Marine Invertebrates of Bermuda Zebra Slug (Hypselodoris Zebra) By Michael T. Collins Dr. James B. Wood - Editor Abstract  Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Abstract


Hypselodoris zebra is a rarely known nudibranch, commonly identified as the zebra slug, which roams the shallow tropical waters in search of sponges (Sterrer, 1992). This nudibranch is part of the Chromodorididae family which includes more than 300 members. It can only be found in Bermuda and the tropical West Atlantic and is usually located in sea-grass fields (Rudman, 1999). The zebra slug has brilliant hues of orange, yellow, and blue which are displayed uniquely between individuals. Their color scheme may be so different from one to the next that they could appear to be different species (Sterrer, 1992). It has foul-tasting chemicals in its mantle which protects it from being eaten by fish and other invertebrates. In combination with its chemical deterrents, it uses its vibrant coloration as a waning sign to ward off predators (Mcclintock, 1997). The deffensive traits that Hypselodoris zebra has evolved over time make it an interesting species to study.


Taxononmy


Phylum: Mollusca
  Class: Gastropoda
    Subclass: Opistobranchia
      Order: Nudibranchia
        Suborder: Doridina
          Family: Chromodorididae



Habitat


Within the subclass Opistobranchia, most nudibranchs are found in tropical regions (Karlsson, 2001). Yet, Hypselodoris zebra in particular is found only in Bermuda and the tropical West Atlantic (Rudman, 1999). You could find this species in grassy shallows up to 20 m where it feeds on Dysidae etherea, a species of Sponge. It lives among Thalassia grass fields in the intertidal zone where it slinks along in search of its prey (Sterrer, 1992).


Hypselodoris zebra is usually be found foraging and is dependant on the chemicals it derives from sponges for its defense. It is not usually hidden from view as it has unique defensive mechanisms that allow it to roam where it pleases without being disturbed by predators (Mcclintock et al., 1997). It can also be found out of grass beds as long as there is enough sponge around to support its feeding and chemical intake for defense mechanisms. This implies that H. zebra can be found anywhere in Bermuda and the tropical West Alantic that there are enough sponges which contain the chemicals it needs to derive for its defense.


Ecology


Hypselodoris zebra belongs to the subclass Opistobranchia along with some of the most unique and remarkable mollusk species which have adapted specific traits relating to foraging and defense. It is hermaphroditic, both male and female simultaneously, and spawns year round releasing gelatinous bands of red eggs that hatch as free swimming veliger larvae. This larvae stage is one common trait that links this nudibranch to other mollucks (Karlsson, 2001). All Gastropods undergo a process known as torsion during their early development. This is a 180° anticlockwise twisting of their body that leaves their posterior region positioned over their head (Pechenik, 2000). The more advanced Gastropods, most Opistobranchs in particular, have developed detorsion in their adult stage which undoes the torsion process leaving them with an elongated and unprotected body which contains all of its major organs on the right side (Karlsson, 2001). Unlike most mollusks, Opistobranchs also lack or have a reduced shell giving them a slug-like body form (Sterrer, 1986). This unique strategy may provide some advantages, but it has left H. zebra exposed to predators (Pechenik, 2000). Phylogenic research studying the reduction of the shell shows how this was an adaptation to allow mobility for exploration of new food sources, as this may be in correlation with its defense (Long, 1999). The loss of a heavy shell gives it the advantage to explore new habitats which are more difficult to reach when being protected by a shell (Wägele and Klussmann-Kolb, 2005). Along with the morphology of its detorsion response and shell loss, the most important and unique design is their chemical morphology and warning coloration which allows it to openly forage with almost complete immunity from fish predation (Long, 1999).


Hypselodoris zebra has an extensively long body that can reach lengths up to 18 cm making it one of the largest nudibranchs in its order (Rudman, 1999). It also possesses a large extruding foot on its oral surface and has a slight lateral compression. This design gives it a good hold on the substrate and allows it to glide along the bottom with some speed when compared to other Gastropods (Sterrer, 1992). H. zebra’s most intriguing and identifiable characteristic is its beautiful color patterns which serve as a warning sign to it’s defensive traits (Judd, 1998) It's body is composed of complex straight and wavy blue lines overtop of a yellow-orange background. It's notum has an irregular yellow lining, while the branchiae are dark blue and edged with yellow (Sterrer, 1986). Although it has been observed that the colors provide no actual defensive traits, it is a constant reminder to its attacker of its unpalatable taste.


Hypselodoris zebra has high concentrations of longifolin, a chemical stored in the exterior mantle region, which may be part of a defensive chemical system derived from the Ethereal Sponge, Dysidae etherea (Walters and Pawlik, 2005). When agitated it releases a bluish-white material which causes immediate negative reactions by almost all fish and invertebrates (Judd, 1998). As this secretion is seen to come from the mantle edge, and this is where the longifolin is stored, it is probable that this chemical is the source of its defense. Along with the secretions, H. zebra seems to have a foul odor even when not agitated. Most organisms will avoid or not come in close contact with it because of this. Although many people think that the colors of nudibranchs are what keeps predators away from it, the coloration is not a direct defensive factor as studied specimens that were stained red or blue were still avoided to the same extent (Judd, 1998). Their defense is based on chemical deterrents, rather than coloration, providing them with almost complete immunity (Wägele and Klussmann-Kolb, 2005). Despite the fact that their color was found to have no direct links to predation, it is said that startling colors “have arisen through internal forces under the immunity of the organism from the action of selection on its characteristics” (Judd, 1998). H. zebra may therefore use its chemical deterrents in combination with its brilliant warning coloration to successfully achieve immunity.


Recent Research


Some recent studies have been conducted on nudibranchs focusing on their phylogeny, chemoreception, and feeding preferences. A 2002 study of the nudibranch family Chromodorididae reviewed their comparative sperm ultrastructure to try and find linking evidence to fill in gaps in their phylogeny (Wilson and Healy, 2002). As some nudibranchs are very hard to link with others, this was an attempt to use untapped sources of informative traits to better describe and correctly decipher the genera of nudibranchs. They showed that some sperm traits, such as morphology of the nucleus, had substantial connections and could potentially be used as taxonomical information (Wilson and Healy, 2002).


Another study looked at the role of the dorsal horn of Ceratosoma nudibranchs as a defensive strategy (Mollo et al., 2004). They studied the concentrations of deterrent chemicals in the tissue of the nudibranchs and found that the highest concentrations were located near the mantle glands which is next to the horns. They supported the theory that the horns act as a defensive lure, attracting potential preadators to the most concentrated areas of its deterrent chemicals (Mollo et al., 2004).


Most species of nuibranchs are hypothesized to feed exclusively on a single species of prey. A recent study demonstrated that these hypotheses may be wrong (Gemballa and Franka, 2004). By videotaping Peltodoris atromaculata, the dotted sea slug, in the wild and in the lab, they showed that it feeds not only on the expected sponge species, but at least one other species of sponge with similar chemical defenses. This implies that chemicals used by nudibranchs as deterrents, which were once thought to come from one source, may come from a combination of multiple sources. This new information on the phlogeny, chemoreception, and feeding preferences of nudibranchs lets us better understand their ecology (Gemballa and Franka, 2004).


Commercial Importance


New chemical compounds are being derived from organisms to study their biological properties and possible importance (Faulkner, 1999). Sponges are the dominant source of newly discovered metabolites, but there is increasing interest that other organisms produce these kinds of compounds. Some recent studies show that some nudibranchs, such as Hypselodoris zebra, synthesize various chemicals for defense when they can not derive them from their diet. These chemicals are being extracted for in-depth investigations of their biological properties. From studying the chemicals produced by nudibranchs, there are properties that may have biomedical and ecological importance (Faulkner, 1999).


Bermuda Laws


Under the Fisheries Act passed in 1972 there were 29 marine protected areas created to give sanctuary to Bermuda's marine life (Wood and Jackson, 2005). They range from a radius of 300 to 1000 m and promote species diversity as well as overall increases in population number. Although these areas mainly enforce against commercial disturbance, they still provide safe havens. To further protect individual species, the Protected Species Act was passed in 2003. This gives the Department of Agriculture and Fisheries power to enforce against the taking of species that are not considered important to commercial or recreational fisheries. Hypselodoris zebra is safe in marine protected areas, and if it becomes endangered it could become specifically protected under these laws (Wood and Jackson, 2005).


Personal Interest


I became specifically interested in Hypselodoris zebra from the first time I observed it in the environment. Its brilliant coloration and ability to stand out in the open with almost complete immunity to attack is astounding. It also came to my attention that there has not been much research conducted on this specific species. Its fascinating behavior is enjoyable to watch, and I believe that their importance is underestimated. After working with H. zebra in a laboratory environment and studying them closely I have become intrigued and would like to continue working with them. As their feeding is not well understood, I am currently conducting tests to identify the species of sponge that they prefer to feed on, and if they will feed on other species of sponge when their main selection is removed. By extending my studies on Hypselodoris zebra I hope to discover new information that will inspire others to study nudibranchs as well.


References

Faulkner, J.D. (1999) Marine Natural Products. Nat. Prod. Rep. 16: 155-198

Gemballa, S; Franka S. (2004) Cytotoxic haplosclerid sponges preferred: a field study on the diet of the dotted sea slug Peltodoris atromaculata. Marine Biology. 144: 6: 1213-1222

Judd, T. (1998) The Sequestering of Secondary Compounds from Sponges by Nudibranchs. Article. Colorodo State University.

Karlsson, L. (2001) A Taxonomic and Biological Review with Emphasis on the Families Chromodorididae and Phyllidiidae Together With Field Notes From South East Sulawesi, Indonesia. Thesis. 52

Long, S.J. (1999) Additional Data on the food-preferences of the Nudibranch Facelina bostoniensis. Opistobranch Newsletter. 25:11:33

Mcclintock, B; Swenson D; Trapido-Rosenthal H; Banghart L. (1997) Ichthyodeterrent Properties of Lipophilic Extracts from Bermudian Sponges. Journal of Chemical Ecology. 23:6: 1607 - 1620

Mollo, E; Gavagnin, M; Carbone, M; Guo, Y; Cimino, G (2004) Chemical Studies on Indopacific Ceratosoma Nudibranchs Illuminate the Protective Role of their Dorsal Horn. Chemoecology. 15: 1: 31-36

Pechenik, J. A (2000) Biology of the Invertebrates. Boston: McGraw-Hill Companies, Inc. pp. 224-225

Rudman W.B. (1999) Hypselodoris zebra. Sea Slug Forum. http://www.seaslugforum.net/factsheet.cfm?base=hypszebr

Sterrer, W. (1992) Bermuda’s Marine Life. Bermuda: Black&white illustrations. pp. 145-146

Sterrer, W. (1986) Marine Flora and Fauna of Bermuda. New York: John Wiley & Sons. pp. 452

Wägele, H; Klussmann-Kolb, A. (2005) Opisthobranchia (Mollusca, Gastropoda) – more than just slimy slugs. Shell reduction and its implications on defence and foraging. Front. Zool. 2:3

Walters K.D.; Pawlik J.R. (2005) Is There a Trade-off Between Wound-healing and Chemical Defenses Among Caribbean Reef Sponge? Integ. Comp. Biol. 45: 352-358

Wilson, N.G; Healy, J.M. (2002) Comparitive Sperm Ultrastructure in Five Genera of the Nudibranch Family Chromodorididae. Journal of Molluscan Studies 68: 133-145

Wood, J.B; Jackson, K.J. (2005) Bermuda.  Caribbean Marine Biodiversity; the Known and the Unknown.In: Miloslavich P. and Klein E. Eds. Pennsylvania. DEStech Publications, Inc 19-36.


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