Coral Reef Science:  Development Highlights

Eric Borneman

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.

Abstract:

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.

Comments:

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 encourages division.

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.

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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.

Abstract:

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 in question.

Comments:

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 frame.

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.

Invertebrate Tidbits

Ronald 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. 248: 85-97.

Abstract:

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, Fm, Fv/Fm) 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 occurs.

Quote from the discussion section of the paper:

"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."

Comments:

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 corals.