|Plants living on the surface of the mud provide food for the herbivores. Several types of algae (sea lettuce, mermaids hair, and sea spaghetti) are easily seen while microscopic forms create a green-golden sheen on the mud flats.|
Sea lettuce, Ulva expansa, is a type of sea lettuce that can continue growing when a piece is broken and free-floating in quiet waters.
Sea lettuce growing in the mud flat. (GA image)
Mermaid's hair, Enteromorpha tubulosa, may cover large areas of the exposed mud.
Mermaid's hair growing in the mud flat. (GA image)
Sea spaghetti, Gracilariopsis sjoestedtii, grows half-buried in the mud and is a common food for many of the mud flat herbivores.
Sea Spaghetti growing in the mud flat. (GA image)
The golden sheen on the surface of the mud is a layer of diatoms. They are microscopic producers that photosynthesize. These mud flat diatoms are elongated (pennate) and come to the surface of the mud when the tide is out. They can fix atmospheric nitrogen so they contribute greatly to the productivity of the mud flat ecosystem.
Diatoms on the surface of the mud at low tide. (GA image)
Animals on the surface of the mud include snails, slugs, mussels and sand dollars.
Miniature barrel snails, Acteocina sp., are tiny gastropods (snails) related to slugs. They are found just under the surface of the mud and crawl along leaving tell-tale 'trails.' The shell is about the size of a grain of rice.
A barrel snail and its trail (left). Barrel snail eggs (right). (GA images)
Bubble snails, Haminoea sp., have a thin shell with just over one coil. They can be found plowing under the surface of the mud like the miniature barrel snail but they are much larger (a half inch or so). Their yellow jelly-like egg masses are often laid on the seaweeds growing on the surface of the mud (sea lettuce, mermaid's hair, or sea spaghetti).
Bubble snail with egg mass on sea lettuce (left). Bubble snail stretched out and crawling (right). (GA images)
A moon snail with its foot extended in the position it uses to plow through the mud. (GA image)
Moon snails, Polinices sp., are also surface plowers. They inflate the front part of their foot to act as a big plow. These snails are quite large, with shells as big as a tennis ball, and lay a characteristic egg case called a sand collar where they release their eggs and cement them together with sand grains. The developing eggs are between the sand grains. Moon snails are carnivorous, scraping a distinctive 'drill hole' in their prey (often clams) that they find just under the surface of the mud. This drill hole is funnel-shaped because of the scraping of the band of rough teeth (radula) in the mouth of the moon snail leaving a hole that is always wider at the top than at the bottom.
A moon snail (left), plowing in the mud, and its egg case(right), called a sand collar. (GA images)
Moon snail drill hole in a bivalve shell (above), notice how it is like a funnel, wider at the top than at the bottom. (GA image)
A Navanax slug. (GA image)
Predaceous sea slugs, Navanax inermis, up to almost a foot in length may be found crawling on the surface of the mud. These slugs prefer to eat other slugs (like nudibranchs) and are able to follow the slime trail of the nudibranch with chemical sensors in the front of their body. They have a suction-like mouth that sucks up the nudibranch prey (like opalescent sea slugs).
A Navanax slug on the mud flat with its yellow ink. (GA image)
Opalescent sea slugs, Hermissenda crassicornis, and other nudibranchs may be present seasonally on the mud flats.
Opalescent sea slugs. (Image, with permission, from Western Marine Lab)
Shag rug slugs, Aeolidia papillosa, are some of the nudibranchs that might be found on the mud.
The shag rug nudibranch. (GA image)
Sea hares, Aplysia californica, are often found in the mud. They are herbivores and consume the sea lettuce, mermaid's hair, and sea spaghetti that grows on the mud. The sea hare can get over a foot in length. If you tickle the back of the sea hare it may let out a purple ink as a smoke screen for itself. The egg masses of the sea hare look like piles of spaghetti noodles without the sauce.
A sea hare inking (left) and a sea hare with its spaghetti-like egg mass (right). (GA images)
Horse mussels, Modiolus rectus, are an unusual mussel species living in the mud instead of attached to a solid surface (as most other mussels species do). They are usually found with one end of their two shells sticking above the mud. Mussels are filter feeders and circulate water in and out of their shells at this exposed end.
A horse mussel on the mud flat. (GA image)
Tiny clams, Transennella tantilla, may be present at the surface of the mud.
Tiny clams, each line on the ruler is only a millimeter. (GA image)
Sand dollars, Dendraster excentricus, are abundant at the surface of the mud in some years. They tend to settle in groups, grow up, and disappear. Thus, when present in the mud flats, the sand dollar beds usually have sand dollars of all one size. It may be years between sets of these sand dollar beds when there are no sand dollar beds at all.
A sand dollar (left) and another one moving through the mud (right). (GA images)
Kids enjoying the mud. (GA image)