An unusual structure, and a conjecture, attached to a tissue section

 On one of the slides of Millepora sp., this unknown object was observed last night.  It reminded me of a possible egg or egg mass I had observed on M. platyphylla on Guam, probably in 1986.  

 

 

 

 [Next day: I am thinking about Foraminifera, but this is minute.  Perhaps a developmental stage?  Calcarea?]

 Coincidentally, the day after I first observed this object, I stumbled upon this description of the putative egg I observed on Guam.  I cannot validate the nature of either object, at this time. 

  
An egg cluster was observed on colonies of Millepora platyphylla on
Guam, in clusters the spacing of which seemed to mimics the pattern of pores of the corallum.  These clusters, extremely difficult to see
on the surface of the colony, seem an almost perfect example of
camouflage mimicry.

The egg or egg cluster resembled a doily with the spherical egg set in the middle. Each egg was attached to the surface of the corallite by root-like projections from the doily, which apparently is an  attachment device, and might, as well, protect the egg from nematocysts of Millepora platyphylla.

It is not known what kind of fish (sic) lays this egg. The spacing pattern of the egg cluster, the its near invisibility on Millepora platyphylla, and several repeated observations conspire ta suggest a specific association.

Thinking this to be a fish egg, I placed it in a vial, probably in ethanol, and passed it to Professor Steve Amesbury at UOG Marine Lab, the ichthyologist at the time.  I have not been able to locate that vial in recent correspondence.  

 

Further observations 

  Note that the darker spots, organized into an array are spaced similarly to what are possibly holes or pores in the paler dish-shaped structure.  Note the shaft of a nematocyst.   The mystery deepens.

 

 

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

 First: here's a phase contrast image of a medusoid, zoomed out, surrounded by tissue of the ampulla.  Notice the proximity to the surface.  The dip at the top is due to degrading tissue, accompanying decalcification of the cover of the ampulla.

 

 

A drawing from Hyman, The Invertebrates, Vol. 1. of a medusoid within the ampulla, in situ.   The other images here are from earlier in the cycle.  There is evidence of decalcification.  

The life cycle of Millepora spp. is a multipart symphony.  The developmental sequences of Zooxanthellae, the skeleton, the gametes, and the medusoid all must  proceed as one.  A large number of Zooxanthellae, below, are dividing, as they accumulate near the dense part of the ovum.  

Ex Hyman:  Medusoid within ampulla with ova.

 

 

 

 Now a panorama across a similar cross section on the same slide. 

 

 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 to occur.

 

Medusoid of Millepora sp.  The velum (v) projects inward from the umbrella margin.