Native Hawai'ian seagrass in the fishpond

While looking for fish and crabs along the seawall this weekend, I found a few small plants like these growing among in cracks in the makaha. They belong to the genus Halophila, which is the only true seagrass found in the Hawai'ian Islands, and they are delicate plants usually found in sand flats. Patches of Halophila grow in other parts of Kaneohe Bay, so it's possible that fragments or seeds and pollen from these neighboring plants got caught in the seawall and took root. This seagrass is also found in completely marine waters (instead of estuarine environments, like some other seagrasses), which goes to show how much marine influence there is on the ocean side of the fishpond.

There are two species of Halophila present in Hawai'i: Halophila hawaiiana, which is native to Hawai'i, and Halophila decipiens, which is from Thailand. The only way to distinguish the two species from one another is by checking for the tiny spines that grow on the outside edge of the leaves of H. decipiens. It was hard to see these leaves very well, but it looks like this plant is H. hawaiiana: a rare endemic seagrass occupying the fishpond! This has exciting implications for pond rehabilitation: maybe if limu removal projects are successful, native plants like this can take over those niches. Seagrasses stabilize sediment and provide food for fish and turtles.

Life at the Sea Wall

Water rushing over the sea wall on a rising tide. You can see the orange sponge Mycale sp. growing on the bricks under the water.

Much of the fishpond has calm waters with low visibility. However, when the tide is rising, water rushes through the cracks in the seawall, bringing in saltwater, plankton, and small fish... and making it possible to see for more than a few feet underwater. Many filter- and suspension-feeding organisms take advantage of the high-flow environment. Fish gather here to eat plankton and other small invertebrates. Below are a few snapshots of life at the ocean break during a high tide. Many of these organisms are marine, and not found further in at the pond. Among these is the puffer fish Arothron hispidus, a species that has been mysteriously dying around the Hawai'ian Islands.

Feather duster worms (Sabellestarte spectabilis) growing among sediment-laden Gracilaria near the ocean break. These marine suspension feeders are found in areas of the pond with high flow, salty water, and access to marine plankton.

Two alien species side by side: a colony of orange sponge next to a feather duster worm.

A white-spotted puffer fish (Arothron hispidus) in the sea wall. This individual is healthy, but others have been found dead in Kaneohe Bay and other parts of the Hawai'ian Islands. Dr. Thierry Work, a wildlife disease specialist from USGS, is studying what may be a virus affecting these fish.

A portrait of life at the makaha. Featherduster worms extend their tentacles in the foreground. Convict tangs, or manini, are eating plankton brought in by the high tide, and picking microalgae from the rocks and seaweeds.

A swimming anemone (Boloceroides mcmurrichi) nestled in Gracilaria near the wall. This species can live on sand, mud, or seaweed, and can swim away from predators by paddling with its tentacles.