Panaeolus on horse dung

I've been puzzling over these mushrooms, growing on a pile of horse dung on the outskirts of Small Dole, West Sussex.

The rounded caps – and the habitat – took my mind towards the Egghead Mottlegill Panaeolus semiovatus.

The jet-black spores would be right for Panaeolus...

And this young collection (left hand side), seems to be doing a pretty convincing half-egg impression.

The slightly sticky ('viscid') cap is evidenced by the little bits of detritus which have stuck themselves there.

Initially pale, several of the more mature fruit bodies had taken on a pale grey hue.

The dark brown gills have a white edge.

UPDATE: 27/08/2018 - Here's a better shot of some fresh fruit bodies I collected this afternoon.

New collection 27/08/2018

All of these features fit with P. semiovatus. Except for one thing. There is no ring on the stem. 

P. semiovatus should have a 'persistent' ascending ring around the stem. Some authors describe this as 'fragile' or 'fleeting', suggesting it could have got washed away. (I don't see how it can be 'persistent' AND 'fleeting', but hey.) Even on the younger specimen (left), I can detect no sign that a ring was ever present, which makes me doubt that my collection is P. semiovatus...

UPDATE: 27/08/2018 - Having been back for a second look, I can confirm none of the fruit bodies I've observed have a ring around the stem.

New collection 27/08/2018

Funga Nordica includes a description of a variety of this species, P. semiovatus var. phalaenarum which "differs in having a dentate or bordered cap margin and a ringless, longditudinally striate stem."

Well, my collection does show striations on the stem... Could it be P. semiovatus var. phalaenarum?

It has also been suggested to me that my collection could be Panaeolus antillarum: another species found on dung heaps which looks very much like this, described in Funga Nordica as "adventive in Europe, common in the tropics and sub-tropics".  There are just a couple of previous British records on the old FRDBI.

Time to get the microscope out.

Illustrations from Funga Nordica (2008) showing the difference in spore shape, cheilocystidia and 'sulphidia' (i.e. yellow pleurocystidia).

Spore shape

Mature spores (from spore print) mounted in water. 40x magnification + digital zoom.

 

Spore shape looks to be a better fit for P. antillarum.

Spore size

Mature spores (from spore print) mounted in water. 40x magnification + digital zoom. I only measured the biggest-looking ones which looked to be lying flat in my field of view (i.e. they were reasonably in focus from end to end).

Spore size fits with P. antillarum. I think this rules out P. semiovatus, as that species has larger spores (18-24 microns in length).

Sulphidia

'Sulphidia' are a new thing for me, described as "chrysocystidia-like pleurocystidia", i.e. yellow things located on the face of the gills.

I think I've found these:
 

And here's another one which I've stained with Melzer's reagent.

These also fit with the description of P. antillarum, having "stipitate-clavate" sulphidia. I found another description here (Wang, 2015) which talks about the sulphidia occasionally having a "broadly mucronate apex", i.e. they sometimes have a little pointy bit on the end (shown in the illustration from Funga Nordica, above). I think you can see that in my third image.

These sulphidia are around 50 - 60 microns in length. That is much longer than the length given in Funga Nordica for P. antillarum sulphidia (25 - 30 microns). However, Wang describes the sulphidia as being 38.8 - 60.1 microns; my observations are within that range.

Cheilocystidia

I haven't had much joy finding these. I staining a section of gill edge in PlaqSearch and have found some faint outlines of things which I think could be cheilocystidia.

Unfortunately my collection's turned to mush, so I think that's as far as I can get for now. I'll try and get permission to return to collect another specimen for my fungarium.

UPDATE: 27/08/2018 - I've got some fresh fruit bodies! 

Here is an image of the gill edge, stained with PlaqSearch and mounted in water. 40x magnification + digital zoom.

Loads of cheilocystidia! They look about right for P. antillarum. I think the blue blob must be another sulphidium. I'm never sure how to measure cheilocystidia, because it's so hard to see where the bottom is.

Basidia

My fresh sample is also showing some fine four-spored basidia.

The basidia aren't illustrated in Funga Nordica. There are line drawings in Wang (2015).

Reproduced from Wang (2015).

Not sure if it's down to the drawing or the specimen, but I can't help thinking the basidia I've observed are rather more elegant and symmetrical than those illustrated.

Caulocystidia

Stained with Congo Red and mounted in water. 40x magnification + digital zoom.

 

References
Knudsen, H. & Vesterholt, J. (2008). Funga Nordica
Wang, Yen-Wen. (2015). Dung-associated, Potentially Hallucinogenic Mushrooms from Taiwan. Taiwania. 60. 160-168. DOI: 10.6165/tai.2015.60.160.

I would also be interested to read the following article, so if anyone's able to email me a pdf that would be much appreciated!

Halama et al. (2014). An Adventive Panaeolus antillarum in Poland (Basidiomycota, Agaricales) with Notes on Its Taxonomy, Geographical Distribution, and Ecology. Cryptogamie, Mycologie 35(1):3-22. https://doi.org/10.7872/crym.v35.iss1.2014.3

For the record
Date: 23 August 2018
Location: Small Dole, West Sussex (private site)
Grid reference: TQ214139