Reef Science: Development Highlights
This month, I cover a couple of articles
on anemone fission and metal toxicity...
Mire, Patricia, and Stacy Venable. 1999.
Programmed cell death during longitudinal fission in a sea
anemone. Invertebrate Biology 118(4): 319-331.
Little is known about the cellular events
regulating fission in metazoans. In sea anemones, longitudinal
fission begins with stretching of the body column and culminates
in ripping apart of the animal. Previously, we found that
mechanical stretching of the animal plays a regulatory role
in early fission events. In this study we use histology, TUNEL
cytochemistry, and TEM to analyze the possible spatio-temporal
relationship between stretching of tissues during fission
and programmed cell death (apoptosis) within stretched tissues.
We report that enhanced apoptosis occurs in specific tissue
regions apparently most affected by stretching during fission.
In stretched animals we find a significant induction of apoptosis
at the junctions of body wall and particular mesenteries that
begins in the axis parallel to stretch and then progresses
to the axis perpendicular to stretch as fission progresses.
Based on these results, we propose a model whereby stretching
induces apoptosis in populations of cells, allowing tissue
to thin and thus facilitating successful fission.
I came across this article as I work with
the process of apoptosis in coral disease. Years ago, I had
come across an article by Japanese researchers that suggested
that injury to the hypostome (mouth) of Galaxea fascicularis
induced polyp division. Aquarists have frequently asked what
they can do to encourage division for propagation. Previously,
I had suggested that slightly injuring the mouth of corallimorphs
Apoptosis is one of the hottest areas of
research today, and appears to exist in all animal systems,
although very little is known of the process in primitive
metazoans, like the cnidarians. However, normal development,
including what was previously thought to be simple differential
cell growth, may frequently have differential cell death as
part of the process. Here, the act of stretching acts as a
trigger to set off the apoptotic cascade and help induce fission.
I suspect this is the case in many corals, as well, based
on aquarium observations of many asexual reproductive behaviors.
Anemone propagation has long been a dream
of aquaculturists, and I think the material in this article
provides some good basis for experimentation. I would suggest
that careful stretching of the mouth and body wall using a
blunt tip pliers or probes might be a possible way to encourage
division by fission and does not involve injury that so often
results in mortality when cutting-type efforts have been employed
in the past.
Mitchelmore, C.L., Alan Verde E, Ringwood
A.H., Weis V.M. 2003. Differential accumulation of heavy metals
in the sea anemone Anthopleura elegantissima as a function
of symbiotic state. Aquat Toxicol. 64(3):317-329.
The accumulation of metals by the North
American Pacific Coast temperate sea anemone Anthopleura
elegantissima, and its dinoflagellate-algal symbiont
Symbiodinium muscatinei was examined following laboratory
metal exposures. Both, naturally occurring symbiotic and symbiont-free
(aposymbiotic) anemones were used in this study to investigate
differences in metal uptake due to the symbiotic state of
the animal. The effects of metal exposures on the anemone-algal
symbiosis were determined using measures of algal cell density
and mitotic index (MI). Anemones were exposed to either cadmium,
copper, nickel or zinc chloride (0, 10, 100 microg l(-1) for
Cd, Cu and Ni; 0, 100, 1000 microg l(-1) for Zn) for 42 days
followed by a 42-day recovery period in ambient seawater.
Anemones were analyzed for metal content using inductively
coupled plasma mass spectroscopy (ICP-MS) at various time
points during the study. Symbiotic anemones accumulated Cd,
Ni and Zn to a greater extent than aposymbiotic anemones.
A dramatically different pattern of Cu accumulation was observed,
with aposymbiotic anemones accumulating higher levels than
symbiotic anemones. Following recovery in ambient seawater,
all tissue metal levels were reduced to near pre-exposure
control levels in most cases. No changes in algal cell density
or MI were observed in symbiotic anemone tentacle clips at
any dose or time point in the Cd and Cu exposures. However,
significant reductions in algal cell densities were observed
in the Ni-exposed and some Zn-exposed animals, although levels
returned to control values following recovery. There were
no changes in mitotic index (MI) following Ni or Zn exposures.
These results demonstrate that the extent of heavy metal accumulation
depends upon cnidarian symbiotic state and the heavy metal
During the past year, much has been learned
about the level of metals resulting from both food additions
and salt mixes used in aquariums. It is apparent that metal
levels may be abberantly elevated in our aquariums, and arguments
have been made to postulate why metal levels may be comparatively
tolerated by otherwise metal-sensitive species. This article
addresses several of the metals which have been found to be
present at elevated levels in aquariums, and indicates that
the effect of metals may be multi-factorial and depend on
the presence of zooxanthellae, the type of metal, and perhaps
the species of the animal. It is also interesting to read
that recovery is possible and within a reasonably rapid time
Anthopleura elegantissima may be
a species with a significant heavy metals tolerance. It is
found in areas where runoff may contain large amounts of heavy
metals due to igneous rock deposition. The demonstrated rapid
recovery may be one reason why this is the case.
L. Shimek, Ph. D.
This month, I will discuss an interesting
article on temperature and its effect on coral...
Saxby, T., W. C. Dennison, and O. Hoegh-Guldberg.
2003. Photosynthetic responses of the coral Montipora digitata
to cold temperature stress. Marine Ecology Progress Series.
Coral bleaching events have become more
frequent and widespread, largely due to elevated sea surface
temperatures. Global climate change could lead to increased
variability of sea surface temperatures, through influences
on climate systems, e. g. El Niño Southern Oscillation
(ENSO). Field observations in 1999, following a strong ENSO,
revealed that corals bleached in winter after unusually cold
weather. To explore the basis for these observations, the
photosynthetic responses of the coral species Montipora
digitata Studer were investigated in a series of temperature
and light experiments. Small replicate coral colonies were
exposed to ecologically relevant lower temperatures for varying
durations and under light regimes that ranged from total darkness
to full sunlight. Photosynthetic efficiency was analyzed using
a pulse amplitude modulated (PAM) fluorometer (F0,
and chlorophyll a (chl a) content and symbiotic
dinoflagellate density were analyzed with spectrophotometry
and microscopy, respectively. Cold temperature stress had
a negative impact on M. digitata colonies indicated
by decreased photosynthetic efficiency (Fv/Fm),
loss of symbiotic dinoflagellates and changes in photosynthetic
pigment concentrations. Corals in higher light regimes were
more susceptible to cold temperature stress. Moderate cold
temperature stress resulted in photoacclimatory responses,
but severe cold stress resulted in photodamage, bleaching
and increased mortality. Responses to cold temperature stress
of M. digitata appeared similar to that observed in
corals exposed to warmer than normal temperatures, suggesting
a common mechanism. The results of this study suggest that
corals and coral reefs may also be impacted by exposure to
cold as well as warm temperature extremes as climate change
Quote from the discussion section of the
"Symbiotic corals exposed to cold
water conditions and high light intensity had significant
decreases un photochemical efficiency and reduced numbers
of viable dinoflagellates compared to the controls. Decreased
photochemical efficiency is indicative of photoinhibition,
while reduced dinoflagellate density is a response that is
typical of general stress among reef-building corals."
The cold temperatures that the corals were
exposed to were 53.6°F, 60.8°F and 68°F. The corals
were exposed to the temperatures for 1, 3, 6, 12, and 18 hours.
Light regimes were 100% of normal intensity, 50% of normal
intensity, and total darkness.
Effects were noticeable at all tested
temperatures and the effects persisted for long periods,
particularly for the lower temperature treatments.
While this obviously has implications for
hobbyists maintaining tanks at low temperatures, it has more
serious implications for all hobbyists in general. Temperatures
as low as the ones examined undoubtedly occur during the shipment
and transport of corals, both immediately after collection
in the transit from collection areas to the United States
(in the baggage compartments of aircraft) AND during mail
order during ALL seasons (baggage compartments where mail
is shipped may not be heated), but especially during winter.
Much transit stress and subsequent mortality
may be related to the low temperatures encountered by the