|
Periwinkle Snails, Fingernail Limpets and
Buckshot Barnacles (G. Anderson)
Periwinkle
Snails
The little periwinkle snail prefers to crawl up above
the highest water level to the area that gets just the smallest splash from
the highest high tide waves. It has the record for the marine animal that
can stay out of the ocean the longest. Some remain above the splash of the
ocean for two to three months. Periwinkles (scientific name Littorina planaxis)
are rarely over three-quarters of an inch. Yet they come in an unbelievable
assortment of shell colors and patterns.
Periwinkle Snail shell diversity (G. Anderson)
From uniform dull gray to shiny shells checkered or striped with white,
these snails often cluster together in a crack or crevice. They secrete a special mucus around the opening to their shell. This
hardens, cementing them to the rocky shore. They spend days like this
without expending any energy - just 'hanging out' on the rocky surface.
Periwinkle Snails cemented to the rock (G. Anderson)
When they are hungry they emerge, first eating the
hardened mucus and then they crawl about leaving slime trails, like land
snails. They glide along; scraping the plant scum off the rocky surface
with a special structure in their mouth called a radula.
Periwinkle Snail crawling in search of food (G. Anderson)
This radula is common to many molluscs
and is similar to a mini chainsaw - having rows and rows of sharp, hooked
teeth for scraping. In fact, they are so efficient that they wear away the
rock in some areas, deepening the high intertidal pools. You can imagine
that the teeth get pretty dull quickly. This is no problem for a mollusc that
continues to produce new rows of radular teeth
its entire life - dropping off the old dull ones at the end of the radula. Periwinkles can replace up to seven rows
of teeth a day.
These critters are real couch potatoes - many only eat every two to three
weeks, spending the bulk of their life cemented to the rocks, with a rare
splash of seawater. Hot sun, rain and wind do not bother them. They have a
'door,' called an operculum, which keeps them protected. It is on their
tail and closes their body inside their shell while they are resting -
keeping in moisture on the driest days.
Periwinkle Snail underside showing operculum closed (G. Anderson)
Once a year they expend extra energy in reproduction.
These snails are separate sexed - the male needing to find a female for
mating. In this species, the males appear unable to distinguish the
opposite sex until actually trying to mate. In spring and summer males become very active - trying all neighbors
for a possible mate and even fighting. Sometimes two males will be fighting
over a third snail only to discover that the third snail is also a male.
Eventually they are successful and, after mating, the female lays her
fertilized eggs in a mucus bundle in high pools. These hatch as planktonic larvae and are taken
away by the extreme high high tide waves.
Fingernail
Limpets
Fingernail Limpets
(G. Anderson)
Only the size of a fingernail, the fingernail limpet (also called the rough
or ribbed limpet, Collisella scabra and Collisella
digitalis) is also a grazer, just like the periwinkle snail. Limpets are closely related to snails, but lack the coiled shell
and operculum. There are several species of fingernail limpets, some with
rough edges, and some with smooth edges.
Lacking an operculum, (the structure that periwinkles
use to avoid desiccation), these fingernail limpets have a neat trick to
avoid desiccation. They make the edge of their cap-shaped shell the exact
configuration of the rock where they live and just pull down for a tight
fit, keeping water inside. This special spot on the rock is
known as their home scar.
Fingernail Limpets sealed on their home scars (G. Anderson)
Fingernail Limpet species with
smooth edges (G. Anderson)
When covered with a high high
tide, these critters come out cruising. Having only about six hours each
day under the water, they travel along the rocky surface near their home
scar, scraping the algae off the rocks with their radula,
just like the periwinkles. After eating, these limpets usually return to
their home scar before the tide recedes, so they can pull down and seal in
moisture. It is unknown exactly how they find their home scar, but their
tentacles appear to be more important than their eyes.
Although they are separate sexed (like periwinkles) they
have no interest in mating. They have a unique reproductive mechanism,
called broadcast spawning, to ensure fertilization. This is
commonly found in many marine species and is accomplished by simply
releasing eggs and sperm into the ocean. The swirling ocean water is where
fertilization occurs. In order to assure fertilization, broadcast spawners release thousands to millions of eggs and
sperm with each spawning to ensure just one offspring. Limpets have a planktonic larval form, like periwinkle snails, that
develops from the fertilized egg.
Buckshot
Barnacles
No bigger than the buckshot in a shotgun shell, the buckshot barnacle, Chthamalus spp., can live the highest of all the
barnacles along our shoreline, often covering the rocks with over 8,000 per
square foot.
Buckshot barnacles (G. Anderson)
Each tiny barnacle is enclosed in grayish-colored
shells that can completely close. Once they begin life on the rock, they
cannot relocate as these shells are attached
permanently to their substrate.
When dead, the outside shell remains as an empty volcano until it degrades
but the body and closing shells fall out - leaving empty volcano shells for
a time. Without the living barnacle, the volcano shell eventually weakens
and falls off. You might wonder how so many can be found together in such a
severe environment. One reason is that few predators venture up here to eat
them. They can also close their shells to avoid desiccation.
Live barnacles with closed shells versus dead ones with empty shells
(G. Anderson)
They have an incredible reproductive style. They are what we call
hermaphroditic in biology, meaning that they are both male and female in
the same body. Each animal makes both eggs and sperm, but usually cannot
fertilize itself. Instead, barnacles
have an inflatable penis that is used in mating
with a neighbor. This penis can inflate and extend up to 2 inches from the
tiny barnacle. For many of these buckshot barnacles this is 20 times the
size of their full body!
Buckshot Barnacles, one with simulated inflated penis (G. Anderson)
They mate during all seasons, except winter, and each
may produce up to 16 broods a year. After mating, the barnacles' fertilized
eggs (usually several hundred to several thousand in each brood) develop to
a planktonic larval form that is
shed into the water. Most of these never survive as they get eaten by filter feeders in their planktonic
stage, or never find a place to settle and become an adult barnacle.
Lone Buckshot Barnacle (G Anderson)
Now what about the 'lone'
buckshot … without a neighbor to mate with? It seems that, in
nature, if a buckshot barnacle is greater than two inches from any other
buckshot then it can fertilize its own eggs (in-breeding).
|