The deep sea is regarded by many as “other-worldly”; a description that in part owes itself to the existence of the alien-like organisms that reside there. From nightmare-inducing anglerfish to the infamously ugly blobfish, the deep sea is often the first place that springs to mind when we think of nature at its weirdest.

Xenophyophores (pronounced zee-no-fie-oh-fors) are just one deep sea oddity that simultaneously live up to the deep-sea’s reputation, and yet, are unheard of by many.

So, what makes these creatures so peculiar?

Xenophyo-what?

Xenophyophores are neither plant nor animal, but rather single-celled organisms belonging to the foraminifera phylum. Roughly translated from Greek, ‘xenophyophora’ means ‘bearer of foreign bodies’; a reference to this organism’s characteristic shell or ‘test’.

Xenophyophores were first discovered in 1883 by British micropalaeontologist, Henry Bowman Brady (1835-1891), and are found in all of the world’s oceans at varying depths.

Syringammia fragilissima, one of the largest known unicellular organism, found off the coast of Scotland.

As with all forminiferans, xenophyophores are unicellular. In fact, these odd little organisms have the largest individual cells found on the planet, often exceeding 10 centimetres in diameter.

Currently, 15 genera and 75 species of xenophyophores have been described, each varying greatly in size and shape. Syringammia fragilissima, found along the Scottish coast, is a strong contender for the largest known coenocyte (multinucleate cell), with a 20-centimetre diameter.

The Test of the Xenophyophore

Xenophyophores are predominantly shells/tests, which account for over 99% of their total body mass. As such, from pictures alone, it is difficult to judge where the test stops, and the organism begins.

They create their tests by extracting minerals and other particles from the surrounding water and gluing them together using organic cements. The agglutinated particles which form the test are aptly called xenophyae and their constituents vary between individuals. Sponge spicules, sediment particles, and even the tests of smaller foraminifera have all been found within xenophyae.

The seafloor is home to xenophyophores of all shapes and sizes. The softness and structure of tests is largely species-specific as well as being influenced by the particulate content of surrounding waters.

A recent (2020) remotely operated expedition of the deep Pacific Ocean, uncovered several new species of xenophyophore (shown below) which clearly demonstrate the group’s structural diversity.

Two newly found species of xenophyophore, Moanammina semicircularis (left) and Abyssalia sphaerica (right). Pictures taken from the DeepCCZ project.

Some species, such as Psammina, possess a compartmentalized test with multiple chambers. As expected, the exact composition of the test varies between species, but the group generally possesses a branching system of organic tubules.

Light micrograph of a section of Psammina limbata wall, showing the light grenellare strings against the dark stercomare mass. Picture by Andrew J. Gooday et al. 2018.

Common among all groups of xenophyophore are small crystals of barite known as granellae, which are suspected to support the individual by reducing both cell volume and the metabolic demands of maintaining a complex test.

Lead and uranium are other minerals that are often extracted from the water. Shinkaiya, a Japanese species of xenophyophore, have been found with high concentrations of mercury within its granellare (the plasma body and associated tube).

We’ve explored the unusual structure of these fascinating creatures. Now we ask ourselves: how do xenophyophores reproduce?

Mysterious Mating

Xenophyophores are found between 500 and 10,600 metres deep. This in conjunction with the fragile test, makes studying xenophyophores particularly difficult.

As such, growth and reproduction in these organisms are, for the most part, a mystery. It is assumed that xenophyophores follow a life cycle similar to that of other foraminifera which undergo metagenesis i.e. reproduction in which generations alternate between sexual and asexual reproduction. This is largely speculation and yet to be confirmed.

A process diagram, detailing metagenesis, or alteration of generations. It is believed, but not confirmed, that xenophyophores reproduce in this manner.

Gametes (sexual cells) are formed in a specialised region of the grenellare. In some species, such as Psammetta, this may look like a swollen branch of the cell. Gametes measure in at 20 micrometres, possess two flagella for swimming.

As with most other forms of sexual reproduction, when two gametes come into contact, a zygote is formed and the diploid stage of the xenophyophore life cycle begins.

Studies indicate that growth in xenophyophores is of an episodic nature; phases of 2-3 days in which an individual has grown to tenfold its original body mass have been observed.

Each growing event typically occurs in three stages: widening and flattening of the base, return to the original form, and finally a return to the original surface texture. These periods of growth are interrupted by resting phases which typically lasts a couple of months.

As in all organisms, a key factor of growth is diet. But how does a unicellular organism feed?

The Xenophyophore Diet

As you may have guessed by now, the diet and feeding mechanisms employed by xenophyophores is a topic of speculation and debate. Upon their discovery, several theories were put forward: filter feeding, bacterial farming, and deposit feeding being just some of the ideas put forward by scientists.

However, recent studies have revealed that xenophyophores uptake food from surrounding sediments with the use of pseudopodia; a projection of the cell membrane that acts as an arm. These functional “arms” drag sediment which has been trapped inside the test and any food particles found within are soon absorbed into the cytoplasm.

One study which analysed lipid concentrations within xenophyophores, showed a high level of bacteria within the stercomata (waste pellets). This has led to the suggestion that bacteria, nourished by xenophyophore waste, grow in number, and become a significant source of food for xenophyophores. This is known as bacterial farming.

However, a 2021 study utilising isotopic labels discovered that diatoms and dissolved organic matter comprise the majority of the xenophyophore’s diet.

No evidence was found to support the bacteria farming theory, instead arguing that the increased surface area provided by the test’s intricate structure, serves to enhance phytodetritus collection. If the latter is true, xenophyophores may play an important role in ocean-floor biogeochemical cycling.

In addition to this, it is believed that xenophyophores act as ecosystem engineers. Not only do the tests provide refuge and an abundant food source for a number of species, the presence of xenophyophores has been found to directly correlate with the presence of other benthic organisms; areas lacking xenophyophores have up to four times fewer crustaceans, molluscs, and echinoderms.

Xenophyophores are just one of many unheard-of, yet undeniably important groups of marine organisms. As mankind continue to delve into the depths, we may yet discover something even more bizarre than these unicellular curiosities.

Written by Lucas King