Thor amboinensis

Famous for its unique bobbing motion, the Sexy Shrimp is a tiny, social crustacean that adds fascinating activity to the lower levels of a peaceful saltwater aquarium. These very small shrimp are not solitary creatures and thrive when kept in small groups, where they feel more secure and display their characteristic swaying behavior. Their name comes from the way they constantly wiggle their abdomens back and forth while they walk around the tank. This makes them a captivating and entertaining addition for any marine enthusiast.

Considered easy to care for, they are an excellent choice for aquarists of all levels, including beginners with smaller, well-established nano or pico reef tanks. As omnivores, they are not picky eaters and will readily consume a variety of prepared foods, such as pellets and flakes. They also serve a practical purpose as part of the cleanup crew, constantly foraging for leftover food and organic debris on the rocks and sand bed. Their impact on the aquarium's biological load is minimal, meaning they produce very little waste.

To keep them healthy, it's important to maintain a stable, warm saltwater environment with gentle water movement, as strong currents can be difficult for them to navigate. They are peaceful inhabitants but are quite small and defenseless, so they should only be housed with other small, non-aggressive tank mates. With proper care, these engaging little shrimp have a relatively short but active lifespan.

Thor amboinensis, commonly known as the Sexy Shrimp or Squat Shrimp, is a diminutive and fascinating decapod native to the warm waters of the Indo-Pacific region. In its natural habitat, this species is renowned for forming a commensal relationship with sea anemones. The shrimp finds sanctuary from predators among the anemone's stinging tentacles, to which it is immune, and feeds on the host's mucus and trapped food particles. While a host anemone can be a beneficial addition to an aquarium housing these shrimp, it is not a strict requirement for their survival in captivity, provided there is ample live rock with complex structures, caves, and crevices for security.

The most notable characteristic of the Sexy Shrimp is its unique and perpetual swaying motion. It holds its abdomen high and rhythmically pulses it up and down, a behavior that is both intriguing and the source of its memorable common name. This behavior is most prominent when the shrimp are comfortable and kept in appropriate social groups. They are shoaling animals that should be maintained in groups of three or more. A solitary individual will often be reclusive and stressed, while a group will be more active, confident, and visible as they forage together along the substrate.

Setting up an ideal aquarium for Sexy Shrimp involves ensuring a mature, stable system with very low water flow. Their small size makes them susceptible to being tossed around by strong powerheads. Water parameters, including temperature, pH, and salinity, must be kept stable, as all crustaceans are particularly vulnerable to fluctuations during their molting process. Due to their peaceful nature and small size, tank mates must be chosen with extreme care. They are easy prey for most fish, even those not typically considered aggressive, as well as predatory invertebrates. Ideal companions include other micro-species like small gobies, tiny hermit crabs, and other peaceful shrimp.

As opportunistic omnivores, their diet in an aquarium should be varied. They are excellent scavengers that actively consume detritus and graze on an invisible layer of organic matter known as biofilm. This natural foraging should be supplemented daily with high-quality, finely-crushed foods. Appropriate offerings include micro-pellets, crushed flakes, and frozen options like cyclops, rotifers, and finely minced mysis or brine shrimp. Providing a diverse diet ensures they receive all necessary nutrients for health and successful molting. Given their very low waste production, they are an excellent choice for adding life and activity to small-capacity reef systems without significantly increasing the bioload.

Thor amboinensis (De Man, 1888) is a small crustacean belonging to the order Decapoda and the family Thoridae. It is widely distributed across the tropical and subtropical reefs of the Indo-Pacific Ocean. The species exhibits a distinct morphology characterized by a laterally compressed, or compressiform, body plan and a small, hinged rostrum. A key identifying feature is its posture and behavior; the animal typically holds its abdomen arched dorsally and engages in a constant, rhythmic swaying motion. This unique kinetic display is a subject of interest regarding its potential function in intraspecific communication or predator evasion.

Ecologically, T. amboinensis is a well-documented commensal organism, most notably forming symbiotic associations with various species of sea anemones, including those from the genera Stichodactyla, Heteractis, and Entacmaea. Within this relationship, the shrimp gains critical protection from predation and access to a consistent food source, consisting of host mucus and trapped organic particles. The shrimp's immunity to the host's nematocysts is a key physiological adaptation enabling this lifestyle. While often found with anemones, it can also inhabit corals and other sessile invertebrates, demonstrating a degree of plasticity in its choice of microhabitat.

As a benthic omnivore, T. amboinensis occupies a trophic position as a micro-predator and detritivore. Its feeding apparatus is adapted for scavenging and grazing on a variety of organic materials, including biofilm, algae, and meiofauna found on the substrate and its host. Physiologically, the species is adapted to its environment with a low metabolic rate and very low oxygen consumption, which contributes to its minimal biological load in closed aquatic systems (a low bioload factor). This efficiency makes it a suitable subject for studies in nano-reef energetics.

Reproduction in T. amboinensis involves the female brooding a clutch of eggs on her pleopods until they hatch. The resulting offspring undergo a planktonic larval phase before metamorphosing and settling into a benthic lifestyle. This complex life cycle presents challenges for aquaculture but is crucial for its dispersal in the wild. Its current IUCN Red List status is Not Evaluated (NE), which is common for small, widespread marine invertebrates that are not commercially targeted, though they remain vulnerable to the broader threats of coral reef degradation and habitat loss.

Successfully breeding Thor amboinensis in a home aquarium is considered a moderately difficult endeavor. While conditioning a healthy group to spawn is relatively common and straightforward, the primary challenge lies in the successful rearing of the tiny, planktonic larvae through their delicate developmental stages.

To begin, establishing a proper breeding group is essential. A group of at least five or six individuals is recommended to increase the probability of having both males and females. Sexual dimorphism is subtle; however, females are generally larger and possess a wider, more curved abdominal carapace to create a brood chamber for carrying eggs. Males tend to be smaller and more slender. A dedicated, species-only breeding tank is highly advised. This ensures the adults are not stressed by other tank inhabitants and, more importantly, prevents the larvae from being predated upon or lost to filtration systems upon hatching.

Once comfortable in a stable environment with ample food, mature females will molt and subsequently mate. Following fertilization, the female will transfer the eggs to her swimmerets (pleopods), where she will carry them for the duration of the incubation period. She will diligently fan and clean the egg clutch, which is visible as a small mass under her tail. The time until hatching is temperature-dependent but generally takes several weeks. As the embryos develop, their eyes will become visible through the egg casing, signaling that hatching is imminent.

The hatching event releases a cloud of minuscule, free-swimming larvae. This stage is the most critical and demanding for the aquarist. The larvae are phototactic (attracted to light), a behavior that can be used to gently siphon them out of the parent tank and into a separate, purpose-built larval rearing system. A circular tank, such as a kreisel or similar design with gentle, upward-flowing water, is ideal for keeping the delicate larvae suspended in the water column and preventing them from settling and perishing. Standard filtration must be avoided; gentle aeration from an air stone is sufficient for circulation.

The larvae have minimal yolk reserves and require an immediate external food source. The aquarist must have live cultures of microscopic foods ready. A diet of phytoplankton and rotifers is essential for the initial larval stages. As the larvae grow and molt through several zoeal stages over weeks, their diet can be transitioned to slightly larger foods, such as newly hatched brine shrimp (Artemia nauplii). Once they successfully undergo metamorphosis into post-larval shrimp—which resemble miniature adults—they will adopt a benthic lifestyle and can be moved to a grow-out system with a more conventional setup.