# Nanomia bijuga

Nanomia bijuga, first described by Stefano Delle Chiaje in 1844 and originally named Physsophora bijuga, is a species of mesopelagic siphonophore in the family Agalmatidae As with all members of the siphonophorae order, it is a colonial organism composed of individual zooids. N. bijuga has a fairly broad distribution, and has been observed in the coastal waters off of North America and Europe. The species has been found to occupy both epipelagic and mesopelagic depths. They utilize specialized swimming zooids for both propulsion and escape behaviors. Similar to other siphonophores, Nanomia bijuga employ stinging tentacles for hunting and defense. They primarily feed on small crustaceans, especially krill.

## Anatomy and morphology

Nanomia bijuga, like other siphonophores, is made up of genetically identical, but highly specialized, zooids. The organism is composed of two main body segments: the nectosome on the anterior end and the siphosome as the posterior. The nectosome contains a gas-filled pneumatophore at its end as well as nectophores, bell-shaped structures that assist in locomotion. The siphosome contains zooids specialized for feeding, digestion, reproduction, and protection. These zooids are organized in repeating sequences called cormidia. Each cormidium contains one feeding zooid, the gastrozooid, with multiples of the other zooid types. Gonophores and gonodendrons, the male and female reproductive zooids respectively, occur together in pairs.
Each gastrozooid has its own tentillum, which is used to capture and subdue prey. These tentilla house nematocysts, stinging cells that deliver toxins into the prey organism. There are four different types of nematocysts found within the tentilla. Heteronemes, the largest of the nematocysts, possess a wider stinging apparatus than the other types and are primarily found at the proximal end of the tentilla. Haplonemes, the most abundant type, are smaller than heteronemes and structured similarly with open tips for stinging but no distinct spiny shaft. The final two types are desmonemes and rhopalonemes which are both used for adhesion to prey in order to prevent it from escaping as the stinging cells perform their function.
A matured pneumatophore of N. bijuga contains five different tissues, two layers of ectoderm, two layers of endoderm, and a layer of ectodermal cells that are not connected to any basement membrane. One set of ectoderm/endoderm layers exists on the outside of the pneumatophore while the other set exists inside the external layer and acts as the gas chamber.

## Distribution and habitat

Nanomia bijuga is widely distributed across all major oceans of the world except the Antarctic Ocean. A few of its sighted locations are the Monterey Bay, the Gulf of Mexico, the Sagami Bay of Japan the Hansa Bay of Papua New Guinea, and the Bantry Bay of Ireland.
Nanomia bijuga is an epi/meso-pelagic species that can vertically migrate up to 1000 in depth, though it predominantly thrives at depths of 200-400m. Their ability for long range vertical migration makes them key contributors to the deep scattering layer.
Their abundance is significantly correlated with seasonality and primary production. Notably, sightings coincide at spring phytoplankton blooms in the Sagami Bay. Similarly, collection rates of Nanomia bijuga in the Bantry Bay of Ireland heightened in the months of May-September, with peak density in May/June, which correlated with the annual phytoplankton blooms in the region.

## Behavior

Nanomia bijuga employs several strategies to evade predators such as medusas, which typically move linearly. This includes sweeping areas, changing direction, and pulsing rapidly. This rapid escape is facilitated by their use of nectophore thrust, which are specialized swimming subunits aiding in propulsion. At certain depths, Nanomia bijuga can change their body shape by retracting their tentacles when disturbed which can also assist in their rapid escape from predators. Another predator escape behavior that Nanomia bijuga use is diel vertical migration (DVM). It is thought that the shallow water and the amount of light may drive this pattern and is a method used to avoid predation.
In addition to their escape techniques, Nanomia bijuga exhibit hunting behaviors similar to their escape mechanisms. They are filter feeders that extend their tentacles and when they contact prey, the siphonophore begins its rapid movement and contracts its tentacles with nematocysts, specialized stinging cells, to capture small organisms, like plankton, and bring them to their body. Unlike other siphonophores, Nanomia bijuga uses rapid swimming to extend their tentacles. Furthermore, they frequently relocate every few minutes to seek out new prey.

## Diet

Some information is still unknown about how deep sea siphonophores consume food, but at this time, it is believed that these creatures can be broken down into 3 dietary groups. In a study led by Purcell, it was discovered that distinct diets could be associated with 3 unique suborders of siphonophore: the Cystonectae, Physonectae, and Calycophorae.
In comparison to the other suborders, organisms in the Physonectae have been shown to consume a larger volume of copepods than fish, ranging anywhere from 14% to 91% of their total diet. Nanomia bijuga, or the common siphonophore, is a member of the Phsyonectae suborder. Despite its small size these creatures can play a substantial ecological role in deep-sea food systems. According to MBARI, when there are robust populations of Nanomia bijuga concentrated in one area, they can collectively eat more krill than several adult whales. Aside from copepods, shrimps have also been documented to make up a large proportion of this siphonophore's diet, as well as decapod larvae and chaetognaths.

## Taxonomy

Nanomia bijuga was first scientifically documented by the French zoologist Félix Dujardin in 1843. Its initial description marked a significant milestone in the understanding of these colonial marine organisms, shedding light on their complex biology and ecological roles within oceanic ecosystems.
Nanomia bijuga exhibits a distinctive morphology characterized by its elongated, slender colony structure. It consists of a succession of specialized zooids organized in a linear fashion, each component fulfilling a specific function essential for the colony's survival. These functions encompass prey capture, propulsion, and reproduction, all orchestrated within a translucent or transparent body, aiding in camouflage amidst its oceanic habitat.
Over time, advancements in genetic analyses, morphological studies, and classification methodologies have prompted revisions in the taxonomy and nomenclature of siphonophores, including Nanomia bijuga. These revisions reflect evolving understandings of their evolutionary relationships, genetic diversity, and ecological adaptations. Consequently, updates in nomenclature serve to refine our comprehension of the intricate relationships between species and their broader taxonomic contexts.
Nanomia bijuga is closely related to other siphonophores in its genus, such as Nanomia cara and Nanomia gracilis. These species share similarities in colonial structure and ecological niches.

## Conservation status

As of January 2022, there is no specific information available regarding the conservation status of Nanomia bijuga on the IUCN Red List. Siphonophores, including Nanomia bijuga, are generally not individually assessed for conservation status due to their widespread distribution and lack of direct threats from human activities However, it's important to note that marine ecosystems, including those inhabited by Nanomia bijuga, face various threats such as habitat degradation, pollution, climate change, and overfishing. These threats can have indirect impacts on siphonophore populations by altering their habitats, disrupting food webs, and affecting oceanic conditions. Population changes in Nanomia bijuga would require specific scientific studies and monitoring efforts, which may be limited due to the challenges of studying and tracking marine organisms, particularly those with pelagic lifestyles like siphonophores.
Conservation efforts aimed at protecting marine ecosystems, reducing pollution, mitigating climate change impacts, and implementing sustainable fishing practices indirectly benefit species like Nanomia bijuga. However, targeted conservation efforts for this species are likely limited by the lack of specific data on its population status and distribution.
While Nanomia bijuga may not be individually assessed for conservation status, its well-being is intricately linked to the health of marine ecosystems. Conservation actions targeting broader marine conservation goals are essential for safeguarding the habitats and resources upon which Nanomia bijuga and other marine organisms depend.