Seasons in the Sea - A month-by-month guide to Central California sea life

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Sea star: Image credit-Alaska Dept. of Fish and Game

Rocky intertidal areas

in May

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Central Coast tidepools are busy places in May. Red algae are lush from their early spring growth spurt, while brown algae are just reaching their stride. The result is a diverse algal community that provides a variety of types of food and shelter for young tide-pool animals.

Many intertidal animals are reproducing or getting ready to reproduce in May. Algae (especially red algae) are lush and diverse. Clouds of microscopic larvae--young barnacles and other animals that have been drifting around the ocean since early spring--are settling onto vacant spaces on intertidal rocks.

These settling larvae must compete with recently hatched young of grazing animals. May is the peak spawning period for many common tide-pool snails and their multi-shelled relatives, the chitons. Many of the snails lay eggs over a period of several months during late spring and early summer. Chitons, on the other hand, are broadcast spawners, who all spawn during the same week, or even the same day, usually in May or June.

With all this reproductive activity, by the end of May, a variety of young grazing animals, including periwinkles, chitons, and sea urchins, can be found crawling over or hiding under intertidal rocks, taking advantage of the late-spring seaweed salad bar. Hatching out at the same time as these young grazers is a new crop of young predatory snails, who will soon be drilling into the shells of and devouring their tidepool neighbors.

Sand moves toward shore, smothering some algae and animals

With the winter storm season essentially over in May, sand that was carried out into the kelp beds by winter waves begins to move back toward the shore. As pulses of sand moves across the shallow seafloor, they may fill surge channels and cover rocky flat areas.

As this sand moves toward shore it smothers and scours some of the algae that have colonized bare rock areas during the winter months. However, other algae, as well as surf grasses, thrive under such conditions of partial burial.

Algae grow lush and diverse

Algae grow lush in spring, despite in some cases, being partly buried by sand that is moving back toward the beaches from offshore. By May, perennial brown algae are growing rapidly, and will soon start shading some intertidal areas. Meanwhile, annual red algae, which put on their growth spurt earlier in spring have reached peak size, but have not yet begun to grow old and battered. With both brown and red algae growing well, May is a good time to see a diversity of algae in the tidepools.

Some of the red algae that have grown to full size by May or June include Iridaea flaccida, Iridaea cordata, Gastroclonium coulteri.

May also sees some species of algae that are adapted to partial sand burial, but may be absent or reduced to mere films on the rock at other times of year.

Some red algae thrives under sand

Some algae are well adapted to sand burial, and can survive an entire summer while buried beneath five or six feet of sand. One red alga (Ahnfeltiopsis linearis, formerly known as Gymnogongrus linearis) grows best when partially buried by sand. Its stout, ropy, branching blades withstand sand scour and grow back rapidly when worn down.

Although technically a perennial alga, Ahnfeltiopsis linearis essentially goes into hibernation during winter, growing little, if at all, during the winter months. With the increasing sunlight of spring, Ahnfeltiopsis, like many red algae, puts on a growth spurt. By May, the alga may already be entirely buried in the sand. By late summer, this amazing alga may be hidden under five or six feet of sand. Surprisingly, this alga not only tolerates being buried, but requires being buried much of the year in order to survive.

Callithamnion red algae also thrives being partially buried

Algae in the genus Callithamnion, also thrive while being partially buried by the return of sand to the beaches. This filamentous red alga puts on a growth spurt in early spring and becomes most luxuriant between May and July [Is it annual or perennial?].

Stiliger sea slugs lay eggs on Callithamnion

Living and grazing on Callithamnion red algae is a small (less then 3/8-inch-long) sea slug called Stiliger fuscovittatus. Like many sea slugs and nudibranchs, this sea slug is a specialist, which feeds on and makes its home mostly around a single type of algae. Not surprisingly, the slug's life cycle closely follows the seasonal changes in its host.

In this case, the slug, like the algae, becomes most abundant in between April and June. It lays eggs on the alga from May through July, which ensures that its young will hatch [release larvae or live young?] before the algae is buried as the beaches build up in late summer.

Periwinkles mate and lay eggs

Close-up view of two pairs of periwinkles mating. Each snail is less than 1/4 inch across.
(Source: Kim Fulton-Bennett)
Periwinkles (Littorina planaxis) are one of those amazingly hardy intertidal animals that are so common it's easy to overlook or ignore them altogether. But, as in dealing with many human beings, when you get into the details of their personal lives, you find that there's more than meets the eye.

For example, who would have thought that these dusky colored, pea-sized snails would spend March through July engaging in orgies of mass mating that make a San Francisco sex club look like a Boy Scout picnic.

As he often did in matters of this nature, the intertidal biologist Ed Rickets described periwinkle mating both succinctly and picturesquely. He pointed out that, although a few periwinkles may be observed mating at any time of year, during their mating season, “ is difficult to find, among thousands, a dozen specimens that are not copulating." Still, you might have to use a hand lens to tell the mating periwinkles from the merely sociable ones, because even under normal conditions, periwinkles cluster together in cracks and corners of the rocks at low tide.

Apparently, female periwinkles initiate the process. At or just before peak high tide the female periwinkles release chemicals into the water of their tidepools that induce males to climb up on the females' shells and try to mate with them. With hundreds of periwinkles sometimes inhabiting a single tidepool, it's easy to imagine how this process might get out of hand.

Another marine biologist (Morris) describes some of the complications that can arise during this mating frenzy:
"A male periwinkle, even when aroused, cannot distinguish the sex of another snail with which it is in contact, until it has attempted to mate with the partner... Sometimes two males attempt to mount the same snail at the same time. A battle follows, lasting [several] minutes, in which the two males push at each other with the front ends of their shells till one is dislodged. Usually such a battle occurs over a female, but occasionally over a male, when neither battler has bothered to determine the third party's sex before the battle.”

Perhaps this simply shows that, in both humans and mollusks, boys will be boys.

At dawn or dusk on the day after the orgy, each female periwinkles finds a cozy tide pool and extrudes several inch-long, worm-like egg capsules, each containing thousands of little eggs. At the highest high tide of the day, the capsules dissolve, releasing the eggs into the water. Those eggs that are not eaten by other tidepool animals will hatch two to nine days later, releasing tiny swimming snail larvae called veligers.

After an unknown period of time drifting in the plankton, the veligers will join their parents grazing on rocks that are only wetted by the highest tides. At extreme low tides, the periwinkles will sometimes move down to stay in the splash zone. But this puts them at risk of being eaten by carnivorous snails (who also breed at this time of year).

From an evolutionary standpoint, periwinkles are in the process of making their way up from the ocean to the land. They can survive two to three months out of the ocean, but will drown if forced to stay underwater for a long time [check how long].

Unlike many tidepool animals, periwinkles can survive in fresh as well as salt water. This is fortunate because some of their favorite foods are the green algae (Ulva and Enteromorpha) that grow where freshwater runoff flows across intertidal rocks. In addition to green algae, periwinkles graze on diatom films and bluegreen algae that grow on damp rocks.

As they scrape the algae with their file-like "tongues" (radula), periwinkles also wear away the rock on which they are grazing. In an area of relatively soft rocks and abundant periwinkles, one researcher estimated that periwinkles eroded a quarter of an inch from the surface of the rocks over a ten-year period.

Young predatory snails hatch from eggs

At the same time as tender little periwinkles are hatching out of their eggs, so are several types of carnivorous tide-pool snails, including unicorn snails, dogwinkles, and hornmouths.

Young unicorn snails hatch

Web page source:
Close-up view of two unicorn snails mating. These snails are about l/3 inch long.
(Source: Mila Zinkova / Creative Commons)
Despite their cute name and beautifully coiled shells, spotted unicorn snails (Acanthina puntulata) are avid carnivores that drill into other snails' shells and eat their insides.

Like periwinkles, spotted unicorn snails mate in large groups of 30 to 40 animals. In May or June you may see these wild unicorn-snail parties in mid-level tidepools or underneath mats or holdfasts of the brown alga Silvetia. After the party each female unicorn snail lays 400 to 500 eggs.

The bloodthirstiness of unicorn snails begins early. Within each batch of eggs, only about one in ten actually develop. Those unicorn snail embryos that do develop spend their time eating all their siblings, which are known as “nurse eggs.” After two to three weeks, the surviving embryos hatch, and out crawl tiny little unicorn snails, all ready to find and drill into hapless young periwinkles.

Each unicorn snail has a sharp spine on one end of shell, which it uses as a "can opener" on the shells of other intertidal animals, especially barnacles, turban snails, and periwinkles. Periwinkles do their best to stay up high, on dry rocks that are above the normal range of unicorn snails. However, periwinkles become fair game if they move closer to the water to feed or are swept off the upper rocks by wave splash.

When a unicorn snail approaches a group of turban snails or periwinkles, the grazing snails will try to "out-run" it by crawling as fast as they can in the opposite direction. As a last resort, if captured, a periwinkle may actually try to crawl up on top of the unicorn snail's shell, out of reach of its deadly spine and proboscis.

Both periwinkles and turban snails have hard, round trap doors called opercula, with which they can seal up their shells. However, the operculum is no match for a unicorn snail's file-like tongue, which can drill through the trap door with the help of special chemicals that dissolve the door.

Since both predators and prey are snails, the process of pursuit and consumption takes place in slow motion--A unicorn snail may spend 15 to 60 hours drilling into and eating a single turban snail. One wonders why the turban snail doesn't simply walk away during this process. Presumably if the turban snail opens up its operculum to run away, it becomes even more vulnerable. Also, there is some evidence that unicorn snails may secret a toxin that paralyzes their prey.

To feed on a barnacle (which can close up its shell but can't run away) the unicorn snail uses a more devious approach. At low tide, the snail crawls up on top of a barnacle whose shell is sealed to hold in moisture. When the tide rises, the barnacle eventually opens the armor-plated "doors" at the top of its shell in order to feed. At this point, the unicorn snail inserts its spine between the plates like a can-opener, keeping them open despite the barnacle’s efforts to close them. Finally the unicorn snail inserts its proboscis inside the barnacle shell and eats the animal inside of its own shell.

Young channeled dogwinkles hatch

Another predatory snail, the channeled dogwinkle (Nucella canaliculata) also breeds in late spring or summer. Like the unicorn snails, young dogwinkles emerge directly from eggs, without going through a drifting larval stage.

Channeled dogwinkles live a bit lower down in the tidepools than do unicorn snails, and eats mainly mussels, with a few barnacles on the side. Like unicorn snails, dogwinkles drill into mussels with their file-like radula and secrete chemicals that soften the mussel's shell—a process that takes a day or two. Since the mussel can't run away, apparently speed is no object. Like the mussels they feed on, dogwinkles often gather in large groups—hundreds of snails can sometimes be found in a single tidepool.

Young leafy hornmouths hatch

A third predatory snail, the leafy hornmouth (Ceratostoma foliata), lays its eggs in early spring (February or March). Hornmouths are typically solitary, but gather in large numbers to mate. After mating, each female hornmouth lays about 2000 eggs. The young snails develop inside their eggs for about 4 months. Like the other predatory snails, the young hornmouths hatch and are ready to drill into prey by late June or July.

Chitons spawn and settle onto rocks

Continuing a trend that started in April, many Central Coast chitons spawn in May and June. These include Stenoplax heathiana, Mopalia muscusa, Chaetopleura gemma, and Katharina species chitons.

In general, chitons that spawn earlier in spring are likely to feed on thin films of diatoms of microscopic algae, which colonize bare rock surfaces exposed by winter storms. In contrast, chitons that spawn later in spring mostly eat seaweeds. In either case, the diverse and abundant populations of intertidal algae of May and June provide plenty of food for young chitons.

Woody chitons spawn

Woody chitons (Mopalia lignosa) spawn throughout the spring, from February to June. These chitons typically graze on the fast-growing diatom films and sea lettuce that colonize exposed rock surfaces in spring.

Like most chitons, woody chitons lay eggs that hatch fairly quickly (after about 24 hours) to release microscopic larvae. Instead of drifting around for months, the larvae of woody chitons settle back to the sea floor within a week of hatching. This generally means that the young end up settling close to home--within a few miles (or less) of their parents.

Katharina chitons spawn

Katharina tunicata chitons spawn relatively early from March through May on the Central Coast (May until July in Washington state). This early spawning may make sense because young Katharina chitons feed mostly on diatom films, which are abundant on the bare rock surfaces available earlier in the spring.

Adult Katharina chitons live on top of tide-pool rocks instead of underneath them, and will even forage in direct sunlight. However, they are very well camouflaged. Their shells are almost entirely covered by a fuzzy "tunic," which looks exactly like the tufts of brown and red algae that the adult chitons eat (they will also munch on diatom films when larger algae are scarce).

Mossy Chitons spawn

Mossy chitons (Mopalia mucosa) spawn in April or May in Monterey Bay (July through September in Northern California). They spawn on calm spring evenings, when they gather in large tidepools to release eggs and sperm. The eggs hatch in about 20 hours, releasing microscopic larvae that swim for up to 12 days, until they find rocks with the appropriate red or green algae on which to settle.

Mossy chitons eat red algae such as Gigartina papillata or Endocladia muricata as well as green alga in the genus Cladophora. They also consume small animals that they come upon during their grazing. Their relatively long (3.5 inch), flat bodies are typically well camouflaged by a thick coat of algae, which gives these chitons their common name.

Mossy chitons spend their days resting in a home base, then head out to graze at night, when they and their grazing area are submerged by high tide. Like owl limpets, each chiton has its own grazing territory, which may be several feet across, and includes a set of pathways extending outward from the home base.

Mossy chitons can survive being buried by sand for days at a time. This may give young mossy chitons and advantage when colonizing bare rock surfaces that are present in early spring, but which will be covered by sand as spring turns to summer.

Stenoplax chitons spawn

Like many chitons, Stenoplax heathiana is a synchronous, broadcast spawner. All the chitons up and down a large stretch of coast release sperm and eggs on the same day, often within an hour of one another.

Stenoplax chitons in Central California usually spawn in May or June, during one of the early morning low tides of that occur at this time of year. This is when the tidepools are most calm because wave action is likely to be dissipated on offshore rocks and reefs. At this time of year, the mornings are typically calm and the sun is often obscured by fog. These conditions allow the chitons' eggs and sperm to remain and reach high concentrations in tide pools where the chitons are spawning.

Note: In some ways, chitons use an opposite spawning strategy from that of periwinkles, which release eggs at high tide, when their young are most likely to be carried out of the tidepools and dispersed by waves and currents.

As male chitons release sperm, this triggers females to lay their eggs. Each female lays between 100,000 and 200,000 eggs in a series of long, gelatinous strings. Often the females continue to lay eggs for several hours, but they stop as soon as rising tide disturbs the surface of their tide pools.

After about a week, the chiton eggs hatch, releasing tiny, swimming larvae. But these larvae aren't equipped to drift on the open sea. They swim just long enough to find the perfect rock or fragment of seaweed on which to settle. Once they find a home, the larvae bide their time for about two weeks, while they finish developing into miniature chitons.

Adult Stenoplax chitons are pale gray with spots, which helps them blend into their daytime hiding spots in the sand underneath intertidal rocks. At night, however, they emerge to feed on bits of loose algae that they find drifting around the tide pools (a feeding strategy similar to that of abalone and sea urchins).

Gumboot chitons spawn

Web page source:
A gumboot chiton, about eight inches long, grazing on algae.
(Source: Steve Lonhart / SIMoN NOAA )
Last but definitely not least, another chiton that spawns in spring is the aptly named gumboot chiton (Cryptochiton stelleri). With a fleshy, reddish-brown body that ranges from eight to twelve inches long, this animal really does resemble an old rubber boot, especially after it has been washed up on the shore (a common occurrence following winter storms).

Despite its amazing size, this chiton's reproductive habits are pretty similar to the other chitons described here--some time between March and May, all the female gumboots along a stretch of coast lay strings of reddish-brown eggs in spiral strings that can be up to three feet long. As wave action breaks up these strings, the male chitons all release sperm into the water.

The fertilized gumboot-chiton eggs hatch after 5 days, releasing microscopic larvae that swim around for only about 20 hours—long enough to spread themselves locally, but not enough to drift very far from shore.

Young gumboots settle down just below the lowest tide pools, where they happily chomp on red algae such as Gigartina, Iridaea, and Plocamium, as well as coralline red algae. All of these algae are typically lush and abundant in May. In a pinch, the young gumboots will also eat other common algae such as sea lettuce, giant kelp, and oar weed.

As the little gumboot chitons grow, they will move up in the tidepools to graze on algae growing on the sides of intertidal rocks and surge channels. Some adults live in deeper water, in the kelp beds, but migrate into the tidepools to spawn.

In addition to achieving impressive size (they are the largest chiton in the world), gumboot chitons live a very long time for a mollusk-up to 20 years.

Gumboot chiton scale worms spawn

Like those of many tide-pool animals, the bodies of gumboot chitons involuntarily provide living space for several species smaller animals. Among these uninvited guests is a scale worm (Arctonoe vittata) that lives within the gumboot chiton's palatial pallial grooves (which lie beneath the chiton's body, between the foot and the mantle). Growing to a few inches long, these scale worms colonize large intertidal animals such as gumboot chitons, keyhole limpets, and sea stars.

Like many other "commensal" organisms, the Arctonoe scale worm is believed to spawn at about the same time of year as its host—in spring or early summer. However, the larvae of the worms drift around in the open ocean for two or three months before returning to the tidepools in late summer or fall to find a home on some lucky gumboot chiton or other animal.

Perhaps the scale worms tend to leave their hosts so spawn in spring. By May, only a one out of four gumboot chitons have scale worms on their bodies. By August or September, when the new crop of scale worms is settling out of the plankton, one of all gumboot chitons will harbor scale worms. By November, about two thirds of the gumboots will have been colonized.

When a young scale worm first settles into a gumboot chiton or a keyhole limpet, it lives by nibbling on bits and pieces of algae and other drifting debris that come its way. In fact, because both host and "guest" feed on the same types of red algae, they both turn a similar reddish color over time.

There is so much room in the bodies of gumboot chitons that the scale worms sometimes share this space with several other types of animals, including a small pea crab. However, an Arctonoe scale worm will not share "its chiton" with other worms of the same species.

Arctonoe scaleworms may seem like freeloaders. But, in an impressive display of loyalty, not to mention self defense, a scale worm living inside a keyhole limpet has been observed defending its host against one of the most fearsome tide-pool predators—the Pisaster sea star.

Here's how it works: When a sea star tries to engulf a limpet inhabited by a scale worm, the worm moves around the base of the limpet (hidden under the edge of the limpet's shell) until it reaches the side nearest the sea star. Then sticks its head out from under the shell and bites the sea star’s tube feed or ambulacral area. According to the marine biologist who witnessed this process, it “usually causes the sea star to withdraw.” Such heroism is even more impressive when you consider that the scale worm is attacking an animal easily ten times its own size.

Black abalone spawn

Another herbivore that gathers in the tidepools to spawn in spring is the black abalone (Haliotis cracherodii). Like red sea urchins, black abalone may spawn twice a year—in spring, from April to June, and then in fall (August or September). Black abalone are synchronous broadcast spawners, with all the animals in an area releasing eggs and sperm at more or less the same time. A minimum population density of black abs is necessary for spawning to be successful. Unless there is at least one black abalone per square yard of seafloor, the clouds of tiny sperm released by the males may be dispersed before they reach the clouds of eggs released by the females.

Those abalone eggs that are fertilized and are not eaten by blue rockfish or filter feeders such as mussels, develop into tiny larvae that drift with the ocean currents for up to three months. This is in contrast to the larvae of red abalone, which only spend a week or two drifting in the plankton. In late fall, the young black abalone will settle near adults of their own kind, and spend most of their time hiding underneath boulders, eating thin films of bacteria. More on this in October.

Giant anemones spawn

Giant green anemones (Anthopleura xanthogrammica) start spawning around the peak of the upwelling season, beginning in late spring and continuing into summer. Male and female anemones release eggs and sperm. The eggs and sperm merge to form larvae that look like little jellyfish. These larvae spend months drifting around the open ocean, feeding on plankton. This allows giant anemones to spread their young over large stretches of coastline.

It is not known exactly how long the anemone larvae spend in the plankton. Once they are done drifting, they apparently settle (or at least survive well after settling) within dense beds of mussels. As the anemones mature, they move out of the mussel beds and down into surge channels just below the mussel beds.

Lying in wait at in these low intertidal areas, adult giant anemones are well positioned to catch hapless mussels, snails, or crabs that fall down from above. When turban snails and other animals “stampede” away from an encroaching sea star, they are likely to lose their grip on the rock and be washed off the rock by waves. Similarly, when a sea star pulls a clump of mussels from a rock, some mussels may be washed away by wave action, and fall into surge channels or lower tidepools.

Giant green anemones position themselves in just the right places (e.g. below dense mussel beds where sea stars hunt) to catch the hapless creatures falling from above. Giant anemones can survive being buried under sand for months at a time, and thus are well adapted to living in surge channels and lower tidepools that may be filled with sand during the summer and fall.

Although giant green anemones may not be as numerous as their smaller cloning cousins (the small, proliferating anemones), they make up for this in their longevity. If a giant anemone finds the right niche, it can live for up to a century.

Two deep-water animals (red octopus and plainfin midshipmen) move into the tidepools to spawn

Some deep-water animals, including red octopus and plainfin midshipmen, migrate into intertidal areas in spring to bear their young. Like coastal estuaries, tide pools (particularly in summer) contain an abundance of animal and algae to provide food for young predators. Tidepools also contain relatively warm water, which may help young animals develop and grow more rapidly than they would in deeper water.

Red octopus move into tidepools

A red octopus in the deep sea.
(Source: MBARI)
Along with the giant Pacific octopus, red octopus are the most common octopus along the Central California continental shelf. However, red octopus are much smaller, reaching an arm-span of only about 18 inches when they mature (at about two years old). Red octopus spend most of their lives hunting small animals on the continental shelf and slope. Some live in water that is thousands of feet deep.

Female red octopus mate for their first and only time during January or February of their second year. Between March and May, the female octopus move from deep water into the tidepools, where they will lay their eggs in August of September. Perhaps moving into the tidepools in spring allows the females to find plenty of snails and crustaceans to eat, giving them the extra energy the need to reproduce.

Plainfin midshipman spawn in tidepools

Like the red octopus, plainfin midshipman (aka toadfish, Porichthys notatus) spend most of their lives in deep water, but migrate into the tidepools to reproduce. These six-inch-long, inconspicuous fish normally live in sandy bottom areas of the continental shelf (as well as San Francisco Bay). They spend day time in burrows, and emerge at night to ambush small shrimp and crabs.

Attracted by the incessant crooning, the female midshipmen come to the nests and lay eggs attached to the underside of the protecting rock. The male then swims around the eggs and releases his sperm, fertilizing them.

For about a month afterward, the male will guard the nests against predators. By June or July the eggs will hatch, releasing tiny larval fish. However, instead of drifting off into the ocean as do the larvae of most fish, the midshipmen larvae remain attached to the underside of their protective rock until they are become juveniles, and can swim off into deeper water on their own.

Between April and June, the midshipmen migrate to the intertidal zone to lay their eggs. The male midshipmen arrive first and pick out well-protected nesting spots, typically underneath large boulders. Settling into their temporary love nests, the males emit a continuous croaking or humming sound to attract females. These love songs are so loud that they can sometimes be heard by nearby boaters (as well as by potential predators, such as harbor seals).

Attracted by the incessant crooning, female midshipmen come to the males' nests and lay clumps of 200 to 1,000 bright-yellow eggs. Each quarter-inch-diameter egg has a adhesive disk that allows the female to "glue" her eggs to the underside of a protecting rock. The male then swims around the eggs and releases his sperm, fertilizing them.

Apparently some male plainfin midshipmen do not build nests, but sneak up on the nests built by other males, then surreptitiously fertilize the eggs in these nests. Apparently these "sexually parasitic" males are able to get away with this because they are much smaller than the nesting males, and make grunting noises similar to those of sexually active female midshipmen.

For about a month after the eggs are laid, the male fans the eggs with his tail and pectoral fins to keep them well oxygenated, and guards them against predators. Nearly exposed at low tide, the nesting midshipmen are at risk of being eaten by a variety of birds, such as gulls, crows, and herons. The spawning midshipmen also provide a close-to-shore, easy-to-catch food for female harbor seals, who nurse their pups on Central Coast beaches at this time of year, and must find food for themselves without leaving their pups alone too long on shore. In June or July the midshipman eggs hatch, releasing inch-long larval fish. However, instead of drifting off into the ocean as do the larvae of most fish, midshipmen larvae remain attached to the underside of their protective rock until they develop into juveniles, at which time swim off to find homes in the sand of the continental shelf.

    All text © Kim Fulton-Bennett                About            Contact            Disclaimer