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.  

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 Footnotes

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