The first photo is a mating pair of Halgerda wasinensis nudibranchs. Nudibranchs get their name from the fact that their gills are exposed – ie “naked lungs”. The two stalk like appendages on their heads are called rhinophores (which are used as chemosensors) and the fuzzy looking appendage at the back of their body are their gills which also surround their anus. Nudibranchs are also called seaslugs and are very similar to land slugs. They move using a foot under their body and some of them even leave a “snail trail” which can be seen on the reef.
Nudibranchs are hermaphrodites with their sexual organ on the right side of their heads. They join sex organs and pass across sperm packets to each other. They store the others sperm until they are ready to fertilise their own eggs. The eggs are laid in mucous on the substrate. Most of them lay their eggs in a spawn ribbon which can be seen in the second photo. The ribbon is from the same species of nudibranch. This ribbon is about 2cm across and the eggs can clearly be seen. Once the eggs hatch they drift in the ocean until they come in contact with their food source where they settle and then turn into juveniles and then adults.
A number of nudibranchs eat toxic animals like sponges or stinging animals like hydroids which they absorb and then use as protection. Many of these nudibranchs have bright colours to warn the fish to leave them alone. Other nudibranchs rely on camouflage to hide away and others are nocturnal to avoid predators. Nudibranchs have a very short life span – varying from a month to 2 years depending on the species.
Sodwana currently is warm during the day with max temps of about 28°C and min temps of 18°C. Water temp is currently 27°C. No current with 12-15 metre visibility. Swell is about 2 metres.