The Japanese species epithet usagi means “rabbit”, and the common name “Rabbit Sap-sucking Slug” reflects the animal’s upright rolled rhinophores with distinctive black tips. Adults typically measure only 3–10 mm in length, making the species one of the smaller sacoglossans on Indo-Pacific reefs. Like its congener the Leaf Sheep, Costasiella usagi is easily overlooked despite its highly patterned dorsum, and finding one requires careful macro searching of Avrainvillea algae in shallow sheltered bays.

Sacoglossans belong to the order Sacoglossa, a distinct order of gastropod sea slugs separate from the nudibranchs, and share the specialised feeding mechanism of piercing individual algal cells with a needle-like radula and sucking out the cytoplasm. Costasiella usagi is an obligate specialist on green algae in the genus Avrainvillea, and like other members of Costasiella it sequesters functional chloroplasts from its algal food and retains them inside cells lining the digestive gland extensions in its cerata — the phenomenon called kleptoplasty.

This guide explores the biology, morphology, feeding, ecological role, and identification of Costasiella usagi, alongside the conservation and training context provided by Black Turtle Conservation on Koh Tao.

Common Names: Rabbit Sap-sucking Slug, Usagi Sea Slug, Rabbit Costasiella

Family: Costasiellidae Order: Sacoglossa Phylum: Mollusca

Learn About Costasiella usagi

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.
Night dive for nudibranch research during the Sea Slug Ecology course at Black Turtle Conservation, Koh Tao.
Close-up photo of a nudibranch taken during the Nudibranch & Sea Slug Ecology course at Black Turtle Conservation, Koh Tao.
Divers conducting sea slug monitoring surveys to study species diversity and distribution around Koh Tao.
Sea slug and nudibranch research surveys in Koh Tao, documenting marine biodiversity through underwater species monitoring dives.
Conservation divers performing underwater research and sea slug identification training in Koh Tao, Thailand.

Overview of the Species

Costasiella usagi is a member of the family Costasiellidae within the order Sacoglossa. The Sacoglossa are a distinct order of gastropod sea slugs characterised by a specialised radula bearing a single row of piercing teeth, herbivorous diets based on algal cell contents, and the widespread ability to sequester functional chloroplasts from their prey. They are separate from the nudibranchs despite being similarly informally called “sea slugs”.

The species was formally described by Masashi Ichikawa in 1993 in “Saccoglossa (Opisthobranchia) from the Ryukyu Islands”, published in the Publications of the Seto Marine Biological Laboratory. Ichikawa’s original description remains the primary reference for the species, and Costasiella usagi is often cited as the most readily recognised species among the multiple new Costasiella species described from Ryukyu waters in that paper.

Size and Appearance Highlights

Costasiella usagi is a very small sea slug, typically measuring 3–10 mm in length. Sources vary somewhat on the exact size range, with Ichikawa’s original description giving specimens of about 3–7 mm and other observers reporting individuals up to 10 mm. The overall body shape is short and stout, with a broad rounded head, dense clusters of cerata across the dorsum, and a small tapered tail.

The colour pattern is highly variable but recognisable. In Ichikawa’s original description the head and tail are unpigmented (translucent white or cream), the rhinophores are black, the median cerata across the middle of the dorsum are grey while the outer cerata along the flanks are black, and all cerata bear a distinctive white end capped with a black tip. Longitudinal striations run from the base of each ceras to the white apical portion, giving the animal an intricate patterned appearance. Some smaller specimens have the median cerata coloured green rather than grey, and colour variation between individuals can be significant.

Daily Behaviours and Activity Patterns

Costasiella usagi is a sedentary sea slug that spends its entire adult life on individual Avrainvillea algae. Movement is slow and deliberate along the algal blade, and the species does not swim, burrow, or actively hunt. When resting on the alga, the cerata are held upright and the intricate patterning of the dorsum becomes clearly visible.

Like all sacoglossans, Costasiella usagi is a simultaneous hermaphrodite. Mating produces small egg masses laid directly on the Avrainvillea host, so that newly settled juveniles have immediate access to their obligate food source.

Role in Koh Tao Marine Life

Around Koh Tao, Costasiella usagi is documented through ongoing photographic and video records compiled by Black Turtle Conservation divers, particularly on shallow sandy sites where Avrainvillea algae grow. The species joins Costasiella kuroshimae and other sacoglossan species that form a distinctive component of the island’s shallow soft-substrate invertebrate community.

As a specialist grazer of Avrainvillea, Costasiella usagi occupies the same broad ecological niche as its sister species C. kuroshimae, but the two species can co-occur on the same algal patches without direct competition thanks to their small size and the abundance of algal cells available for feeding.

Key Behavioural Adaptations

The most remarkable adaptation of Costasiella usagi, shared with other Costasiella species, is functional kleptoplasty. When the sea slug pierces individual Avrainvillea cells with its needle-like radula, chloroplasts are drawn into the digestive gland and retained inside specialised cells lining the digestive extensions that reach into the cerata. There the chloroplasts continue to photosynthesise, converting sunlight into chemical energy that supplements the sea slug’s diet.

This ability to maintain functional algal chloroplasts inside animal cells is unique to sacoglossans among all metazoans and represents one of the most striking cases of animal-plant integration in the natural world.

Costasiella usagi Morphology

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.

Body Shape and Texture

Costasiella usagi has the short, stout body typical of sacoglossans in the genus Costasiella. The head is broad and rounded, and the dorsum is densely covered with clusters of cerata that create the distinctive patterned appearance. The body proportions are quite different from those of aeolid nudibranchs, which have longer, more slender bodies with more sparsely arranged cerata.

The cerata themselves are finger-like projections containing extensions of the digestive gland. Each ceras houses the sequestered chloroplasts that give the cerata their varied colour, and the characteristic longitudinal striations described by Ichikawa run from the base of each ceras to the white apical portion capped with a black tip.

Sensory and Respiratory Structures

The rhinophores of Costasiella usagi are rolled and upright, standing from the top of the head like small ears. They are typically black in colour, and this distinctive dark pigmentation is one of the most reliable identification features for the species. The rhinophores function primarily in chemoreception, detecting chemical cues from the host algae.

Sacoglossans do not have discrete gills. Gas exchange occurs across the general body surface, with the extensive surface area provided by the dense cerata contributing significantly to respiratory function. The cerata therefore serve a triple role: digestive extension, respiratory surface, and photosynthetic factory.

Size Variations and Defensive Features

Juvenile Costasiella usagi are smaller than the adult 3–10 mm range and have less densely packed cerata. Smaller individuals often display green median cerata rather than the grey coloration of larger specimens, an ontogenetic pattern that may reflect accumulation of chloroplasts as the sea slug grows.

The principal defence of the species is crypsis. The complex ceras patterning blends against the algal host, and the slow sedentary lifestyle further reduces the sea slug’s visibility to visual predators. Some sacoglossans additionally sequester algal secondary metabolites as chemical deterrents, though the specific chemistry of Costasiella usagi has not been fully characterised in the peer-reviewed literature.

Costasiella usagi Diet and Feeding Strategy

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.
Night dive for nudibranch research during the Sea Slug Ecology course at Black Turtle Conservation, Koh Tao.
Close-up photo of a nudibranch taken during the Nudibranch & Sea Slug Ecology course at Black Turtle Conservation, Koh Tao.

Primary Food Sources

Costasiella usagi is reported to feed on green algae in the genus Avrainvillea, specifically Avrainvillea erecta and related species. Avrainvillea is a genus of calcified green macroalgae growing on fine sandy or muddy sediments in shallow, wave-protected areas from the intertidal zone down to about 30 metres depth. The sea slug is not documented off its algal host in any regular way.

Feeding Behaviour in Detail

Sacoglossans have a specialised feeding mechanism unlike any other sea slug group. The radula has been reduced to a single row of needle-like teeth used to pierce individual algal cells. Once a cell wall is punctured, the sea slug uses a muscular pharynx to suck out the cytoplasm and cell contents, including any chloroplasts. This “sap-sucking” feeding mode is the origin of the informal name “sap-sucking slug” used for the whole group.

Because the sea slug consumes only cytoplasm, a single Avrainvillea plant can support a Costasiella individual for extended periods. The sea slug moves slowly along the alga, piercing cell after cell, and its cerata gradually accumulate stored chloroplasts that continue to photosynthesise within the digestive gland extensions.

Chemical Defence Mechanisms

The specific chemical defences of Costasiella usagi have not been fully characterised in the peer-reviewed literature. Some sacoglossans sequester algal secondary metabolites and use them as chemical deterrents against fish predators, and it is likely that Costasiella usagi employs a similar strategy on a small scale.

The species’ principal defence is nevertheless cryptic colouration and sedentary behaviour on its algal host, both of which combine to make the animal extremely difficult for visual predators to spot in the field.

Activity Patterns and Photosynthetic Contribution

Costasiella usagi is active throughout the day and night on its Avrainvillea host, with feeding continuous during active periods. The sequestered chloroplasts continue to photosynthesise during daylight hours, contributing meaningful additional energy to the sea slug’s metabolism.

This dual energy source — direct cell-cytoplasm consumption plus photosynthesis from stored chloroplasts — makes Costasiella usagi, like other sacoglossans, one of the very few animals capable of harnessing sunlight for their own energy needs.

Costasiella usagi Substrate Preference

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.

Costasiella usagi is exclusively associated with its Avrainvillea host, which itself grows on fine sandy or muddy sediments in shallow, sheltered coastal waters. The species is therefore effectively restricted to habitats where these algae are abundant.

Why Substrate Matters

Avrainvillea algae require fine-grained soft sediment for their holdfast attachment and typically grow in wave-protected areas where sediment is not continuously disturbed. Costasiella usagi is found wherever these algae grow, and Japanese sources report the sea slug from sandy bottoms down to about 16–18 metres depth.

Around Koh Tao, BTC diver records of Costasiella usagi come from shallow sheltered bays with fine sandy substrate where Avrainvillea populations are well developed. The sea slug is essentially absent from open reef substrates or coarse sand habitats that do not support its algal host.

Response to Environmental Conditions

Costasiella usagi tolerates the warm, sheltered conditions typical of shallow Indo-Pacific bays and lagoons. The species is sensitive to any disturbance that reduces Avrainvillea populations, including physical damage to soft substrate, sedimentation from coastal runoff, and reduced light penetration that limits algal growth.

The species also depends on adequate sunlight to sustain photosynthesis by its sequestered chloroplasts, so shallow well-lit sites are preferred over deeper or shaded habitats.

Costasiella usagi Ecological Role(s)

Broader Reef Contributions

Costasiella usagi contributes to shallow soft-substrate community dynamics as a specialist grazer of Avrainvillea. By feeding on individual algal cells the sea slug exerts a small but persistent pressure on its host, and populations of the sea slug track the local abundance of the algae.

The species also contributes to the extraordinary story of sacoglossan kleptoplasty. Together with its more famous congener Costasiella kuroshimae, C. usagi illustrates how sea slugs in this family have integrated photosynthesis into an otherwise animal lifestyle, and studies of Costasiella species have contributed to understanding of how chloroplasts can be maintained functional inside animal cells.

Value as a Bioindicator

The presence of Costasiella usagi in shallow bays is a reliable indicator of healthy Avrainvillea populations and stable soft-substrate conditions. Because the species is small, cryptic, and exclusively tied to a single algal genus, it can serve as a fine-scale indicator of the state of shallow soft-substrate ecosystems that are often overlooked in broader reef monitoring programmes.

Declines in local Costasiella sightings can indicate reductions in Avrainvillea cover, which may in turn reflect broader problems such as sedimentation, nutrient enrichment, or physical disturbance of shallow sandy habitats.

Opportunities for Citizen Science

Costasiella usagi is a favourite subject for underwater macro photographers thanks to its intricate ceras patterning and manageable size. Records from Koh Tao dive sites add directly to BTC’s in-house species archive and to wider regional biodiversity datasets. Photographs documenting ceras colour pattern, rhinophore pigmentation, and the associated Avrainvillea host all provide useful data for regional distribution mapping.

Black Turtle Conservation supports this work through training in sacoglossan identification, ethical macro-photography of small subjects, and structured species recording during conservation dives.

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.
Night dive for nudibranch research during the Sea Slug Ecology course at Black Turtle Conservation, Koh Tao.
Close-up photo of a nudibranch taken during the Nudibranch & Sea Slug Ecology course at Black Turtle Conservation, Koh Tao.
Divers conducting sea slug monitoring surveys to study species diversity and distribution around Koh Tao.

Costasiella usagi Fun and Interesting Facts

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.

Discovery and Naming

The species was described in 1993 by Masashi Ichikawa in his study of sacoglossan opisthobranchs from the Ryukyu Islands, published in the Publications of the Seto Marine Biological Laboratory. The Japanese species epithet usagi means “rabbit”, referencing the animal’s upright rolled rhinophores that stand from the head like small rabbit ears. Ichikawa described several new Costasiella species in the same paper, but C. usagi is often cited as the most easily recognisable of them thanks to its distinctive black rhinophores and complex ceras patterning.

The Rabbit Sea Slug

The common name “Rabbit Sap-sucking Slug” directly translates the species epithet usagi, and the association with the rabbit imagery is a nice parallel to the more famous “Leaf Sheep” Costasiella kuroshimae. Both species are similarly diminutive and both are found on the same Avrainvillea host, and the two together have become popular subjects for the specialist macro-photography community.

Solar-Powered Sea Slug

Like other Costasiella species, Costasiella usagi is a functional kleptoplast: it sequesters chloroplasts from its Avrainvillea food and retains them inside cells lining the digestive gland extensions in the cerata. There the chloroplasts continue to photosynthesise, contributing meaningful additional energy to the sea slug’s metabolism. This makes Costasiella usagi one of the very few animals in the natural world that can harness sunlight directly for its own energy needs.

Colour Variation

Costasiella usagi is quite variable in colour. In Ichikawa’s original description the head and tail are unpigmented, the rhinophores are black, the median cerata are grey, and the outer cerata are black, all with white ends capped with black tips. However, smaller specimens can have the median cerata coloured green instead of grey, and populations from different localities can show noticeable colour variation. Longitudinal striations along the cerata are however a consistent feature, and photographs of individual cerata are the most reliable way to confirm identity.

Not a Nudibranch

Despite being widely called a “nudibranch” in popular media and even some field references, Costasiella usagi is not a nudibranch. It belongs to the order Sacoglossa, which is a separate order from Nudibranchia within the wider Heterobranchia. Sacoglossans differ from nudibranchs in radula structure (single row of piercing teeth versus rasping ribbon), diet (herbivorous versus predatory), and the presence of kleptoplasty. The confusion is understandable given the shared common name “sea slug” and superficial similarity, but the two groups are separated by tens of millions of years of evolutionary divergence.

Costasiella usagi Vulnerability Profile

Marine conservation diver photographing nudibranch species on Koh Tao coral reef as part of biodiversity documentation.

Costasiella usagi is currently classified as Not Evaluated on the IUCN Red List. As a small marine invertebrate with no commercial harvest, the species has not been the subject of dedicated conservation assessment, and reliable population trend data are not available.

Local populations depend absolutely on the availability of Avrainvillea host algae, which themselves depend on stable fine-grained soft sediments in sheltered coastal waters. Threats to Avrainvillea populations include coastal development, dredging, physical disturbance of shallow sandy habitats, and pollution that reduces water clarity and suppresses algal growth.

Around Koh Tao, the species is best monitored as part of broader sacoglossan and shallow soft-substrate community surveys, with particular attention to sheltered bays where Avrainvillea populations remain healthy. Ongoing BTC diver records provide baseline data against which future changes in local abundance can be tracked.

How to Identify Costasiella usagi

Identification of Costasiella usagi in the field is based on the combination of the distinctive rhinophore and ceras pigmentation with the certainty of finding it on Avrainvillea algae. Look for a very small sea slug, typically 3–10 mm long, with an unpigmented white head and tail, distinctly black rhinophores standing upright from the head, and dense clusters of cerata across the dorsum. The median cerata are usually grey (or green in juveniles), the outer cerata are black, and all cerata have white ends capped with a black tip. Fine longitudinal striations run from the base of each ceras to the white apical portion.

The species is essentially always found on Avrainvillea green algae, which itself grows on fine sandy or muddy sediments in shallow sheltered bays.

Costasiella usagi can be distinguished from the sympatric Costasiella kuroshimae by rhinophore and ceras colouration. C. usagi has predominantly black rhinophores and grey-to-black cerata with white tips and black caps, while C. kuroshimae has white rhinophores with black tips and predominantly green cerata often with iridescent blue spots. Both species can occur on the same algal patch, and careful attention to head and rhinophore colouration is essential for accurate identification. Photographs of the rhinophores, ceras colour pattern, and dorsal striations all assist in confirmation.

Conservation divers performing underwater research and sea slug identification training in Koh Tao, Thailand.

Mastering Identification with Black Turtle Conservation

Black Turtle Conservation on Koh Tao offers structured sacoglossan, nudibranch, and flatworm identification training as part of our marine conservation pathway. Costasiella usagi is included as a key reference species alongside its more famous congener C. kuroshimae, because the two species together illustrate how morphological differences at the ceras level are used to separate closely related species within Costasiella, and how the sacoglossan lifestyle differs fundamentally from that of predatory nudibranchs.

Our programmes help divers move beyond casual sea slug spotting toward accurate species-level identification across the multiple orders of gastropod sea slugs, and to understand the important taxonomic distinctions between sacoglossans and nudibranchs that shape their fundamentally different ecological roles.

Our Training Approach: From Theory to Survey

Training begins with classroom sessions covering the biology and ecology of Indo-Pacific sacoglossans, the structure of the family Costasiellidae, the contrast between the herbivorous Sacoglossa and the predatory Nudibranchia, the phenomenon of kleptoplasty, and the conservation context for Koh Tao’s shallow soft-substrate habitats. Participants review reference photographs and video from BTC’s archive.

In-water training takes participants to shallow sandy bays around Koh Tao where Avrainvillea algae grow. Practical skills include close macro searching of algal beds for tiny cryptic residents, macro-photography of small subjects, non-invasive observation techniques, and structured species logging using BTC’s standardised templates. Differentiating C. usagi from C. kuroshimae on shared algal patches is a particular focus.

Graduates contribute to BTC’s ongoing invertebrate biodiversity monitoring, helping to refine site-level inventories of shallow soft-substrate species, track changes in sacoglossan abundance over time, and support the long-term documentation of Koh Tao’s remarkable invertebrate biodiversity.

External Link Citations and References

Ichikawa, M. (1993). Saccoglossa (Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory 36(3): 119–139.

Christa, G. et al. (2014). Functional kleptoplasty in a limapontioidean genus: phylogeny, food preferences and photosynthesis in Costasiella. Journal of Molluscan Studies 80(4): 499–507.

Jensen, K.R. (2007). Biogeography of the Sacoglossa (Mollusca, Opisthobranchia). Bonner Zoologische Beiträge 55(3–4): 255–281.

Mehrotra, R. et al. (2021). An updated inventory of sea slugs from Koh Tao, Thailand. ZooKeys 1042: 73–188.

World Register of Marine Species: Costasiella usagi M. Ichikawa, 1993.

Sea Slug Forum: Costasiella usagi species account (Rudman 2004).

Black Turtle Conservation: Advanced Sea Slug, Nudibranch and Reef Ecology Training, Koh Tao.