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The Seashore


Topshells

These are an important group of grazers of micro-algae on the shore. The genus Gibbula is a common one but there are several others, so read on! Topshells

Purple Topshell (Gibbula umbilcalis), underside left, topside right

This is a large group of colourful seashells. The inside lip of the shell has a "Mother-of-Pearl" effect of multiple colours. The Purple Topshell (Gibbula umbilicalis) is probably the most widespread species and is around 15 mm tall. This topshell species has a hollow spiral through the centre of the shell (the hole can be seen when turned over). A horny plate, the operculum, closes across the shell opening reducing water loss at low tide. In topshells as a group, the operculum is round. In Periwinkles it is tear-drop shaped (elongated).

The Grey Topshell (Gibbula cineraria) is 12 mm tall, has thinner, dark grey stripes on a cream background and the hole beneath the shell is small. The Painted Topshell (Calliostoma zizyphinum) is a beautiful cone shaped shell.

The Painted Topshell (Calliostoma zizyphinum)

Painted Topshell spawning. This is stimulated when temperatures rise. Actual fertilsation is unclear as females lay strings of eggs on the shore and is different to the Gibbula by avoiding the plankton. Mini-adults hatch out on to the shore.

The Painted Topshell

The Painted Topshell has a very clean, shiney shell (compared to the other topshells). It lives on the lower shore, so desiccation is not a particular problem, and spends time with the foot moving over the shell keeping it clean.

Topshells are important herbivores. They eat microscopic algae, sporelings and almost certainly ingest organic debris in the process. The sexes are separate and fertilisation is external. The fertilised egg hatches into a planktonic larva (except Calliostoma). Grey Topshell lacks the tolerance to environmental stress like other Gibbula species and is found lower down the shore.

Grey topshell Grey topshell at the base of a kelp holdfast

Grey Topshells are intolerant of high temperatures, hence they are only found low down the shore, however, they are distributed all the way around the UK. Purple and Toothed Topshells are more tolerant of raised temperatures and are often abundant in the middle and upper parts of the shore. The latter two species are only found on the western parts of the British Isles where the sea temperature is raised by the influence of the Gulf Stream.

The Purple Topshell is abundant across the middle seashore on the various seaweeds. Painted Topshells are common on the lower shore, especially amongst holdfasts. The Grey Topshell is also a lower shore species. All live in rocky seashore habitats and are widespread across European coasts. All except the Purple Topshell are found in the Mediterranean

Toothed Topshell (Osilinus - formerly Monodonta- lineata)

Toothed Topshell (Osilinus - formerly Monodonta- lineata)

A large topshell, the toothed topshell has a small bump onthe undeerside protruding slightly into the aperture.

 

 


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