St. Tysilio's Trivias is a very interesting article about the two common species found in the UK

St. Tysilio's Trivias by John Orr

 

"I'm Trivia monacha (Da Costa, 1778). You can tell by my mottled mantle." "You can tell that I'm Trivia arctica (Montagu, 1803) by my transparent mantle."

Photos: Orr

LONDON  Menai Bridge is a sturdy Victorian edifice which spans the narrow, swift-flowing [water in the] straits between the Isle of Anglesey and Caernarvonshire in North Wales. Its web of slender spars and struts is a signpost for travellers heading north to Holyhead, where steamers ferry them across the choppy Irish Sea to the green hills of Ireland. It is also a landmark for naturalists in search of marine flora and fauna which abound along the rocky shore flanking this narrow channel.

Treacherous currents up to nine knots or more combine with swift riptides known locally as the "swellies." When in full spate, these chum the water into a cauldron of foam around islets and rocky shoals. Yet not least of the marine creatures thriving under such turbulent conditions are Trivia monacha (Da Costa, 1778) and Trivia arctica (Montagu, 1803) two of only four marine gastropods closely allied to true cowries known to inhabit UK waters. (The others are Erato voluta Montagu, 1803, and Simnia patula Pennant, 1777.)

Trivia monacha frequents more southerly regions, while T. arctica extends along rugged coasts as far north as Scotland where their empty shells are washed ashore in considerable numbers. Yet the living animals of both species are elusive, often dwelling in deeper water. Not so in the Menai Straits, however. There they can be found at low tide, very much alive and flourishing.

Both species share the same habitats beneath outcrops of heavy rocks which fringe small islands set in extensive mud flats. The mud is deep and tacky, and the water murky and there are forests of kelp and bladder weed. Under such conditions, hip-boot waders are more practical than wet suits; thick gloves and a stout hook are also useful for clearing weed and turning rocks under which the Trivia browse on spongy ascidians beside small cup-corals.

For some ecological reason, they seem unusually abundant around one particular island upon which stands a tiny fourteenth century Celtic church, set among neatly cropped grass and well-tended graves.

The church has another claim to fame besides being the guardian of defenseless Trivia. It was here that its founder, St. Tysilio  the son of seventh century Welsh rulers forsook the traditional family life of soldiering to seek solitude as a monk in AD 630. In terms of evolutionary time, 1350 years represents a mere tick of the clock.

Trivia probably flourished here in his day much as they do now, although the mud might have been less deep, the rocks fewer and the water clearer. Unlike other Welsh saints, who sailed the seas on floating oak leaves, St. Tysilio, between periods of sleep and meditation became a keen naturalist observing the teeming wild life of this isolated spot flocks of herons, cormorants, rare gulls and fishes – and who knows? – perhaps also mollusks. He may even have noticed striking color pattern differences between the animals of these two species, strangely neglected by successive authors who have otherwise taken great pains to faithfully record their specific shell features and functional morphology (Vayssre, 1923; Pelseneer, 1926; Lebour, 1933; Graham, 1949; Fretter, 1951).

The mantle of T. monacha, for instance, with its red-brown mottled coloring and noduled papillae, is quite different from the relatively transparent mantle of T. arctica, which sprouts needle-shape projections tipped with yellow. Moreover, by a strange paradox of nature the brown blotches which often pigment the mantle of artica's colorless shell appear more or less in the same position as the three "beauty spots" which ornament the shells of monacha. Then there are the conspicuous white gossamer stripes superimposed on the orange foot of monacha, yet absent on the pale-lemon foot of arctica.

The radulas are also different; denticles on admedian teeth of arctica are absent on those of monacha. So are their genital organs; arctica's penis is cylindrical, monacha's broad and flat. [T. ] arctica veligers have four-lobed velums, while monacha have only two. Unusual visible features which happens to be common to both species, however, are curious pearly spots which fleck their tentacles and siphons, though the functional role of these may be anybody's guess.



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