Panorama (4 images) of tissue section across top of medusoid

 

 This image shows the tissue across the region of the top of the ampulla that had been decalcifying, prior to liberation of the medusoid.  

 

The following section was cut across what I think is the Velar Plate, which will open up to leave a velum around the periphery of the medusoid, allowing propulsion and escape.

 

 

Synchronized cycles in the live cycle of Millepora spp.

 Coral reproduction has attracted interest since the 19th Century.  This interest has accelerated in recent years.  Most recent work has focused on the Scleractinia, Cnidaria of the  Class Anthozoa, the most common reef-building corals.  Millepora spp., of Class Hydrozoa, Family Milleporidae, are also important reef-builders, yet they have received less attention than their distant scleractinian cousins.  Two species of Family Milleporidae, Millepora platyphylla and M. dichotoma, have been the focus of my interest since 1984.   

Life cycles of the Scleractinia are fascinating, each species (or population) exhibiting some variation upon a single theme: polyps, sedentary adult forms, give rise to gametes that, upon fertilization develop into motile planula larvae.  Planulae, usually planktonic, are agents of dissemination that eventually attach to hard substrates.  After metamorphosis into polyps, most species divide through asexual budding to form often massive colonies.  Reef-building scleractinia are joined by obligatory plant partners, Zooxanthellae, either acquired after fertilization by the larva (horizontal transmission), or carried by the egg from mother to offspring (vertical transmission).   may be ingested by larvae---horizontal transmission---or, in other species, the egg acquires its zooxanthellae from the mother colony---vertical transmission---prior to fertilization.  Variations occur in timing, seasonality, larval transport across seas, or locally, or other parameters of these interesting life cycles.  

Millepora spp. are tropical hydroids with superpowers.  Like reef-building scleractinians, they are composite organisms, holobionts, known in earlier ages as "zoophytes," due to their dual animal and plant natures.  When Linnaeus established three divisions of  the Kingdom of Nature, rocks, plants and animals, he stated  "all three exist in the lithophytes", the corals(1).  In Milleporids, we see the integrations of all three, unlike the scleractinians.

Together with the related family  Sylasteridae, they comprise the hydrocorals.  Milleporids' partners, the Zooxanthellae, give them a special power: the ability to produce robust Calcium Carbonate skeletons.  The life cycles of hydrocorals are far more complex than those of scleractinians, especially of Milleporids.  They exemplify alternation of generations between polyp and medusoid stages.  Their Zooxanthellae are transmitted vertically, from mother to egg.  In a symphony of inter-related, integrated cycles, the development of  medusoid and gametes,  modifications of the  skeleton, acquisition by the egg of  Zooxanthellahttps://millepora.blogspot.com/2024/06/structure-of-ampullae-of-millepora-spp.htmle; ampullae, temporary modifications of the skeleton are tailored to protect the growing medusoid; and, ultimately the Calcium Carbonate covering of the ampulla dissolves, allowing the escape of the medusoid with it's ripe gametes.  

 In previous posts have been presented photomicrographs illustrating the dissolution of the coverings of the ampullae ( Here and 

 

 

https://millepora.blogspot.com/2024/06/structure-of-ampullae-of-millepora-spp.html

 

 

This "liberation" of medusoids occurs in a tightly synchronized dance among the colonies of a local population, and perhaps beyond,     Fertilization follows in rapid succession, for the medusoids live but for a few hours.  

Each of these processes is exquisitely timed and apparently synchronized with all the others,  at every step.  Reproduction is seasonal.  Colonies are gonochoristic (dioecious); therefore the liberation must be synchonized to ensure successful fertilization.  Apparently in response to lunar cues, this is timed for an evening a few nights following full moon, just as it is for many other coral reef organisms.  Since medusoids are unfit for maintenance of life---having no mouth or digestive system, and no balance organs---spawning takes place soon after medusoids escape from their colonial hosts.  It has been widely claimed that they live for only a few hours;  I observed weakly pulsating medusoids in a dish the morning after collection.

With at least four parallel cycles occurring, in sync,  within the host  Over a period of time unknown to me, the medusoid---the gonozoid of the colony---begins to develop; gametogenesis---development of eggs or sperm---is initiated; and the colony begins to form an ampulla, their protective cavities with the skeleton.   Some  incredible cooperation must exist between the systems of this simple organism for all of these to proceed precisely at such a pace toward the ultimate moment of release of the medusoids.  This happens, moreover, in coordination with other members of the same population, and, apparently across a wider areas, or potentially even among members of a species in a geographical region.  These finer points are not understood, despite a daunting amount of previous or ongoing research in the Pacific and the Red Sea, Indian Ocean, and perhaps the Atlantic/Caribbean.  Among even further questions that remain to be understood is are questions about fertilization and larval transport.  Some progress has recently  been made in many of these areas; however, each question answered leads to a multitude of new ones.  

 We may marvel, unequivocally, at the symphony of multiple parts that are embraced by the life cycles of the species of Family Milleporidae.      

 Synchronized Developmental Processes 

 I have been focused lately on the process of the opening of ampullae, clearing the way for  medusoids to escape from the colony.  Here I will propose the existence of  four  tightly synchronized, independent processes---leaving aside for now the astounding synchrony between colonies, necessary for successful fertilization.

Sequences of events have been observed and reported.  A plethora of questions remain: what exogenous and endogenous Zeitgebers, what signalling systems allow this system to maintain seasonal timing, and then to coordinate gametogenesis, development of the medusoid, generation of the ampulla, both within the colony, and among colonies?  Which clocks are hard wired?  What environmental cues invoke the various steps: intitiation of gametogenesis to induction of zooxanthellae, to dissolution of the ampulla's cover,?  What final cues stimulate the exodus of blizzards of medusoids at some exactly timed moment---are they lunar?  If so, how does this happen?  Are they endogenous?  What signals are exchanged among the colonies of the neighborhood?  What signal invokes the induction of zooxanthellae into the medusoid?  Into the egg?  Are these externally coordinated?  What is the master clock?  How is sex determined?  Is a colony faithful to sex from year to year?  I wonder too, which ones are released first, male or female?  Do male medusae seek out colonies of females prior to release of the female medusae?  And do female meduoids respond to the amassing of male medusoids?  Do medusoids congregate at the surface as an automatic consequence of the location of gametes in the apex of the lumen of the medusoid?   

 

 

Background; the cycles

1. Gametogenesis (initiation, seasonality)
2. Budding and growth of the medusoid, encapsuling the developing gametes
3.  Development of the gametes
4.  Creation and maintenance of the ampulla
5.  Invasion of the Zooxanthellae 
6.  Acquisition of Zooxanthellae by the egg (or infection?)
7.  Maturation of all components
8.  Commencement of dissolution of the cover of the ampulla
9.  (As I have observed) intensification of the brown color to a darker shade as liberation approaches
10.  Synchronized liberation, apparent lunar timing
11.  Spawning within hours or less

 

Each of these demands an entire chapter.  This is not a comprehensive list of the processes ongoing, and largely synchronized, at this time.  An incredible number of issues remain unresolved, in spite of numerous publications by researchers at independent sites.  We are attempting to understand a species living in an alien world, into which we have but limited access for observations, and processes that have only been examined through in vitro laboratory experiments.

It seems that Millepora spp. have been given short shrift in comparison with the more common Scleractinia.   

  For now I present some evidence and ideas concerning the process of dissolution of the covering of the ampulla. At the current time I am focusing on the opening of the ampullae, the protective pits surrounding the developing medusae.  Questions of timing are important in their own rite, as well as questions of synchrony.  Currently, I am looking at the amazing fact of demineralization, of the dissolution of the thin covering overlying the ampulla, exposing the mature medusoid and the gametes within, that they may escape to the outside world and join others to spawn, on the same evening, among other members of the population at a particular place.  

 

Maturation and Opening of the Ampulla 

 At the current time I am focusing on the opening of the ampullae, the protective pits surrounding the developing medusae.  Questions of timing are important in their own rite, as well as questions of synchrony.  Currently, I am looking at the amazing fact of demineralization, of the dissolution of the thin covering overlying the ampulla, exposing the mature medusoid and the gametes within, that they may escape to the outside world and join others to spawn, on the same evening, among other members of the population at a particular place.  

 ------------

 Footnotes

(1).  Wikipedia, "Zoophytes", accessed January, 2026. 

Escape of the medusoids: decalcification of the coverings of Ampullae of Millepora platyphylla

Fire corals, Millepora spp.,  cousins of the predominant Scleractinian reef corals, exhibit a more complicated reproductive strategy.   Scleractinians' life cycle is simple, devoid of the alternation of polyp and medusa generations for which the Cnidaria are well known.  In both cases the challenge is resolved by broadcasting either gametes or larvae into the water column, where oceanographic processes can either maintain them on the reef, or carry them to other reefs, sometimes nearby, but sometimes thousands of miles away.  This modality---using currents and tides as agents of dispersal--- is common among marine invertebrates, as well as fishes.   A dizzying number of variations on this scheme have made reproduction of marine animals---and plants---the subjects of hundreds of studies.

 Among Hydrozoa, alternation of generation is the dominant reproductive scheme.  Hydroids are typical examples, and within this group the variations are seemingly limitless.  Millepora spp. are hydrocorals, calcifying hydroids with a remarkable reproductive biology.   

 In 1984--1986, I studied Millepora spp.\ reproduction, on Guam with an eye to using cleaned skeletal specimens as proxies for understanding the reproductive periodicity.  Because Millepora platyphylla was abundant on Guam reefs, this species was the focus of my study.   

 Here, the focus is on a rare phenomenon in coral biology, decalcification, or skeletal modification.  In Millepora spp. this process plays an important role in enabling medusoids to escape their captivity in the ampullae, by degrading the covering to expose medusoids and allowing their escape.  Because medusoids only live a few hours, and it is they who are the adults, who spawn; and because of the challenges of synchronization to support successful mass spawning, harmonious timing of several process is demanded.  Development of the medusoid; maturation of the gametes; and degradation of the cover of the ampulla is must all reach a climax on a single evening---or possibly a few evenings, enabling an en masse liberation of medusoids.    

Another factor comes into play here.  Millepora spp., like hermatypic Scleractinian reef corals, are not simple animals: they are obligatory  holobionts.  Within their tissues dwell symbiotic dinoflagellates, their zooxanthellae.  In a wrinkle that is uncommon among reef holobionts, zooxanthellae are transmitted vertically, directly from parent to offspring: before fertilization, during the development of the egg within the medusoid vehicle, zooxanthellae are acquired by, or infect the egg.  This additional step demands an even greater level of synchronization.  

 Incorporate: 

Early naturalists noticed these peculiarities, and they were classified as Zoophytes---a living hybrid between animal and plant. Their animal nature demanded them to be classified among the animal kingdom, however, and eventually, it was recognized that the comprised a unique kind of amalgam, as animals with single-celled dinoflagellates living within them. This relationship is more than a friendship---the coral cannot survive without it's partner, and the partner is specially adapted for life within animals and animal cells. Such organisms are now identified as Holobionts, similar to Lichens, amalgams of fungus and alga partners.

 I have focused on these events in tissues of specimens I collected during two seasons in 1985 and 1986.  I say seasons, because one of the findings of this study as that reproduction does happen within specific months of the year, on Guam.  (I also found that another species, Millepora dichotoma, undergoes similar processes, in later months.  Populations of Millepora platyphylla undergo events of this nature over two or more months, in my experience, apparently synchronized with lunar cycles (the results of other workers differed as to timing, studying this and other species.). 

 

 

 Millepora spp., like Scleractinians, create massive Calcium Carbonae skeletons, although the size of each polyp is less than 1mm across. The name "Millepora" means, literally, thousand pores: the colony is peppered with numerous minute pores, ordinarily of two kinds: the gastropores---from which the larger of two types of polyps, the gastrozoids, the eating polyps, can access the exterior; and the dactylopores---home to the smaller dactylozoids, the stinging zooids. These two types are arranged in cyclosystems, a central gastropore encircled by five or six dactylopores. The colony, then, consists of one large organism comprised of a multitude of smaller specialized individuals, the zooids. Like other hydrozoans, Millepora spp. also have a third kind of zooid: a medusoid. This zooid develops only at certain times, and it does not have access to the exterior: this zooid is the host to developing gametes; it has two tasks: to swim away from the colony at the appropriate moment, and to spawn. Obviously, this must be the adult, similar to the butterfly; the literal colony serves is a relatively giant host of these minute adults.   Or maybe the metaphor breaks down.  

This adult, the medusoid, develops within a protective cocoon---the ampulla---a pit within the stony skeleton covered by a thin layer of calcium carbonate, protected against the external world and its multitudes of hungry mouths.  (I observed juvenile Oxymonacanthus sp. swarming, with a parent, around a colony of Millepora dichtoma with open ampullae---these gamete-laden vehicles would be nutritious morsels indeed.)  During develomental stages, the medusoid and gametes must, at the appropriate moment of its liberation, be able to escape.  This is made possible because the cover of the ampulla dissolves,  in concert with the maturation of the medusoid and gametes.  This synchronization is even more remarkably, bearing in mind that the ripening propagules of the entire colony are largely synchronized; and, moreover---since Millepora spp. are gonochoric---with those in the entire population (at least among the colonies that are ripening at the time).  In fact, when I surveyed other sites on Western Guam, on the morning after an event at Toguan Bay, it appeared that this event was synchronized on a larger scale, at least on Western Guam: at Fafai Beach and Asan Bay, at least, colonies bore evidence of recent liberation events. 

 
My study had focused on this unique character of Millepora spp., using presence, absence, and condition of the ampullae on collected specimens as proxies for reproductive condition.  My original intention---to statistically analyze t hese parameters across my collection of specimens---was rendered unnecessary on one evening in early April, 1985, when I observed swarms of minute medusoids in a ziploc bag.  I had noticed, a few days earlier at Fafai Beach that some of the colonies of Millepora platyphylla had turned to a darker shade of brown, and that minute white rings peppered their surfaces.   I now understand the white rings as evidence of the dissolving edge of a growing perforation in ampulla coverings.    

My study, at this moment, found a new focus: it was important to observe that 


A very important question arises: how are these developments all synchronized on the reef? Because Millepora spp. are gonochoristic---each colony bearing either male or female gametes---unless this process is tightly orchestrated, fertilization will fail. Not only that, but synchronized timing is necessary to ensure maximal efficiency in success of spawning.

==========================//==========================



I have observed medusoids swarming from a freshly collected piece of Millepora platyphylla on Guam. in a ziploc bag. This was at about sunset in March of 1985. My surveillance of Millepora spp. had been predicated upon the fact that the skeleton bears pits, ampullae, at times when the colony is reproductively active. The hallmark event of Millepora spp. reproduction is the escape of medusoids into the water column, where they spawn. I now had a data point: a significant date for a rep: roductive event. I was a little bit closer to understanding the reproductive timing of this species.

But during the development, of gamete bearing medusoids they are securely enclosed in protective pits with stony covers that may manifest---especially for some species---as obvious deformations about 1mm across, on the surface of the colony, among the normal cyclosystems of dactylopores and gastropores. How, then, is the medusoid to breach this covering?

I had observed minute white rings of <= 1mm in diamter scattered across surfaces of some of the surveilled colonies, a few days prior to the encounter with swarming medusoids. It occurred to me that I had observed an event, the opening of ampullae, to permit the medusoids access to leave the colony.



At this moment, my research experienced a paradigm shift: now that I understood that the spawning was a mass event, at a specific moment in time, I could begin to collect samples for microscopical study, including both skeletal fragments and, whereever possible, decalcified tissue samples from the same colony. Over the next year and a half, this was my focus.

The story of this research project is a protracted one. The study began in 1984, as a casual side-project. I was interested in reproductive periodicity, possible in Tridacnid clams; however, Tridacnids are large, they are prized targets for fishermen, and they were becoming increasingly rare around Guam, according to sources at the Marine Lab. In my first conversation with Dick Randall, this interest came up, and he mentioned Millepora spp. These species produced ampullae in their skeletons, but only when reproductively active. This looked like a perfect side-project, while I investigated other options. GIven the existence of markers of reproductive effort, pits in their skeletons, should I collect, perhaps, one or two fragments of Millepora spp. every snorkeling trip, after a year or two I would have sufficient material for a statistical analysis, to plot out the seasonality of this organism.

Bear in mind, I had no fore-knowledge of this species. My first task would be to begin collection. I developed a system of sorts. I marked each specimen with the date and place of collection, in pencil. I learned from Doug Markell to use a common white 5 gallon bucket in a childrens' swim ring---and later adopted a small inner tube, to two around behind me. In the bucket was a masonry hammer, ziploc bags, and---for the night snorkel---flashlights, at least two spares. Each specimen was placed in a ziploc bag. Back at the Marine Lab, they were thrown into Professor Randall's well-known blue tank of Calcium Hypochlorite---swimming pool chlorine---for cleaning, for a few days. After rinsing and drying, each was placed in a plastic bag and kept in a large drawer in my desk. It was an efficient, self-documenting system, not to mention that the existence of ready made proxies for reproductive condition in the skeletal fragments themselves.

The first collecting was done in August, before classes had even begun. By spring, a fair number of skeletal samples had been accrued. Things were moving along, with little effort required. I enjoyed many snorkeling trips---outside of classes, this was probably the central focus of my life---as I began the a priori task of learning the fauna and flora of Guam.*


On a fateful evning in April of 1985, I accompanied a few friends on an evening of spear fishing. They would spear and I would study the nocturnal reef life. I carried my bucket and inner tube, etc., and while they moved on ahead along the reef, I took my time, to study the amazing variety of crabs, fishes, the plankton, corals---and, of course, as I collected a few pieces of Millepora sp., marked them, plopped them into ziploc bags with sufficient sea water to maintain life, and tossed them into the bucket.

Back on the beach, I studied the specimens in the bags, and---to my amazement---observed a multitude of medusoids swarming around inside at least some of the bags. The next morning, Professor Eldredge verified the existence of a velum on these medusae, which were still weakly pulsating after perhaps 8 hours or longer. This was important, as Mayer, in his tour d' force study of world wide medusae, had erected a separate taxonomical category for Millepora---Medusae Milleporae---based on the then-accepted statements of Hickson, and possibly others, that the medusae of Millepora sp. did not possess a velum.

The side-project of mine was now gaining momentum. Indeed, that night in April, 1985, marked a turning point. You see, my original hope had been to study neurosecretory control of gametogenesis in, perhaps, clams. I had never studied microtechnique, but I had read widely on the work of Gabe and others on neurosecretion. I had chosen this area of research because of the relatively unsophisiticated technology required. A light microscope could be used in the field, I had reasoned, perhaps naively. More sophisticated techniques would require much more sophisticated equipment.

Now that at least one mass reproductive event (it is not spawning, in the first instance, although it has been tempting to use that term; rather, it was a mass liberation of medusae, themselves the adults, and subsequent spawning, within hours. The medusae, incapable of eating, was at the time believed to live no longer than three hours.
A doubling of effort
Millepora spp. are simple animals, right? They do experience gametogenesis, somehow, but what controls this? Should I try to move on to a study of neurosecretion, as I had planned. Knowing almost nothing---except what I had read in books---I adopted a new approach to collecting efforts, and began a study of microtechnique. Perhaps I could at least try Gabe's methods, even with this simple animal?


From now on, I would continue collecting, but each specimen would be divided into two: one piece for the cleaning tank, while the other would be immediately fixed on the beach, and later decalcified and embedded in paraffin. I used a number of methods for fixation and decalcification over the course of time.



Over time, I developed an approach to sectioning, staining, and mounting on slides. Professor Doug Smith had allowed me to enroll in an independent study of microtechnique in the well-supplied laboratory on upper campus. He had given me a key, and told me to give him a slide or two at the end of the term. I worked like a man possessed. That microscope lab was my home away from home.


I had moved into an apartment in Merizo, near my study site. I kept SCUBA tanks and a microscope in my apartment, and managed to collect fairly often, even when classes were in session.

When spring came around again, I was ready. I had marked some colonies at the collection site, in pencil. Millepora spp. lend themselves well to this approach. I failed to map these colonies, all of them in a fairly limited area. In the latter part of March, I was rewarded with another mass-liberation, and I went to work, collecting through the months. Both of these events happened a few days after Full Moon, in April, 1985, and March 1986. And each time I observed them after dusk, always in a Ziploc bag. The one time I observed an active event was at Pago Bay, when I observed medusoids bulging out of their ampullae; however, this event was not on the expected schedule, but several days outside of that window; I suspected at the time, that other factors interfered with what I had come to accept as "normal" timing.




After a long dormancy, slow but steady


In about 2002, Dr. Esther Peters encouraged my continued study, and kindly processed a number of my paraffin blocks from my field study of 1985-1986, sectioning, staining, and mounting them.

Years later, I was allowed to section four more blocks---out of about 200, the rest of which have not been processed as of 2025. I chose four, two of which, remarkably, bore medusoids, one female and one male.

Dr. Peters, referring to an image I had taken of one of the sections of the female medusoid, noticed the tissue lying above the medusoid, where the dissolving ampulla would be located, and that this tissue seemed to be in a state of decomposition. Another stage of this research was heralded by this finding.

I am now studying another slide of this same specimen.

Yamashiro and Yamazato presented a paper, in 1996, in which they describe phenomena similar to those observed in my specimen, in an in-depth study of the separation of natient disks from the stalk by which they are attached to the solitary "mushroom coral" Fungia fungites.



Armed with this new perspective, I have continued my microscopic study. The figure, here, shows what I bng cell Millepora spp. elieve to be Calicoblast cells and vesicles that, I propose, are involved in the dissolution of the Calcium Carbonate covering to which I referred above.







--------------

* In my letter of intent attached to my application for admissions to the Marine Biology program at UOG, I stated that my interest was not to earn a degree, but to learn the marine fauna and flora of Micronesia sufficiently well to support my lexicon and traditional knowledge project in Chuuk, FSM.

Stepping back: What are Millepora spp.?

 

Fire corals Millepora spp. are cousins of the predominant reef corals, the Anthozoan Scleractinians.  Scleractinian reproductive biology has attracted attention partly because of mass spawning and multispecies mass spawning events.  Such spawning behavior---synchronized broadcast of eggs and sperm into the "water column" is common in marine species, particularly species that are attached to the sea bottom.  Among coral reef species, broadcast spawning is a prevalent reproductive strategy, partly because the reef is a bulwark of hungry animals, so it is believed that by broadcasting eggs and sperm can facilitate their escape until they are competent to make their way to a reef---the same or even another reef at great distance---and claim their own living spaces.  The literature of coral reef invertebrate and fish reproduction is vast, presenting a panoply of fascinating and diverse life patterns.  

 Millepora species hold a special interest in regard to their life histories.  They hold a large amount of space on coral reefs, wherever they occur.  They live a complex lifestyle, as both consumering---meeting some proportion of their nutritional needs by capturing plankton; and produccers---through their partnership with photosynthesizing zooxanthellae, which provide them with a preponderance of their needs.  They, like other reef-building corals---which likewise could not thrive and build massive reef frameworks without their zooxanthella partners---take on characteristics of both plants and animals.  Like plants, they obtain energy from the sun, and they are immobile, living in the shallow waters where sufficient sunlight exists to support photosynthesis; and at the same time, they possess all the characteristics of animals: their polyps can snag prey, their cells are of the animal type, without cell walls or photosynthesizing plastics.  Early naturalists noticed these peculiarities, and they were classified as Zoophytes---a living hybrid between animal and plant.   Their animal nature demanded them to be classified among the animal kingdom, however, and eventually, it was recognized that the comprised a unique kind of amalgam, as animals with single-celled dinoflagellates living within them. This relationship is more than a friendship---the coral cannot survive without it's partner, and the partner is specially adapted for life within animals and animal cells.  Such organisms are now identified as Holobionts, similar to Lichens, amalgams of fungus and alga partners.  

 Millepora holds a special interest among "corals."  Their general nature is so similar that one might be forgiven for classifying them together with Scleractinia, but a closer leads to a completely different conclusion: these are Hydrozoans, where Scleractinians are Anthozoans, both belonging to the Phylum Cnidaria---known for their stinging cells, their cnidae, or nematocysts.  Moreover, the life cycles of these Hydrozoans is very unlike that of  their cousins,  as Millepora species exhibits the same Alternation of Generations that is characteristic of many comon hydroids, between polyp and medusa.  Like butterflies and caterpillars, these alternate stages seem completely unlike each other.   The reproductive cycle of Millepora spp. invokes a medusoid to carry the gametes---egg and sperm---away from the colony, into the water column where they spawn within a matter of hours, and die (like ephemeral insects).  

 Millepora spp., like Scleractinians, create massive Calcium Carbonae skeletons, although the size of each polyp is less than 1mm across.  The name "Millepora" means, literally, thousand pores: the colony is peppered with numerous minute pores, ordinarily of two kinds: the gastropores---from which the larger of two types of polyps, the gastrozoids, the eating polyps, can access the exterior; and the dactylopores---home to the smaller dactylozoids, the stinging zooids.  These two types are arranged in cyclosystems,   a central gastropore encircled by  five or six dactylopores.  The colony, then, consists of one large organism comprised of a multitude of smaller specialized individuals, the zooids.  Like other hydrozoans, Millepora spp. also have a third kind of zooid: a medusoid.  This zooid develops only at certain times, and it does not have access to the exterior: this   zooid is the host to developing gametes; it has two tasks: to swim away from the colony at the appropriate moment, and to spawn.  Pretty obviously, this must be the adult, like the butterfly, where the colony serves as the relatively giant host of these minute adults.  

 The medusoid develops within a protective cocoon---the ampulla---a pit within the stony skeleton covered by a thin layer of calcium carbonate, isolated from the external world.  The medusoid  cannot access the outside environment; but at the time of its liberation, it is essential that it escape.  This is made possible because the cover of the ampulla dissolves in concert with the development of the gametes and the maturation of the  medusoid itself, as the moment   for the medusoid to carry the gametes away.  Remarkably, this concert is synchronized among other medusoids on this colony, as well as on other, nearby colonies, at the appropriate moment.  

 My study focused on this unique character of Millepora spp., that as the egg and sperm have matured toward the point of readiness, the ampulla dissolves, making possible the escape of the medusoid.  By examining skeletal fragments, collected over a period of months, it should be possible to determine the time of reproductive activity.  

 

A very important question arises: how are these developments all synchronized on the reef?  Because Millepora spp. are gonochoristic---each colony bearing either male or female gametes---unless this process is tightly orchestrated, fertilization will fail.  Not only that, but synchronized timing is necessary to ensure maximal efficiency in success of spawning. 

It's that time of year. A perplexing question: when will be Millepora platyphylla first go off, on Guam, in Spring 2025?

 [This blog is intended to store keepers, both my own work, and found material.  ]

 [Update, 6 April 2025: My gut feeling trends toward the next mass release of medusoids on Guam about three to five days after the next Full Moon--between April 18 and 18, 2025.]  

 

It is the second day of spring, 2025.  I am in California.  In 2023 and 2024, I wrote lengthy pieces around my predictions for potential dates for Millepora platyphylla's reproductive output.  I still have received no affirmation or confutation of my predictions.  

2025 presents a baffling puzzle.  Let me explain briefly.

 In 1984, soon after my arrival at the University of Guam Marine Laboratory, to commence graduate study.  Professor Richard Randall took me aside and suggested that corals would make a good thesis study.   I told him I was interested in reproductive timing, and I probably told him that I was interested in Tridacnid clams.  He called attention to an important detail of the life history, the fact that the hard parts (the skeleton) of these species bear markers of reproductive status: ampullae, small pits in which develop the medusoids---the reproducing adult stage.  I could easily, so I reasoned, collect a few small fragments of these corals every time I snorkeled.  This would make an interesting side project, requiring little effort.  After a year or two, I would have sufficient fragments to study them and perform a statistical analysis of the times of reproductive effort.  

Indeed, I began collecting fragments, cleaning them in Professor Randall's blue tank of Calcium Hypochlorite (swimming pool bleach) on the lanai, behind his lab.  I enjoyed snorkeling, and did so frequently during the coming months.  Over time;  I visited a number of reefs.  Over time a number of fragments (each between a couple of inches and perhaps 8 inches across) accumulated over the months in a large drawer in my desk.  Each of them bore pencil markings recording the place and date of collection. 

In April of 1985, about 3 or 4 days after Full Moon, a fellow student invited me to join his party on a night fishing foray in Southern Guam, and I tagged along.  By this time I had developed an efficient technique for collecting my fragments.  A fellow student, Doug Markell, introduced me to the use of a child's swim ring as a flotation collar for the five gallon white buckets that were ubiquitous and  cheap at local hardware stores.   Spare flashlights and large Ziploc bags, a geologist's hammer, a  masonry chisel, and a handmade acrylic slate made up my kit.  I probably was a hindrance to my friend and his cousins, but by the time I left the water I brought a few bags of seawater and fragments to my car.  

I was amazed: as I prepared to treat the specimens, in the beam of the flashlight could be seen swarms of medusoids in some of the bags!  Statistics would not be necessary to determine at least one date of reproductive effort!  

This was the first event I was to observe.  The next morning, I visited several of my snorkeling spots along the West (leeward) coast of Guam, and at each site, evidence presented itself that indicated recent spawning.  It seemed to me that such an event would be tightly syncronized, to ensure successful fertilization.  (You see, each medusoid is either male or female, and I had read reports that these ephemeral adults did not more than a few hours.)  

I kept on it, collecting fragments into the summer and fall.   I observed the sibling species M. dichotoma, with medusoids popping out of their ampullaee, swarmed by young Oxymonacanthus sp. filefish.  Could it be that these fish timed their production of young to coincide with the availability of such a food source?  The mouths, after all, of these young fishes were undoubtedly the ideal size to suck the medusoids out of their holes. 

As it happened, my study of Millepora spp. intensified, and I began to learn to prepare the tissues for microscopic study.  I had spent time at UCSB, during my last year prior to graduation, learning about microscopic methods that, I reasoned, would be possible even on a remote island.  My interest in reproductive timing led me to research methods for study of neurosecretion, which involved release of hormones that stimulated the development of gametes, a process called gametogenesis.  The next year found me in the microscope laboratory, processing specimens of Millepora spp.  

And the next year, 1986, I was not disappointed.  Millepora platyphylla went off about 4 or 5 nights after Full Moon, but in March.  My collecting and study now became the entire focus of my time, even until I left Guam in November 1986.  

A hypothesis about timing.

These observations  led me to hypothesize that Millepora platyphylla released the medusoids in either late March or early April, three to five nights after Full Moon, just after dusk.  The date of this event would be some time during a window in the calendar, depending on when Full Moon fell.  I did not realize at the time that this mode of timing conincides in at least some details to the timing of many marine invertebrates, including Scleractinian corals, the kind that gets most of the attention, due to mass spawnings at various places around the world.  Easter, in fact, is reckoned by a similar algorithm, sometimes in March  and sometimes in April.

 Perplexing Question

 

This year Full Moon falls near the middle of March and again in April.  One is baffled.  

 

???

 

 

Structure of the ampullae of Millepora spp. is defined by the structure of the tissues.

An earlier post presented an image of the degradation of the tissue overlying the ampulla---and hence medusoid---of Millepora sp. cf. platyphylla.  An image of lower magnification of the identical slide shows the structure that has been compromised, toward the periphery of the hollow that is the ampulla.  Pylons remain beyond the central part of the disk.

Below, another image, this time of a male medusoid gives a better idea of the structure of tissues of an intact covering of an ampulla.  The pylon-like structure is clearly identifiable.  Individual spermatozoa are also clearly visible.

Ampullae of Millepora spp: Decalcification Prior to Liberation/Escape of Medusoids

This image signals a hallmark event in the life history of Millepora spp., as one of several propitious sections with views of a Millepora sp. (probably M. platyphylla) collected in 1986, likely at Toguan Bay, Guam.  This study derived from the plan to use the condition of hard-part specimens---presence of open ampullae---as an indicator of reproductive activity, as an approach to determining the timing of reproductive activity.  Once open ampullae had been observed, and certain signs in  colonies, in situ,  interest was aroused in histological study of the structural changes.  This slide represents a proof of concept of this approach.  e of  image is the beginning of a story, involving timing of reproductive activity of these spp.; development of medusoids---inside protective cavities, the ampullae; and the dissolution of the coverings of ampullae to allow the medusoids to escape.  

Millepora spp. are colonial calcifying hydroids, exhibiting alternation of generations; each colony bears either male or female medusoids. the "adult" form that spawns within a few hours of liberation into the water column, each of them less than 1mm in diameter.  In order to facilitate successful spawning between ephemeral medusoids, which live a number of hours, they must swarm in the water column en masse. This requires a tight synchrony in time of release.    

 

Distal part of medusoid, degrading covering, concomitant with decalicification of ampulla cover.  Two tentacles shown with developing nematocysts.