Aeolid Nudibranchs

Nudibranch. The very word strikes fear into the hearts of sponges and anemones.

Introduction

Aeolid Nudibranchs are a suborder of Nudibranchia, the largest order in the subclass Opistobranchia (Ellis 2001). Nudibranch (pronounced Nudi-brank), means naked gills. Those organisms in the subclass Opistobrachia are hermaphroditic and have the atrium of the heart posterior to the ventricle (McDonald 1999). The order Nudibranchia is characterized by lack of shell, mantle cavity, operculum, and ctenidial gills in the adult form (McDonald 1999). There are four suborders of Nudibranchia: Doridoidea, Aeolidoidea, Dendronotoidea, and Arminoidea (Ellis 2001). Aeolids take their name from the Greek god of the winds, Aeolus because of the waving of their cerata resembles streamers in the wind (Tackett and Tackett 2003).

Distribution:

Nudibranchs are marine slugs living in a range of environments from Antartica to the tropics (Ellis 2001).

Size:

The largest species grow to 40cm. The smallest are microscopic. Most are smaller than 10cm (Ellis 2001).

Life Span:

Nudibranchs live for a maximum of one and a half years. Their lifespan is variable depending on food and environmental conditions like water temperature and climate (Ellis 2001).

Diet:

All nudibranchs are carnivores. Nudibranchs can be parasitic, have a specific diet, feed off the eggs of other nudibranchs, or have a generalized diet (Ellis 2001). Aeolids usually feed on members of the phylum Cnidaria (sea anemones, corals, gorgonians, and jellyfish) (Bertsch 2004). A small group of aeolids use zooanthellae (microscopic green algae) in their bodies to make food (Ellis 2001).

Classification

Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Opistobranchia
Order: Nudibranchia
Suborder: Aeolidacea (Aeolidoidea)
Family: Flabellinidae, Eubranchidae, Aeolidiidae, Glaucidae, Embletoniidae, Tergipedidae and Fionidae

(McDonald 1999)

Anatomy

General Structure:

Aeolid nudibranchs are characterized by having long, narrow bodies with numerous finger-like projections, called cerata. They have a pair of oral tentacles, parapodial tentacles, and rhinophores on their head (McDonald 1999).

Mantle:

The shell is only present during the larval stage of all nudibranchs (Ajtai 2004). The mantle replaces the shell and operculum in adults (Ajtai 2004). The mantle of Aeolid nudibranchs is extended in long projections called cerata (Ajtai 2004).

Cerata:

Cerata are thin, finger-like projections of the digestive gland of the slug. Aeolids do not have any gills, so oxygen exchange occurs through the thin epidermis (Ellis 2001; Bertsch 2004). Some nudibranchs have photosynthetic zooxanthellae from the coral in its cerata (Tackett and Tackett 2003). The Cerata are also used in defense. The tips of some cerata can contain cnidosacs, which can store nematocysts obtained from cnidarian prey (Picton and Morrow 1994). Cerata can also be cast off when the nudibranch is alarmed (Tackett and Tackett 2003). The Cerata distract the predator by wiggling enabling the nudibranch to slip away unnoticed (Tackett and Tackett 2003).

Rhinophores:

Rhinophores are chemical sensors on the head that are used to detect chemicals in the water (Ellis 2001). These structures are primarily chemosensory (smell, taste) in function and can be used to locate food or other members of their species (Picton and Morrow 1994; Ellis 2001). The rhinophores can vary in shape between species of nudibranchs (Picton and Morrow 1994; Ellis 2001).

Oral (cephalic) Tentacles

These processes are sensory feelers used to help the slug feel its way over the terrain (Ellis 2001; Ajtai 2004).

Radula:

The radula is a ribbon of chitin on which rows of teeth are positioned (Ajtai 2004). The number of teeth and their size is correlated with prey specificity (Bertsch 2004). The radula can be useful in species identification; it can function similar to a fingerprint in the identification of species (Ajtai 2004). Aeolid teeth are meat-hook shaped and are in rows of one to three (Bertsch 2004). Although they have few teeth, their jaws are strong and well developed to hold on to cnidarian prey (Bertsch 2004).

Nudi Sex

Hypselodoris bennetti mating. Note the extended reproductive organ on the right hand side of both animals. Not aeolids! (Ellis 2001)

All opisthobranchs are simultaneous hermaphrodites; they have both female and male sex organs (Ellis 2001; Bertsch 2004). The external genitalia consist of a complicated tubing to eject autosperm, receive allosperm, and lay eggs (oviposition) (Bertsch 2004). The genitalia have become highly specialized to prevent self fertilization (Bertsch 2004). Nudibranchs usually copulate in pairs, with both functioning as male and female (Bertsch 2004). They must face opposite directions so that their genitalia (on the right side) are lined up (Bertsch 2004). Copulation can last for several hours, or be brief (Ellis 2001; Bertsch 2004). The sperm is stored until eggs develop, at which point fertilization occurs (Ellis 2001). Eggs are laid in large quanties in mucus sheathed masses in a variety of sizes, shapes and colors depending on the species (Bertsch 2004). Chemical defense can be used to protect the egg mass against predation, but nudibranchs have no active role in the rearing of the young (Ellis 2001). Egg masses can take the form of (1) ribbons, (2) cylindrical capsule filled cords, or (3) small kidney shaped jelly bags (Ellis 2001).

Interesting Information

Aeolid defense and Cnidarians:

Nudibranchs, with their lack of shell have found other means of protection. The aeolid nudibranchs have an interesting mechanism for deterring predators. Cnidaria, which the nudibranchs feed on, have stinging cells (nematocysts) that they use to stun potential prey and deter predators. When aeolids eat the tentacles of cnidarians, they swallow the nematocysts without firing them (Ellis 2001; Bertsch 2004). The nematocysts are then passed to the cerata, where they are stored in a compartment called a cnidosac where they are retained and used for defense (McDonald 1999; Ellis 2001; Bertsch 2004). According to Wayne Ellis (2001), it is believed to be possible to pass the nematocysts through the body and into the cerata because aeolids are physically but not yet physiologically mature.

Solar-Powered Nudibranch

The white nudibranch is a juvenile of Pteraeolidia ianthina. It has not developed its crop of zooxanthellae yet (Australian Museum 2 2005).

Some species of aeolids have an interesting technique for gaining food; these nudibranchs “farm” colonies of symbiotic zooxanthellae. These species feed on soft corals and store, instead of nematocysts, the zooxanthellae from the coral in their cerata (Tackett and Tackett 2003). The zooxanthellae can be observed in the cerata as small gold specks (Tackett and Tackett 2003). The zooxanthellae benefit from the protected environment and flourish providing the nudibranch with nutrients (Tackett and Tackett 2003).

Observe the zooxanthellae in the cerata of Phyllodesmium longicirrum (Australian Museum 2005).

Recommended Sites/ Fun Links

National Oceanic & Atmospheric Administration (NOAA).
The NOAA provides useful information on the oceans; coral reefs, tides, currents, buoys, marine sanctuaries, and estuaries. The site also provides information on protecting marine mammals and habitats. Although there is little information on nudibranchs, the NOAA has useful information about the ocean in general.
A movie on the NOAA of a nudibranch moving.

Department of Fisheries; Introduced Marine Aquatic Invaders
This site provides information on aquatic invasive species. It provides information on the distribution, mechanism of introduction, biology, habitat, impacts, control measures taken, and sources for further reading for the species. There are few nudibranchs in the listing, and it seems little is known about the issues surrounding invasive nudibranchs.

The Vibrant Sea: Underwater Photography of Jeffrey Rosenfeld.
This site is a gallery of photographs of nudibranchs. Jeffrey Rosenfeld has absolutely beautiful pictures. All the pictures are classified by genus and species and are even separated by location. This site while not having a lot of background information has artistic pictures that show nudibranchs in their natural habitat.

Literature Cited

Picton, B. & Morrow, C. 1994. A Field Guide to the Nudibranchs of the British Isles. Immel Publishing Ltd., 20 Berkeley Street, Berkeley Square, London W1X 5AE. ISBN 1-898162-05-0. http://www.pictonb.freeserve.co.uk/nudibranchs/ Accessed 2005 March 28.