As part of our ongoing series of Similan Island Marine Life, Wicked Diving presents:
Nudibranchs
This one is for all the real nerds out there. Just like us! At Wicked Diving we have a philosophy - anyone can find a Whale Shark but it takes talent to find a nudi. You have to have the eye, the patience and the passion. They are small, hard to find and usually fail to impress the beginning diver who expects Sharks and Whales everywhere they look.
But down there, under that coral, inside that sponge are some of the most beautiful of all the marine species. Nudibranchs are marine snails (gastropod molluscs) that have no shell at maturity.
Nudibranch means "naked gills" because their gills are on the rear of the back (posterior dorsal) of some species or along the side covered by a protective flap as these species tend to bury into sand or mud. The gills can be used for identification. Some species wave their gills as they move or feed.
Nudibranchs live in marine environments from Antartica to the tropics. Most spend their adult life on the bottom. Glaucus marginata and Glaucus altanticus float around (pelagic) upside down on the ocean surface. They feed on blue bottles or Portuguese man-of-war̢۪s (Physalia physalis) and By-the-wind sailors (Velella velella).Fiona pinnata feeds on goose barnacles that attach to flotsam and jetsam or Velella.
The largest species grow to 40cm, the smallest, microscopic and are easily able to move through grains of sand. Most are smaller than 10cm.
Nudibranchs appear to live for a maximum of one and a half years, depending on food and suitable conditions. At times there will be population explosions and droughts. When food is plentiful the numbers increase, as the food source is consumed or decreases for natural reasons numbers decline. Water temperature and climatic conditions may play a part in their survival chances.
The life style can be categorized as:
1. Nudibranchs that feed upon hydroids, etc., (ephemeral prey) tend to appear several times each year.
2. Those feeding upon sponges live longer, possibly beyond a year.
NUDIBRANCH FACTS
In most species, the gills are prominently displayed on their dorsal surface.
Some kinds of nudibranchs have a tuft of gills located on their backs at the posterior end, which are used only for respiration. Other kinds of nudibranchs have many finger-like structures on their backs called cerata, which function for gas exchange and often defense
Sea slugs have a pair of tentacles (called rhinophores) located on top of their heads, which biologists believe are used as sensory organs to assist in finding food and seeking a mate.
Many nudibranchs are brilliantly colored to warn predators of their dangers while others are subtly colored to match their background, thereby providing camouflage.
Nudibranchs are benthic organisms, meaning they live on the ocean bottom. They can be found crawling over rocks, seaweeds, sponges, corals and many other substrates.
All known nudibranchs are grazing carnivores and as a whole they feed on a wide variety of animals including sponges, hydroids, tunicates, anemones, corals, sea pens, bryozoans, barnacles, and sometimes other nudibranchs!
Each species of nudibranch tend to be very selective feeders, and may only eat one certain prey species
Most nudibranchs have a ribbon of teeth called a radula, the structure of which is usually adapted to the animal's particular prey items. For example, many dorid nudibranchs (Suborder Doridacea) have broad radulae with numerous teeth for grazing on sponges, while most aeolid nudibranchs (Suborder Aolidacea) have narrow radulae and strong jaws for feeding on hydroids and bryozoans.
Nudibranchs are simultaneous hermaphrodites, which means that they possess both male and female sex organs at the same time. This strategy increases the probability of finding a mate, since every mature individual of the same species is a potential partner (self-fertilization is very rare).
After mating, nudibranchs lay their egg masses either on or near the organism on which they feed. These egg masses vary in shape, size and color depending on species. Some sea slugs lay single coils of eggs, while others are in the shape of a thick ribbon wound into a spiral. The egg masses are often white, but they can also be red, pink, orange or any other colour depending on the species.
Egg development can take between 5 and 50 days, and is strongly influenced by temperature. Warmer waters generally result in a shorter embryonic period.
Usually the eggs develop first into a larval form called a veliger, which drifts in the ocean currents as plankton. Specific environmental conditions trigger the larvae to settle and metamorphose into the adult form. This larval dispersal is important in the successful exploitation of new areas, since adult nudibranchs move very slowly and cannot travel long distances.
Since nudibranchs have lost their protective shells, they require alternative means of defense.
The bright coloration seen in many species is believed to warn potential predators that the nudibranchs contain distasteful or even toxic compounds. Many of the dorid nudibranchs (Suborder Doridacea) are strongly scented, which is thought to be a chemical anti-predator warning.
Many sea slugs feed on sea anemones and hydroids (Phylum Cnidaria), which contain stinging cells called nematocysts. The nudibranchs are able to block the discharge of these nematocysts during digestion, and they are then passed to special storage sacs in the cerata. The adopted stinging cells form part of a defensive mechanism against any predators that attempt to prey on the nudibranchs.
Some nudibranchs can also swim short distances when disturbed by predators - they contract their body muscles and undulate through the water while flapping their cerata.
weird facts
Some nudibranchs in tropical waters feed on corals (Phylum Cnidaria), which contain photosynthetic single-celled algal symbionts called zooxanthellae. These nudibranchs are able to remove the algae intact from the coral's tissues and store them in their cerata, where the algae photosynthesize and provide the host nudibranch with a continuous supply of manufactured sugars.
Beachwalkers can search for nudibranchs in tidal pools at low tide - try looking under loose rocks or near animals that they are known to feed on (e.g. sponges and bryozoans). One of the best ways to locate nudibranchs is by spotting their egg masses, which are usually quite visible - the parents are likely nearby!
If you are lucky enough to find nudibranchs in tide pools or while diving, please do not attempt to pick-up or touch them. These creatures are very soft and easily damaged, and you may hurt them inadvertently.
http://www.seaslugforum.net/ This is THE center of the Universe for Nudi Nerds.
http://en.wikipedia.org/wiki/Nudibranch
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