae (i.e. those from A. aureoradiata) to respond to homogenized host tissue, it was tested whether zooxanthellae from other host species (i.e. cultured heterologous algae) responded. Heterologous zooxanthellae representing 5 clades (A-E) of Symbiodinium were incubated in host tissue homogenate and photosynthate release again measured with 14C. The %translocation varied from 12-51% in A. aureoradiata homogenate and 17-67% in FSW, again suggesting a lack of an active HRF in the homogenized tissues of this sea anemone. Photosynthetic rates amongst the different heterologous algae also varied widely with, for instance, freshly isolated zooxanthellae from A. aureoradiata having 6-fold higher photosynthetic rates than cultured algae from the same clade (clade A). The zooxanthellae of A. aureoradiata are known to be N-sufficient in the field, and studies with other species have demonstrated that N-deficient zooxanthellae release more photosynthate in response to HRF than do N-sufficient ones. Therefore, induction of an HRF effect was attempted by starving sea anemones, and hence their zooxanthellae, prior to incubation of freshly isolated zooxanthellae in homogenized tissue. However, even after 8 weeks of starvation, the zooxanthellae showed no signs of N-deficiency (as indicated by the extent to which ammonium enhanced the rate of dark 14C fixation), meaning that the relationship with HRF activity could not be examined. The ability of these temperate zooxanthellae to maintain their Nsufficiency, even after relatively long periods of food deprivation, may indicate a lower reliance on host feeding for nitrogen than is seen in tropical zooxanthellae, or a greater capacity to use internal stores of nitrogen. The lack of photosynthate release by both homologous and heterologous zooxanthellae in host homogenate, as opposed to substantial carbon released in the intact symbiosis, suggests that control of carbon translocation in A. aureoradiata is not related to the activity of an HRF; alternatively, if an HRF is present, its activity is hindered when the symbiosis is disrupted. Further study is needed to determine what is responsible for the control of photosynthate translocation in the A. aureoradiata-Symbiodinium symbiosis.</p>

scholarly journals The Control of Carbon Translocation in a  Sea Anemone-Dinoflagellate Symbiosis  from New Zealand

2021 ◽  
Author(s):  
◽  
Tiffany Bock
Keyword(s):  
Algal Cell ◽  
Sea Anemone ◽  
Host Tissue ◽  
Tissue Homogenate ◽  
Host Feeding ◽  
Rocky Shores ◽  
Sea Anemones ◽  
Fixed Carbon ◽  
Photosynthetic Rates ◽  
Carbon Translocation

<p>Anthopleura aureoradiata, a common sea anemone of New Zealand's intertidal mudflats and rocky shores, hosts symbiotic dinoflagellates of the genus Symbiodinium. This study investigated the control of photosynthetic carbon translocation in this symbiosis, and in particular the presence and operation of socalled 'host release factor' (HRF). Evidence for HRF exists in a number other algalinvertebrate symbioses, where tissue extracts of the host stimulate carbon release by isolated algal symbionts. However, its identity remains elusive and it has never been studied before in A. aureoradiata. Translocation of photosynthetically-fixed carbon in the intact symbiosis and in the presence of host tissue extract was measured using a 14C label. Zooxanthellae in the intact symbiosis released around 40% of their photosynthetically-fixed carbon to the anemone. Isolated zooxanthellae, however, translocated only 8%, even less than the amount of photosynthate liberated by zooxanthellae in FSW alone (11%). Photosynthetic rates per algal cell were similar in the intact symbiosis and both host homogenate and FSW incubations, meaning that the total amount of photosynthetically-fixed carbon released (in pg C/cell/h) by the zooxanthellae in these different situations reflected the %translocation values. Given the failure of homologous zooxanthellae (i.e. those from A. aureoradiata) to respond to homogenized host tissue, it was tested whether zooxanthellae from other host species (i.e. cultured heterologous algae) responded. Heterologous zooxanthellae representing 5 clades (A-E) of Symbiodinium were incubated in host tissue homogenate and photosynthate release again measured with 14C. The %translocation varied from 12-51% in A. aureoradiata homogenate and 17-67% in FSW, again suggesting a lack of an active HRF in the homogenized tissues of this sea anemone. Photosynthetic rates amongst the different heterologous algae also varied widely with, for instance, freshly isolated zooxanthellae from A. aureoradiata having 6-fold higher photosynthetic rates than cultured algae from the same clade (clade A). The zooxanthellae of A. aureoradiata are known to be N-sufficient in the field, and studies with other species have demonstrated that N-deficient zooxanthellae release more photosynthate in response to HRF than do N-sufficient ones. Therefore, induction of an HRF effect was attempted by starving sea anemones, and hence their zooxanthellae, prior to incubation of freshly isolated zooxanthellae in homogenized tissue. However, even after 8 weeks of starvation, the zooxanthellae showed no signs of N-deficiency (as indicated by the extent to which ammonium enhanced the rate of dark 14C fixation), meaning that the relationship with HRF activity could not be examined. The ability of these temperate zooxanthellae to maintain their Nsufficiency, even after relatively long periods of food deprivation, may indicate a lower reliance on host feeding for nitrogen than is seen in tropical zooxanthellae, or a greater capacity to use internal stores of nitrogen. The lack of photosynthate release by both homologous and heterologous zooxanthellae in host homogenate, as opposed to substantial carbon released in the intact symbiosis, suggests that control of carbon translocation in A. aureoradiata is not related to the activity of an HRF; alternatively, if an HRF is present, its activity is hindered when the symbiosis is disrupted. Further study is needed to determine what is responsible for the control of photosynthate translocation in the A. aureoradiata-Symbiodinium symbiosis.</p>

scholarly journals The Influence of Symbiont Diversity on the Functional Biology of a Model Sea Anemone

2021 ◽  
Author(s):  
◽  
Dorota Ewa Starzak
Keyword(s):  
Coral Reefs ◽  
Host Cell ◽  
Metabolic Cost ◽  
Global Scale ◽  
Sea Anemone ◽  
Host Tissue ◽  
Sea Anemones ◽  
Dominant Type ◽  
Symbiodinium Type ◽  
Symbiont Type

<p>Cnidarian–dinoflagellate symbioses, particularly those between anthozoans and dinoflagellates of the genus Symbiodinium (commonly referred to as zooxanthellae) are widespread in the marine environment. They are responsible for the formation of coral reefs and are thus of great ecological importance. In recent years there has been an increase in the frequency and severity of episodes of coral bleaching resulting in degradation and mortality of coral reefs on a global scale. In order to gain a deeper understanding of how corals can adapt to changing environmental conditions, the effect that symbiont type has on the persistence and physiology of an association needs to be ascertained. The aim of this research was to determine how different symbiont types affect the nutritional biology and intracellular physiology of the symbiosis when in association with the sea anemone Aiptasia pulchella. The specific objectives of the study were to; (1) determine whether different symbiont types are equally as adept at supporting the energetic demands of the same host; (2) determine if internal pH (pHi) is a reflection of symbiont type and whether the optimal pH for photosynthesis coincides with the host cell pHi; and (3) test the influence of Symbiodinium type on host tissue glycerol and glucose pools. In order to answer these questions, aposymbiotic (i.e. symbiont-free) sea anemones were infected with different Symbiodinium types and the relationship between symbiont type, photosynthetic performance and autotrophic potential was tested. A range of ‘normal’ and novel cnidarian–dinoflagellate symbioses was also used to measure host cell pHi and to determine the optimal pHi of isolated intact symbiosomes (i.e. the vacuoles that house the symbionts), as well as to compare the amounts of free glycerol and glucose (metabolites) present in the host tissues. Different host-symbiont combinations were found to have different photosynthetic and respiratory attributes. Earlier onset of full autotrophy (i.e. when all metabolic carbon demands of the symbiosis were met by photosynthesis) and higher CZAR values (i.e. the contribution of zooxanthellae to animal respiration) were demonstrated by symbioses hosting Symbiodinium B1 both from the original (homologous) and different (heterologous) host. The study showed that Symbiodinium types differ in their pH optima and that the optimal pHi for photosynthesis does not always match the actual measured pHi. Symbiont type was also shown to have an effect on host tissue glycerol and glucose pools, with the associations harbouring the homologous Symbiodinium B1 attaining the highest concentrations of both metabolites. Findings from this study suggest that corals may be able to maintain an association with a range of Symbiodinium types, and hence potentially switch as a consequence of bleaching. The new symbiont type may not be as nutritionally advantageous as the original type however, which could have implications for the growth and survivorship of the coral, unless it is able to supplement its carbon demands heterotrophically. The rapid proliferation of some of the heterologous Symbiodinium types (e.g. Symbiodinium E2) inside the host indicates that, after bleaching, there is potential for fast symbiont establishment. The reduced carbon contribution of these heterologous symbionts may not be a major concern should the coral be able to reinstate the more nutritionally advantageous symbiont as the dominant type during bleaching recovery. Finally, the rapid proliferation demonstrated by the heterologous Symbiodinium types and the associated metabolic cost to the host, could be an indication of the opportunistic nature of some of these types and may indicate a shift towards parasitism. It is imperative to extend this type of work to corals in the field to determine how these associations behave in nature. Also, in order to get a clearer picture of the diversity in symbiosis physiology, a wider range of Symbiodinium types needs to be investigated.</p>

scholarly journals The Influence of Symbiont Diversity on the Functional Biology of a Model Sea Anemone

2021 ◽  
Author(s):  
◽  
Dorota Ewa Starzak
Keyword(s):  
Coral Reefs ◽  
Host Cell ◽  
Metabolic Cost ◽  
Global Scale ◽  
Sea Anemone ◽  
Host Tissue ◽  
Sea Anemones ◽  
Dominant Type ◽  
Symbiodinium Type ◽  
Symbiont Type

<p>Cnidarian–dinoflagellate symbioses, particularly those between anthozoans and dinoflagellates of the genus Symbiodinium (commonly referred to as zooxanthellae) are widespread in the marine environment. They are responsible for the formation of coral reefs and are thus of great ecological importance. In recent years there has been an increase in the frequency and severity of episodes of coral bleaching resulting in degradation and mortality of coral reefs on a global scale. In order to gain a deeper understanding of how corals can adapt to changing environmental conditions, the effect that symbiont type has on the persistence and physiology of an association needs to be ascertained. The aim of this research was to determine how different symbiont types affect the nutritional biology and intracellular physiology of the symbiosis when in association with the sea anemone Aiptasia pulchella. The specific objectives of the study were to; (1) determine whether different symbiont types are equally as adept at supporting the energetic demands of the same host; (2) determine if internal pH (pHi) is a reflection of symbiont type and whether the optimal pH for photosynthesis coincides with the host cell pHi; and (3) test the influence of Symbiodinium type on host tissue glycerol and glucose pools. In order to answer these questions, aposymbiotic (i.e. symbiont-free) sea anemones were infected with different Symbiodinium types and the relationship between symbiont type, photosynthetic performance and autotrophic potential was tested. A range of ‘normal’ and novel cnidarian–dinoflagellate symbioses was also used to measure host cell pHi and to determine the optimal pHi of isolated intact symbiosomes (i.e. the vacuoles that house the symbionts), as well as to compare the amounts of free glycerol and glucose (metabolites) present in the host tissues. Different host-symbiont combinations were found to have different photosynthetic and respiratory attributes. Earlier onset of full autotrophy (i.e. when all metabolic carbon demands of the symbiosis were met by photosynthesis) and higher CZAR values (i.e. the contribution of zooxanthellae to animal respiration) were demonstrated by symbioses hosting Symbiodinium B1 both from the original (homologous) and different (heterologous) host. The study showed that Symbiodinium types differ in their pH optima and that the optimal pHi for photosynthesis does not always match the actual measured pHi. Symbiont type was also shown to have an effect on host tissue glycerol and glucose pools, with the associations harbouring the homologous Symbiodinium B1 attaining the highest concentrations of both metabolites. Findings from this study suggest that corals may be able to maintain an association with a range of Symbiodinium types, and hence potentially switch as a consequence of bleaching. The new symbiont type may not be as nutritionally advantageous as the original type however, which could have implications for the growth and survivorship of the coral, unless it is able to supplement its carbon demands heterotrophically. The rapid proliferation of some of the heterologous Symbiodinium types (e.g. Symbiodinium E2) inside the host indicates that, after bleaching, there is potential for fast symbiont establishment. The reduced carbon contribution of these heterologous symbionts may not be a major concern should the coral be able to reinstate the more nutritionally advantageous symbiont as the dominant type during bleaching recovery. Finally, the rapid proliferation demonstrated by the heterologous Symbiodinium types and the associated metabolic cost to the host, could be an indication of the opportunistic nature of some of these types and may indicate a shift towards parasitism. It is imperative to extend this type of work to corals in the field to determine how these associations behave in nature. Also, in order to get a clearer picture of the diversity in symbiosis physiology, a wider range of Symbiodinium types needs to be investigated.</p>

Transfer of photosynthetically fixed carbon between the prokaryotic green alga Prochloron and its ascidian host

1983 ◽  
Vol 34 (3) ◽  
pp. 431 ◽  
Author(s):  
DJ Griffiths ◽  
L Thinh
Keyword(s):  
Light Intensity ◽  
Green Alga ◽  
Host Tissue ◽  
Total Carbon ◽  
Symbiotic Association ◽  
Fixed Carbon ◽  
Low Light ◽  
Ascidian Species ◽  
Dark Fixation ◽  
Efficient Transfer

In the symbiotic association between the prokaryotic green alga Prochloron and three didemnid host species (Diplosoma similis, Lissoclinum bistratum, Trididemnum cyclops), between 6 and 51 % of the total carbon fixed during exposure for 1 h to H14CO3- in the light (150 �E m-2 s-1) becomes associated with the host tissue. Dark fixation of 14CO2 in these ascidian species and in Lissoclinum punctatum never exceeds 6% of photosynthetic fixation at saturating light intensity. The corresponding values for dark fixation of 14CO2 in isolated Prochloron cells fall within the same range. There is very little excretion of photosynthate from whole colonies of the above ascidian species nor from Didemnum molle, Lissoclinum voeltzkowi and Trididemnum miniatum (usually less than 1 % of total photosynthate at saturation light intensity), suggesting an efficient transfer mechanism from Prochloron to host. Evidence from pulse-chase experiments suggests that transfer probably involves the early products of photosynthesis. The extent of transfer of photosynthate between Prochloron and T. cyclops varies with the rate of photosynthetic 14CO2 fixation into the whole colony but there is some transfer even at low light intensities, which strongly limit photosynthesis.

Gametogenesis and the reproductive cycle in the deep-sea anemone Paracalliactis stephensoni (Cnidaria: Actiniaria)

1990 ◽  
Vol 70 (1) ◽  
pp. 163-172 ◽  
Author(s):  
Michel Praet-Van
Keyword(s):  
Organic Matter ◽  
Shallow Water ◽  
Deep Sea ◽  
Phytoplankton Bloom ◽  
Sea Anemone ◽  
Bay Of Biscay ◽  
Sea Anemones ◽  
Spring Phytoplankton Bloom ◽  
Sea Floor ◽  
Ultrastructural Investigation

This ultrastructural investigation of gametogenesis in a deep-sea anemone of the Bay of Biscay trawled around 2000 m depth, contributes to the knowledge of biology and strategy of reproduction of deep-sea benthos.This sea anemone is dioecious. The sperm appears very similar to those of shallow water sea anemones of the genus, Calliactis. The ultrastructural investigation of oogenesis allows the characteristics of the stages of previtellogenesis and vitellogenesis to be defined. The latter begins with a period of lipogenesis correlated with the formation of a trophonema. Mature oocytes measure up to 180 (im in diameter. Study of spermatogenesis and oogenesis reveals that spawning occurs in April/May. In males, the main area of testicular cysts, full of sperm, reaches maximal development from March to May and, in females, the percentage of mature oocytes decreases from 33% in April to 1% in May.Spawning may be induced by the advent in the deep-sea of the products of the spring phytoplankton bloom. This period of spawning, during the increased deposition of organic matter to the deep-sea floor, may be an advantageous strategy for early development of Paracalliactis.

The Behaviour and Neuromuscular System of Gonactinia Prolifera, A Swimming Sea-Anemone

1971 ◽  
Vol 55 (3) ◽  
pp. 611-640
Author(s):  
ELAINE A. ROBSON
Keyword(s):  
Electrical Stimulation ◽  
Mechanical Stimulation ◽  
Motor Units ◽  
Nerve Cells ◽  
Sea Anemone ◽  
Neuromuscular System ◽  
Sea Anemones ◽  
Nerve Net ◽  
Local Contraction ◽  
Electric Shocks

1. In Gonactinia well-developed ectodermal muscle and nerve-net extend over the column and crown and play an important part in the anemone's behaviour. 2. Common sequences of behaviour are described. Feeding is a series of reflex contractions of different muscles by means of which plankton is caught and swallowed. Walking, in the form of brief looping steps, differs markedly in that it continues after interruptions. Anemones also swim with rapid tentacle strokes after contact with certain nudibranch molluscs, strong mechanical disturbance or electrical stimulation. 3. Swimming is attributed to temporary excitation of a diffuse ectodermal pacemaker possibly situated in the upper column. 4. From the results of electrical and mechanical stimulation it is concluded that the endodermal neuromuscular system resembles that of other anemones but that the properties of the ectodermal neuromuscular system require a new explanation. The size and spread of responses to electric shocks vary with intensity, latency is variable and there is a tendency to after-discharge. There is precise radial localization, for example touching a tentacle or the column causes it to bend towards or away from the stimulus. 5. A model to explain these and other features includes multipolar nerve cells closely linked to the nerve-net which would act as intermediate motor units, causing local contraction of the ectodermal muscle. This scheme can be applied to other swimming anemones but there is no evidence that it holds for sea anemones generally.

A new vermiform sea anemone (Anthozoa: Actiniaria) from Argentina: Harenactis argentina sp. nov.

Zootaxa ◽  
2011 ◽  
Vol 3027 (1) ◽  
pp. 9 ◽  
Author(s):  
DANIEL LAURETTA ◽  
ESTEFANÍA RODRÍGUEZ ◽  
PABLO E. PENCHASZADEH
Keyword(s):  
New Species ◽  
Southern Hemisphere ◽  
First Record ◽  
Sea Anemone ◽  
Soft Bottom ◽  
Sea Anemones ◽  
A New Species

During 2007, 2008, and 2010, 23 specimens of an undescribed vermiform sea anemone were collected on Punta Pardelas and Fracaso Beach (Península Valdés, Argentina). The specimens have longitudinal rows of cinclides distally, all mesenteries perfect, tentacles hexamerously arranged without acrospheres, column not divisible into regions, no marginal sphincter and no conchula. We describe these specimens as a new species within the genus Harenactis (family Haloclavidae). Harenactis argentina sp. nov. is the second species of Harenactis; it represents the first record of this genus in the southern hemisphere and the first record of a soft bottom-dwelling sea anemone in the Argentine continental zone. Furthermore, we discuss the familial placement and relationships of the genus Harenactis and other athenarian sea anemones.

scholarly journals Phylogenetic relationships among the clownfish-hosting sea anemones

2019 ◽  
Author(s):  
Benjamin M. Titus ◽  
Charlotte Benedict ◽  
Robert Laroche ◽  
Luciana C. Gusmão ◽  
Vanessa Van Deusen ◽  
...  
Keyword(s):  
Phylogenetic Reconstruction ◽  
Taxonomic Revision ◽  
Sea Anemone ◽  
Coral Triangle ◽  
Sea Anemones ◽  
Ecological Processes ◽  
Holistic Understanding ◽  
The Family ◽  
Genus 2 ◽  
Insight Into

AbstractThe clownfish-sea anemone symbiosis has been a model system for understanding fundamental evolutionary and ecological processes. However, our evolutionary understanding of this symbiosis comes entirely from studies of clownfishes. A holistic understanding of a model mutualism requires systematic, biogeographic, and phylogenetic insight into both partners. Here, we conduct the largest phylogenetic analysis of sea anemones (Order Actiniaria) to date, with a focus on expanding the biogeographic and taxonomic sampling of the 10 nominal clownfish-hosting species. Using a combination of mtDNA and nuDNA loci we test 1) the monophyly of each clownfish-hosting family and genus, 2) the current anemone taxonomy that suggests symbioses with clownfishes evolved multiple times within Actiniaria, and 3) whether, like the clownfishes, there is evidence that host anemones have a Coral Triangle biogeographic origin. Our phylogenetic reconstruction demonstrates widespread poly-and para-phyly at the family and genus level, particularly within the family Stichodactylidae and genus Sticodactyla, and suggests that symbioses with clownfishes evolved minimally three times within sea anemones. We further recover evidence for a Tethyan biogeographic origin for some clades. Our data provide the first evidence that clownfish and some sea anemone hosts have different biogeographic origins, and that there may be cryptic species of host anemones. Finally, our findings reflect the need for a major taxonomic revision of the clownfish-hosting sea anemones.

scholarly journals Identification of microsatellite loci in sea anemones Aulactinia stella and Cribrinopsis albopunctata (family Actiniidae)

F1000Research ◽  
2018 ◽  
Vol 7 ◽  
pp. 232
Author(s):  
Ekaterina S. Bocharova ◽  
Alexey A. Sergeev ◽  
Aleksandr A. Volkov
Keyword(s):  
Population Genetics ◽  
Microsatellite Loci ◽  
Sea Anemone ◽  
Sea Anemones ◽  
Polymorphic Loci ◽  
Dna Libraries

From the DNA libraries enriched by the repeat motifs (AAAC)6, (AATC)6, (ACAG)6, (ACCT)6, (ACTC)6, ACTG)6, (AAAT)8, (AACT)8, (AAGT)8, (AGAT)8, for two viviparous sea anemones Aulactinia stella and Cribrinopsis albopunctata, 41 primer pairs were developed. These primer pairs resulted in the identification of 41 candidate microsatellite loci in either A. stella or C. albopunctata. Polymorphic loci were identified in both sea anemone species for 13 of the primer pairs and can be applicable for population genetics researches.

The cell cycle of symbiotic Chlorella. I. The relationship between host feeding and algal cell growth and division

1985 ◽  
Vol 77 (1) ◽  
pp. 225-239
Author(s):  
P.J. McAuley
Keyword(s):  
Cell Division ◽  
Cell Growth ◽  
Host Cell ◽  
Algal Cell ◽  
Host Cells ◽  
Digestive Cell ◽  
Host Feeding ◽  
Digestive Cells ◽  
Division Factor ◽  
Cell Mitosis

When green hydra were starved, cell division of the symbiotic algae within their digestive cells was inhibited, but algal cell growth, measured as increase in either mean volume or protein content per cell, was not. Therefore, control of algal division by the host digestive cells must be effected by direct inhibition of algal mitosis rather than by controlling algal cell growth. The number of algae per digestive cell increased slightly during starvation, eventually reaching a new stable level. A number of experiments demonstrated that although there was a relationship between host cell and algal mitosis, this was not causal: the apparent entrainment of algal mitosis to that of the host cells could be disrupted. Thus, there was a delay in algal but not host cell mitosis when hydra were fed after prolonged starvation, and algae repopulated starved hydra with lower than normal numbers of algae (reinfected aposymbionts or hydra transferred to light after growth in continuous darkness). Two experiments demonstrated a direct stimulation of algal cell division by host feeding. Relationships of algal and host cell mitosis to numbers of Artemia digested per hydra were different, and in hydra fed extracted Artemia algal, but not host cell, mitosis was reduced in comparison to that in control hydra fed live shrimp. It is proposed that algal division may be dependent on a division factor, derived from host digestion of prey, whose supply is controlled by the host cells. Numbers of algae per cell would be regulated by competition for division factor, except at host cell mitosis, when the algae may have temporarily uncontrolled access to host pools of division factor. The identity of the division factor is not known, but presumably is a metabolite needed by both host cells and algae.

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