The genus Nembrotha is a group of shell-less, small and colourful nudibranchs, that belongs to the family Polyceridae (Alder & Hancock, 1845) and that was first described by the German malacologist Bergh in 1877¹. It is comprised of twelve species, with two of them only recently described by Pola, Cervera and Gosliner in 2008 (Nembrotha rosannulata and Nembrotha aurea)², and that has Nembrotha kubaryana as the type species of the genus, becoming the most extensively studied organism in this group^2,3.

These hermaphrodite animals are endemic to the warm and shallow tropical and subtropical waters of the Indian and Pacific Ocean². They inhabit coral reefs and rocky benthos, where they prey only on a certain group of tunicates, the Atapozoa genus (Brewin, 1946)⁴. These nudibranchs, like many others, sequester toxic chemical compounds found on their prey and use it to their own advantage by producing a poisonous mucous layer on their bodies that fends off predators^4,5. They evolved their colourful body as a warning signalling system that communicates their toxicity to attackers⁵.

They sense their surroundings using two distinguishable horn-like chemoreceptors called rinophores, that are located on the top of their head². Another striking body feature is the external feather-like branchial plumes, located on the dorsal part of the body, and that the animals use to breathe².

Although some studies have been carried out on this particular genus of nudibranchs, there is still a considerable amount of information lacking on their ecology, behaviour and evolution, that makes these organisms intriguing and fascinating.

• Size and Colouration Patterns 

This genus of dorid opisthobranchs⁶ has a considerable variance on their length,  with some organisms being quite small, measuring as few as 5mm, to some of them being on the larger side of the spectrum, reaching sizes of up to 130mm, and anywhere in between². 

These small animals tend to catch people’s attention due to their brightly coloured bodies. This genus is no exception. The animals belonging to this group have a wide palette of colours that indicate their toxicity to possible predators (aposematism)⁷. In this particular genus, the most widespread coloration is the combination of dark greens and black, followed by browns, oranges and white/cream⁸. Colour patterning observed can be alternating longitudinal ridges and stripes of bright colours (Figure 1), as well as colourful round pustules all over the body⁸ (Figure 2). The distinctive patterning has also been used by researchers to further divide the genus into 2 informal sub-groups (longitudinal bands vs round pustules)². 

• External Morphology

The species of the genus Nembrotha are characterized by a limaciform body shape (slug-like), that is shell-less, robust and highly muscular^2,9,10 (Figure 2). They present a set of paired rinophores on the frontal part of the body, usually on top of their rounded head (Figure 1). The rinophores are retractable, hiding into a compartment-like structure called rinophoral sheath^2,9,10. The rinophores present the characteristic lamellae structure, that is use in many cases to ID the species¹¹, and that varies between 10 and 35 per rinophore² (Figure 1).  This genus is also characterized by the presence of 2, quite thick and circular, oral tentacles in the anterior part of the body^2,9,10 (Figure 1). A specific characteristic of dorid nudibranchs is the presence of bipinnate branchial plumes (gills), which are external and are involved in respiration¹². In Nembrotha spp, this structure is located in the dorsal, middle region of the body, and ranges between 2 to 5 plumes (Figure 2). This genus has branchial plumes that, compared to other dorid nudibranchs, are non-retractile^2,9,10. Moreover, the genital pore of these animals is located on the right side of the organism, between the gills and the rinophores^2,9,10. Likewise, the anus can be found in the middle of the gill circle, although it is observed on top of an structure called papilla^2,9,10. The posterior part of the foot is long and narrow, contrasting with the shorter and wider anterior-to-middle section of the body^2,9,10 (Figure 2). 

• Feeding, Defence and Predation

The feeding and defence mechanisms of this genus, like many other in the nudibranch order, are tightly linked together. This is due to the ability that Nembrotha species have of synthesising toxic chemical compounds from the toxins of the animals they prey upon^4,5,13. It is believed that this group of animals predate only upon very specific species of tunicates, more specifically species from the genus Atapozoa (Brewin, 1946)⁴. It is unclear if there are other preferred prey animals for this genus due to the lack of information on this ecological aspect. However, it has been suggested that they could predate on the bryozoan Bugula dentata (Lamouroux 1816)^4,14,15, as this species has been found to produce the same toxic metabolite found on atapozoids, that is in turn sequestered by nembrothids^4,14,15. Information on the predators of this genus is rather scarce, and there is a possibility that there are not any due to their toxicity. This is surprising, as their coloration and shell-less body is nothing but an invitation to be predated upon^5,16.  Furthermore, failed attempts of predation by sea anemones (family Edwardsiidae Andres, 1881) have been observed on N. lineolata¹⁷ (Figure 3). Reasons behind failed attempts are unclear. In addition, there has not been reports of cannibalism within the genus.

• Habitat

No specific habitat has been described for this group of animals. They can be found in the waters of the Indian and Pacific Ocean, from South Africa to Australia^2,8,10, suggesting a preference for warm tropical and sub-tropical conditions. The specimens that have been collected for different previous studies have been found on rocks and on coral reefs of this region^{2,5,6,8,10,17}. They have been observed and collected on varying depths, from just 2 metres and as far down as 44 metres deep^{2,5,6,8,10,17}, which indicates that there is no particular preference for bright or dark light conditions.

• Life History

These animals, like most nudibranchs,  are simultaneous hermaphrodites, that fertilize internally after mating¹⁸ and have a reduced lifespan of no longer than a year¹⁹. However, very little is known of the life history of the animals in this genus. The larval developmental stages, settlement and metamorphosis are yet to be described by malacologists.

• Reproduction 

Because of their short lifespan, nembrothids only reproduce once in their lifetime. Aside from this, little is known on the precise mating behaviour of this genus¹⁹. However, the similar hermaphroditic condition that is shared with most nudibranchs make a generalization of the mating mechanics possible. Nudibranchs align one next to each other, facing opposite directions until the gonophores of each animal are aligned reciprocally²⁰. The sperm transfer is reciprocal but there are very few cases of simultaneous fertilization²⁰. The exact method of sperm transfer is species specific and unknown for the genus¹⁹. Moreover, most species of nudibranchs have different mechanisms put in place to avoid self-fertilization, that are related to the method of sperm transfer of the animal^19,21. Eggs of this genus have the same representative spiral shape as the rest of nudibranchs and it has been visually documented (Figure 4)

• Locomotion

The specifics on the locomotion of this genus of animals have not been studied. However, it is safe to assume that crawling through the benthos is their primary locomotion method, especially since only very few species of nudibranchs have been reported to swim as well as crawling, swimming is a rarely documented behaviour in nudibranchs²². These animals crawl using two distinctive methods. Through cilia located underneath their foot to propel themselves over a layer of excreted mucous or through muscle use, where the foot of the animal attaches to substratum, then contracts and pulls the rest of the body²³. Animals of the genus Nembrotha would crawl through the reef using one of the aforementioned methods, or a combination of both.

• Detorsion 

Opisthobranchs undergo a curious developmental process by which they return to the most ancestral bauplan stage of the Mollusca. This process is called detorsion, and as its name indicates, it reverses the torsion undergone by the mantle in the evolutionary history of molluscs²⁴. This means that Nembrothids have a more classical bilateria distribution of their mantle and internal organs, rather than them being tightly compressed like in the more traditional gastropod bauplan²⁴. Detorsion is also the reason why the reproductive organs and anus are located on the right side of the body, instead of on top the head like in other gastropods. Detorsion is also believed to be the cause of the origination of the external gill system²⁴.

• Digestive System

The oral tube is the beginning of this system, that connects directly with the buccal mass, which is surrounded by two salivary glands that aid in the grinding of prey on the radula, and it is connected to the oesophagus of the animal². The entrance of the digestive tract is covered with a chitinous lining to protect the nudibranch from the defences of its tunicate prey². The rachidial teeth (repetitive sets of plates with denticles that make up the radula) (Figure 5) are mostly rectangular, but it can vary in shape for some species, with triangular denticles asymmetrically placed on the top². The laterals of the radula contain between 5 to 12 rows of triangular plates that work as denticles² (Figure 5). The digestive system is quite similar among species in the genus, with the only marked differences on the overall shape of the radula and the number and shape of denticles². Radula can be used as an identification method for the genus, but other morphological traits or molecular analysis are preferred^2,25. 

• Circulatory and Respiratory System

Information on the circulatory system of this genus is scarce. However, like other nudibranchs, they have an open circulatory system. That is, all the oxygenated hemolymph (transparent blood-like substance) is pumped out of the two-chambered heart into the hemocoel (body cavity where all organs are found)²⁴. The hemolymph also works as a transportation method for gas, nutrients and waste²⁴. Then, vein-like structures uptake the hemolymph and take it to the nephridium for waste control and then back to the gills for reoxygenation (for more information on respiration and gills, see external morphology)²⁴. Moreover, the hemocoel acts as a semi rigid hydrostatic skeleton for the animal²⁴.

• Reproductive System

This is probably the most extensively studied system of this genus, since it is used in the identification of species within genus and with other families¹⁹. The overall architecture of the reproductive system within the genus is quite similar if not identical¹⁹. Within genus, the only differences arise from the morphology of the penial spines and vagina of the animals¹⁹. The prostate gland and vas deferens are large and wrap around the bursa copulatrix but lack a vaginal gland¹⁹. The penis is encapsulated inside the vagina and it tends to be considerably larger than other nudibranchs’ (3mm)¹⁹. The main differences come from the shape, location and type of penial spines, that vary within the genus (Figure 6)². The spines are made of chitin, they can be basal or non-basal, long or short and located in different areas of the penis, in varying numbers. They are believed to serve an anchoring function inside the vagina if there is any type of disturbance during mating². The morphology of the vagina also varies within the genus, and it can be short, long, wide, straight or convoluted^2,19. 

• Nervous System

Although there are no specific studies on the nervous system of this genus, the nervous system of nudibranchs is quite a simple one. The cerebral ganglia and the pleural ganglia are fused together to form two cerebral-pleural ganglia²⁶. Additionally, this complex is connected to the pedal ganglia, and in some families of nudibranchs they fuse altogether²⁶. In some other families of nudibranchs, exists an extra set of ganglia called labial ganglia, which can be separately connected to the rest of the complex or fused as well²⁶.

Nudibranchs manage to sense their environment through two main anatomical features. These are rinophores and oral tentacles. Rinophores are located at the front, dorsal part of the head (Figure 1) and their main function is waterborne chemoreception and hydrodynamic sensing, that is achieved thanks to several flat gill-like structures called lamellae²⁷. On the other hand, oral tentacles, which are frontal, ventrally located (Figure 1), are in charge of contact chemoreception and mechanoreception²⁷. 

This group of nudibranchs can be found in the Indian and Pacific Ocean. More specifically, they have been found in Palau², Indonesia^9,46,(Bali⁴⁶), Mauritius Islands^43,47, Australia^2,44,47,48, (Great Barrier Reef^2,44,48, Western Australia^2,48, Northern Territory^47,48, Norfolk Island⁴⁸ and New South Wales²), Japan⁵ (Kerama Island² and Ryukyu Island²), Papua New Guinea^2,48, Marshal Island², Malaysia², Solomon Island², Philippines^2,9, Maldives^43,47, Fiji⁹, Seychelles⁴⁷, Comoros Island^2,49,50, Tanzania^2,49,50, South Africa^2,49,50 , the Red Sea^8,47,48, Vietnam⁴⁷, Mozambique^2,49,50. A map with more specific information on the sights of Nembrotha species can be seen below (Figure 7).

• Evolution and Phylogeny

The study of the evolution of nudibranchs is a complicated one²⁸. Because of the nature of their soft bodies, there are little to none fossil records available^6,28. This has resulted in their taxonomy being heavily based on morphological traits, which is not one hundred percent reliable all the time due to variability within same species⁶. However, in more recent years there have been more studies on the phylogenetic relationships of the Nembrothinae subfamily and their genomes^3,25, which has shed some light on the genetic relationships of the genus Nembrotha and its sister genera; Tambja and Roboastra (Figure 8). Thanks to genetic sequencing and aided with morphological traits, it has been concluded that Nembrotha is a monophyletic genus (Figure 8)²⁵. Genetic analyses have also supported the hypothesis of the existence of two subgroups (spotted and lined, see physical description) within the genus²⁵. The synapomorphies used to differentiate this genus from the others are: 

• weak labial cuticle with a strong internal edge²⁵ 
• prostate spreading over the bursa copulatrix²⁵

• Classification and Systematics

The taxonomic classification of the genus Nembrotha, and the species that belong to the genus, is presented below: 

Kingdom: Animalia (Linneaus, 1735)²⁹

Phylum:         Mollusca (Linneaus, 1758)³⁰

Class: Gastropoda (Cuvier, 1795)³¹

Subclass: Heterobranchia (Burmeister, 1837)³²

Infraclass: Euthyneura (Spengel, 1881)³³

Subterclass: Ringipleura³⁴

Superorder: Nudipleura (Wägele & Willan, 2000)³⁵

Order: Nudibranchia (Cuvier, 1817)³⁴

Suborder: Doridina (Bouchet, (2017)³⁴

Infraorder: Doridoidei (Bouchet, 2005)³⁶

Superfamily: Polyceroidea (Alder & Hancock, 1845)³⁷

Family:         Polyceridae (Alder & Hancock, 1845)³⁷

Subfamily: Nembrothinae (Burn, 1967)³⁸ 

Genus:         Nembrotha (Bergh, 1877)¹

Species: Nembrotha aurea (Pola, Cervera & Gosliner, 2008)²

                        Nembrotha chamberlaini (Gosliner & Behrens, 1997)⁹

      Nembrotha cristata (Bergh, 1877)¹

      Nembrotha kubaryana (Bergh, 1877)¹ 

      Nembrotha lineolata (Bergh, 1905)³⁹

      Nembrotha livingstonei (Allan, 1933)⁴⁰

      Nembrotha megalocera (Yonow, 1990)⁸

      Nembrotha milleri (Gosliner & Behrens, 1997)⁹

      Nembrotha mullineri (Gosliner & Behrens, 1997)⁹

      Nembrotha purpureolineata (O'Donoghue, 1924)⁴¹

      Nembrotha rosannulata (Pola, Cervera & Gosliner, 2008)²

      Nembrotha yonowae (Goethel & Debelius, 1992)⁴²

In past literature certain species names have been used as synonyms. To avoid any confusion they are as follows:

• Nembrotha guttata (Yonow, 1993)⁴³, as synonym of N. yonowae (Goethel & Debelius, 1992)^2,42.
• Nembrotha nigerrima (Bergh, 1877)¹, as synonym of N. kubaryana (Bergh, 1877)^1,2.
• Nembrotha rutilans (Pruvot-Fol, 1931)⁴⁴, as synonym of N. purpureolineata (O'Donoghue, 1924)^2,41.
• The species Nembrotha caerulea (Eliot, 1904)⁴⁵ is of nomen dubium and its taxonomical resolution is unclear².

The conservation of nudibranchs is an interesting topic, and this genus is no exception. Because of the lack of information and data regarding their local and global distribution, and their complex and short-lived lifecycles, there is little knowledge regarding their conservation status and population numbers⁵¹. Some of the more broad challenges they could face in the future are shared amongst other invertebrates groups, such as water pollution due to anthropogenic influence or habitat loss due to human development⁵². Additionally, any reduction on their specific prey can lead to extinction as research have shown they will not eat other prey⁵². Nonetheless, the presence of nudibranchs in a tropical ecosystem is used as an indicator of the health of such regions and can also be used to understand the impact of climate change on the system⁵¹.  This is because of their high diet specificity and high sensitivity to chemical and physical changes of the water⁵¹. 

This group of nudibranchs is of no economic importance to humans and as such, little studies have been conducted to understand their conservation status. However, more resources should be allocated on research of these invertebrates, not only because of their biodiversity value, but also because new studies have shown potential for their toxic mucous as component for new antibacterial substances and analgesic development²⁵.