Jellies. Of course.

An aquarium crowd pleaser and anathema of the beach, jellies are by no means rare animals, yet they never cease to fascinate and terrify their human audience. Their simple design and alien appearance combine in awe-inspiring beauty – a diaphanous bag of cells operating at the most basic level of functionality. It is this simplicity of jellies that amazes. It is hard for us to believe that these are actually animals, yet they manage to thrive with the most primitive tissue structure, lacking viscera, and possessing only a rudimentary nervous system. The thick, brown sea nettles that contrast so attractively with a blue aquarium background and the heavy, dinner-plate shaped moon jellies that wash up on the beach are actually far more complex than many jellies in the ocean. Although they resemble their stinging cousins, the animals of Phylum Ctenophora, are usually classified separately from the more well-known jellies of Phylum Cnidaria.

At the New England Aquarium we distinguished the groups as true and comb jellies. The true jellies, members of Phylum Cnidaria, tend to have more robust bodies and possess venomous cells called nematocysts which can paralyze prey and sting a human interloper. The animals of Phylum Ctenophora are totally painless yet proliferate so aggressively that reaching into a tank feels like dipping your hand into a jar of, believe it or not, jelly. Comb jellies can bloom in extremely dense masses, flowing in to shore with the high tide where we would meet them with buckets, scooping them up with sore arms by the thousands. After an accidental introduction in the Black Sea, one type of comb jelly, Mnemiopsis, even caused the collapse of local fisheries by eating both fish larvae and fish prey and expanding to a devastating population size.

Comb jellies come in all shapes and sizes – from the marble sized gooseberry (Pleurobrachia) to the nearly flat, meter-long Venus’ Girdle (Cestum). The relatively few species of the phylum all have transparent bodies that appear as fine as a snowflake. The Venus’ Girdle, for instance, is so thin that divers can look through its body to see undistorted images on the other side. The only observable action of these animals is the rapid flutter of tiny cilia, organized in eight long stripes, which propel the animal through the water. Their swimming movement appears nearly effortless compared to the strenuous flex of a true jelly’s bell. The flutter of the cilia can also scatter light, creating a prismatic effect in many species. Most Ctenophore species are capable of bioluminescence, lighting up the dark ocean with flashes of blue and green. These nearly invisible oceanic lanterns add to the extraordinary experience of diving at night, which is at once strange and beautiful.

Read more about Phylum Ctenophora and one diver's encounter with the nighttime glow of Venus' Girdle here.

Photo: Mnemiopsis photographed by Herb Segars

Species Profile: Leopard Shark

Interested in San Francisco Bay wildlife? This is a species profile that I wrote on Triakis semifasciata, the Leopard Shark, for our Fall 2009 newsletter. This is the original, soon to be hacked away by the cruel hand of the redactor.

Where are Leopard Sharks found?
Leopard Sharks are found along the Pacific Coast - from Central Mexico to Oregon. One of the most common sharks in California, there is a large population of Leopard Sharks living in the San Francisco Bay. Most of them live in the Bay year round, with a few individuals migrating out in the fall. Leopard Sharks are often spotted near the bottom in the shallow waters of the sloughs and mudflats along the Bay margins. The Bay offers a safe haven for the Leopard Shark, because the water is too shallow and warm for predators, such as the Great White Shark. Plus the Leopard Shark finds abundant prey in the muddy bottom of the Bay. They often follow the high tide up to the shoreline to feed on animals in the shallow mudflats; then they move back out as the water recedes.

What do they look like?
The Leopard Shark is a slender fish with silvery-bronze skin and dark ovals arranged in neat rows across its back. Leopard Sharks are quite small. Their average length is around three to four feet, although they can grow up to seven feet. The Leopard Sharks in San Francisco Bay are more likely to be between two and three feet long.

What do they eat?
Leopard Sharks feast on small invertebrates, such as clams, worms and crabs that they find along the muddy bottom of the Bay. They also like small fish, eggs and the occasional Bat Ray. Sometimes the Leopard Shark can pluck prey right off the mud with its bottom-facing mouth. In other instances, the shark will shovel its nose into the bottom and toss the sediment away, exposing hidden clams or worms. Leopard Sharks may go to extreme lengths to eat small animals, but they will not attack humans.

How do they reproduce?
Leopard Sharks have ovoviviparous reproduction, which means that the baby sharks, or pups, develop in eggs that are retained within the mother’s body. The eggs hatch in the mother’s body, and then the pups are born live. A female Leopard Shark can have up to 29 pups in one litter.

What are the threats to Leopard Sharks in San Francisco Bay?
Leopard Sharks are fished both commercially and recreationally, with recreational fishing accounting for the majority of the catch. Even though it is an abundant species, Leopard Sharks grow so slowly that overfishing could deplete the population. Concerns about overfishing lead to the implementation of size limits by the Department of Fish and Game - Leopard Sharks smaller than 36 inches must be released. Leopard Sharks also contain high levels of mercury in their tissues. These animals have a greater exposure to mercury than other fish species because they spend so much time feeding in contaminated Bay sediments. It is unknown if the mercury is harmful to the Leopard Sharks, but it certainly exceeds the accepted safe limit for humans.

Photo credit: Peter J. Bryant

Hospitality Returning to the Bottom of the Bay

We must consider many factors before settling our lives into a new place. Whether it is the school system, safety or convenience of transportation, people have a list of requirements for their suitable habitat. It is the services and infrastructure after all that distinguish our comfortable communities from hard life on the frontier. Whether dispersing from their population or remaining exactly where they were born, organisms seek to fill similar needs when settling into their habitat. Some plants and animals, known as foundation species, are specially equipped to satisfy these requirements for many different species. By virtue of their physical characteristics, foundation species provide three-dimensional habitat for entire ecological communities, and in doing so dramatically enhance biodiversity in a relatively small area. Reef-building organisms like corals and oysters, trees that form a dense canopy layer, and the impenetrable underwater forests of kelp are all examples of foundation species that host robust ecosystems.

As we have learned from the worldwide destruction of coral reefs, declines in foundation species pose an enormous threat to their associated, denizen species. For the same reason, the protection and restoration of foundation species could be the most important step to maximize the conservation of global biodiversity. This conservation strategy is currently underway in San Francisco Bay, as collaborative effort between Audubon California, Save the Bay, NOAA, and San Francisco State University strives to restore and enhance one of the most important foundation species in San Francisco Bay – Zostera marina, or eelgrass.

Eelgrass is a type of seagrass – a marine flowering plant (not a seaweed!) that grows submerged in shallow water areas worldwide. Seagrasses form vast meadows in coastal environments that resemble terrestrial grasslands, their dense root system and tall leaf canopy creating complex habitat in areas that are otherwise unvegetated. Seagrass beds provide food, cover, and spawning ground for a wide array of invertebrates, fishes, birds, and mammals. Found in tropical as well as temperate regions, seagrasses play host to hundreds of associated species including green sea turtles, manatees, seahorses, and countless species of fish. As a biodiversity hotspot, seagrass beds also attract hordes of predators that come to feed on their residents. Unfortunately, seagrasses are declining worldwide at an astounding rate. Like all foundation species, the destruction of seagrasses can have severe impacts on the many associated species that rely on seagrass bed habitat. A recent study by A. Randall Hughes of UC Davis found that nearly 15% of all seagrass species worldwide are currently listed as threatened in some portion of their range. For every species of seagrass, there is at least one associated species of concern, and in total there are 74 species of concern that are associated with seagrasses worldwide. These results come from a preliminary study, however, and the true conservation costs of seagrass declines have likely been underestimated. Moreover, there is a general lack of awareness of the importance of seagrass bed ecosystems on biodiversity.

Over the past few decades within the San Francisco Bay, the size and number of eelgrass beds has been steadily declining, mostly due to reduced light availability. Like all plants, eelgrass requires sunlight for photosynthesis; however, conditions within the Bay have severely limited the depth to which light can penetrate the Bay water. Dredging operations in the Bay destroy eelgrass beds either by physical disturbance or by stirring up sediments to increase turbidity. Construction activities in the Bay watershed release sediments to streams that eventually reach the Bay, smothering eelgrass beds. Even if there is enough light for the plants to growth, excess nutrients can accelerate algae growth beyond the feeding rates of grazers, allowing the algae to over take eelgrass leaves and block out light.

Biologists of the eelgrass restoration team are putting in a great effort to enhance existing eelgrass beds and restore this habitat to the fullest extent of its San Francisco Bay range. Richardson Bay in North San Francisco Bay is the ideal location for eelgrass restoration. It harbors the second largest eelgrass bed in the estuary, with plants that have the greatest genetic diversity of all beds sampled. Given its sheltered location and distance from dredging operations, Richardson Bay has the model environmental conditions for large eelgrass beds, but the genetic diversity of its plants also gives hope for successful transplanting to other sites in the Bay. Using a variety of techniques, biologists are hoping to discover the restoration method with the highest success rate. Both mature plants and seedlings are transplanted to sites at four different depths where they are either hand-planted by divers or tied to special grid-like frames which sink to the muddy bottom. At the same time the team is modeling Bay circulation patterns to better understand the potential for seed dispersal from existing beds. Divers are collecting mature eelgrass flowers from donor beds and using them for targeted seed dispersal at other sites in the Bay. Seeds can be deployed either by hand or from mesh bags attached to buoys which hold the flowers and distribute the seeds in a circular pattern, moving with the current.

The restoration team and Bay Area naturalists alike hope that these efforts will reverse the Bay’s long-term decline in native biodiversity, giving us an example of the multi-layered ecosystem that this foundation species once supported. At the base of its complex food web, dense mats of eelgrass roots hold sediments in place and keep them well-oxygenated to support the growth of important bacteria. Eelgrass leaves provide substrate for algae and epiphytic plants which are grazed upon by a number of invertebrates. Even dead eelgrass plants that settle within the bed, develop a film of bacteria, fungus and detritus, which also feeds small invertebrates. During low tides, eelgrass beds hold moisture, so that these small organisms are protected. As a result, waterfowl arrive in droves to feast on a surfeit of invertebrate prey. The Pacific Herring, the largest commercial fishery in the Bay which has seen recent population declines, depends on eelgrass beds for spawning and cover. The herring lay their sticky eggs on eelgrass leaves so that the young will be protected until they reach maturity. The success of these restoration efforts could enhance the Pacific herring population, allowing for increased takes by local fishermen. Birders would also be happy to see the return of the diversity of native and migratory waterfowl that hunt the in eelgrass beds. To learn more about the progress of the San Francisco Bay eelgrass restoration, visit www.tiburonaudubon.org

Cited: Hughes, A. Randall, Susan L. Williams, Carlos M. Duarte, Kenneth L. Heck Jr., and Michelle Waycott. 2009. Associations of concern: declining seagrasses and threatened dependent species. Frontiers in Ecology and the Environment. 7:242-246

Photo credit: www.ceoe.udel.edu