Snails hoist yel­low flags

Canal Boat - - Waterside Wildlife -

yel­low flags in­clude Ja­cob’s Sword and Segg – the An­gloSaxon word for a short sword.

Up to three deep yel­low blooms per stem are pro­duced suc­ces­sively in the sum­mer months, each with three up­right petals (“stan­dards”) and three “falls” – coloured sepals that turn down to­wards the stem.

The flow­ers are pol­li­nated by bum­ble bees who fol­low the dark brown lines ra­di­at­ing along the large pe­tal-like sepals from the nec­tar source. Af­ter pol­li­na­tion, green seed cap­sules, look­ing like minia­ture cu­cum­bers, are formed which split when ripe to re­lease smooth, flat seeds – ideally shaped to float away with the cur­rent to a new site.

Irises also spread lo­cally by growth of the rhi­zomes (thick, un­der­ground stems) from which this peren­nial plant re-grows each year. The boiled roots were some­times used for sooth­ing bruises and cramps and were pow­dered to make snuff.

Many fresh­wa­ter snails are found among the tan­gled veg­e­ta­tion that grows along the banks of our canals and they can be spot­ted at the sur­face in sum­mer. The great pond snail (shown above) has a shell with a sharply pointed spire that can grow to 6 cm high; the ramshorn snail is smaller and has a flat, coiled shell. These two com­mon snails are both pul­monates that need to come to the sur­face to breathe in air through a clos­able breath­ing pore.

An­other group of wa­ter snails, the op­er­cu­lates, breathe through gills, like fish, tak­ing their oxy­gen di­rectly from the wa­ter. They can be recog­nised by a plate, known as an op­er­cu­lum, at­tached to their foot which they use to close their shells. They need run­ning fresh wa­ter, rich in oxy­gen, so are not found so com­monly in canals.

The shell of a snail has two layers: the horny outer layer pro­vides a wa­ter­proof cov­er­ing for the tough chalky layer be­neath. Snails do not need to shed their shells in or­der to grow but lay down new ma­te­rial at the mouth of the shell. Lit­tle wrin­kles or slightly dif­fer­ent patches of colour – a bit like tree rings – may show the bound­aries be­tween pe­ri­ods of dif­fer­ent rates of growth.

The diet of wa­ter snails largely com­prises the thin layer of mi­cro-or­gan­isms, such as al­gae, that grow over the un­der­wa­ter sur­face of veg­e­ta­tion. The tongue (radula) of a snail is cov­ered with rows of mi­cro­scopic teeth that ef­fi­ciently scour the al­gal film. De­cayed re­mains of pond weed are also eaten and the great pond snail will eat small, dead fish and soft­bod­ied in­ver­te­brates. The mouth of a snail is ac­tu­ally on its foot (the flat sole on which they glide along): hence gas­tro (mouth) pod (foot) – snails are mol­luscs from the class Gas­tropoda.

Snails are pro­lific breed­ers, which is as well since they are eaten by water­fowl, fish, newts, great div­ing bee­tles and even other pond snails. Each great pond snail (they are her­maph­ro­dite) lays up to about 100 eggs, at­tach­ing the large gelati­nous mass to weeds and other ob­jects.

En­joy the beau­ties and cu­riosi­ties of sum­mer.

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