Shrimp keepers

Longfin damselfish have their algal farms fertilized

Longfin damselfish grows algae with help of shrimp

Every intruder is chased away from the algal farm of the longfin damselfish, but a swarm of opossum shrimp is allowed stay and even receives protection. For good reason, Rohan Brooker and colleagues report.

The longfin damselfish (Stegastes diencaeus) is an aggressive, territorial fish that lives on coral reefs. It grows its own food by creating a farm of a few square meters where palatable algae grow. It tends its algae and defends its territory fiercely; all animals are chased off.

Or rather: almost all animals. Rohan Brooker and colleagues show that during the day, a swarm of opossum shrimp (Mysidium integrum) can be found in many algal farms. A farmer not only tolerates the shrimp’s presence, but it also protects them from their predators, although it takes some extra effort. Apparently, the tiny animals are worth it.

Flourishing farm

Why would an algae-farming longfin damselfish care about the shrimp, Brooker wondered. Although the fish supplements its algae diet with some small animals, it does not eat these shrimp. Perhaps, Brooker supposed, the shrimp are fertilizing the algae with their feces.

And that turned out to be the case. A farm with a swarm of shrimp does better than a farm without such swarm, thanks to nutrients excreted bythe shrimp. It hosts more large brown algae that form a structure on which turf-algae, which the damselfish prefers to eat, grow well. This translates into a better condition of the fish: damselfish with a shrimp swarm on their farm have a larger energy reserve than colleagues with a non-fertilized farm.

Brookers conducted his research on coral reefs off the coast of Belize, Central America.

Domesticated

Thus, the farming fish benefits for its flourishing crop, the shrimp for its guarded refuge. There are no shrimp swarms to be found outside farms during daytime. The shrimp leave the farm at night, when it is safe, to filter food from the water at the surface. Then they return to their permanent residence.

The fidelity to this place is so strong that young shrimp remain in their parents’ farm. Therefore, the authors consider the relationship between longfin damselfish and opossum shrimp as an early stage of domestication. The fish ‘keep’ the shrimp as livestock.

Willy van Strien

Photo: Longfin damselfish Stegastes diencaeus. Mark Rosenstein (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

The research explained on YouTube

Source:
Brooker, R.M., J.M. Casey, Z-L. Cowan, T.L. Sih, D.L. Dixson, A. Manica & W.E. Feeney, 2020. Domestication via the commensal pathway in a fish-invertebrate mutualism. Nature Communications 11: 6253. Doi: 10.1038/s41467-020-19958-5

Sprouted food

Common ragworm caches seeds and consumes the seedlings

The common ragworm sprouts seeds

Common ragworms bury seeds of cordgrass for future use: when the seeds have germinated, the worms eat the sprouts. Sprouting seeds is a newly discovered gardening strategy in animals, as Zhenchang Zhu and colleagues point out.

The seeds of cordgrass, Spartina species, are protected by husks that make them inedible for the common ragworm, Hediste diversicolor. Yet these worms take the trouble to drag the large seeds to their burrows and pull them inside. This is deliberate behaviour, according to Zhenchang Zhu and colleagues. The buried seeds will germinate and the worms can eat the sprouts. In fact, the seeds are a nutritious dietary supplement for the ragworms, which feed mainly on low-quality sedimentary organic matter, because they are high in protein and vitamins.

Waiting

Common ragworms live in the seabed of intertidal flats. Each animal inhabits a self-made burrow in the sand or the mud.The species is native to the north-east Atlantic.

In experiments, the researchers noticed that the ragworms wouldn’t eat intact cordgrass seeds. But if sprouted seeds were offered, they did eat them. Experiments also showed that the worms grew much better when given a diet including cordgrass sprouts than on a diet without sprouts.

It is striking that they cache the seeds, because that behaviour pays off only in the long term. The cordgrass produces seeds from October to March, and these seeds will germinate from April until July. So, the worms have to wait a few weeks or months before their stored food supply will be usable.

Seed dispersal

Sprouting seeds to consume the seedlings is a form of agriculture. More examples of agriculture in animals exist, such as the well-known fungus gardens of ants and termites. The sprouting strategy of common ragworms, however, differs from fungus gardening. While the fungus is the main food source for the ants and termites, the sprouts are a superior supplementary food for the ragworms: superfood instead of staple food.

Also, a mutual relationship exists between termites or ants and the fungus they grow: the animals are dependent on their crop, but the fungus is also dependent on its growers. Common ragworms and cordgrass, in contrast, have a predatory relationship, as the cached seeds are eaten after germination. The ragworms may help in seed dispersal, though. Buried seeds will not be displaced by water currents, retain their viability and can produce new plants when the ragworm that cached them dies or is eaten itself. This often happens, as common ragworms have many predators: birds like avocets and curlews, and fish like plaice and sole.

The authors suggest that common ragworms and their relatives may bury seeds of plants like seagrass and glasswort to sprout them as well.

Willy van Strien

Photo: Common ragworm. © Jim van Belzen

Source:
Zhu, Z., J. van Belzen, T. Hong, T. Kunihiro, T. Ysebaert, P.M.J. Herman & T.J. Bouma, 2016. Sprouting as a gardening strategy to obtain superior supplementary food: evidence from a seed-caching marine worm. Ecology 97: 3278-3284. Doi: 10.1002/ecy.1613

Mini garden

Some arboreal ants grow useful plants

A Squamellaria major plant on macaranga, grown by ants

Gardening is an art – and there are ants that master this art. On the branches of trees they cultivate plants to live in or to strengthen their nests, as research teams of Guillaume Chomicki and Jonas Morales-Linares report.

Many ants and plants are partners in a mutualism: the plants provide the ants with a place to live or with nectar, and the ants deposit their droppings as fertilizer or protect the plants from herbivorous insects. Some tropical arboreal ants go a step further and cultivate the plants they live with. As these plants grow upon tree branches (they are epiphytes), it is more difficult for them to obtain nutrients than it is for plants that root in the soil, so the ant-plant mutualism is a good strategy. Many of the ant-grown plants are completely domesticated and would perish without the ants.

Seed collection

Philidris nagasau, native to Fiji, inhabits the hollow stems of Squamellaria species, bulb-shaped plants that grow on trees. The ants live nowhere else, and six Squamellaria species are always inhabited by these residents. The ant fertilizes the plants, as Guillaume Chomicki and colleagues had previously shown.

workers of Philidris nagasau inspect seedlings of SquamellariaNow, they discovered that the ant makes sure that plants are available by farming them. The researchers observed ant workers collecting exclusively the seeds of these six Squamellaria-species, and not those of any other species. They take them out of the unripe fruits, insert them in fissures and cracks in the bark of a tree and patrol the planting sites. Soon after, the seeds germinate and seedlings appear on the tree, and as soon as they form a cavity, a few ants will enter it, likely to leave their droppings. By doing so, they grow the plants they need to live in.

So, this ant-plant mutualism is more intimate than previously thought. The plants need the ant partner not only for nutrition, but also for seed dispersal.

Beautiful flowers

A different kind of plant nurseries can be found in Central and South America: conspicuous little gardens that hang from some trees. They are the overgrown carton nests of certain ant species. The ants collect seeds of epiphytes and insert them in the walls of their nest, where of the seeds germinate and grow up. The plant roots strengthen the nest and take up water when it rains, so that the nests don’t disintegrate. In return, the ants fertilize the plants and protect them against herbivorous insects. Some plants are exclusively dispersed by the ants and only germinate in an ant nest.

Hanging garden of Azreca gnavaAzteca gnava from southern Mexico and Panama is such a gardening ant. His gardens are frequently found in plantations, as Jonas Morales-Linares and colleagues report, mostly on cocoa, mango, sapote and orange trees. The gardens contain twelve plants on average, typically of two or three different species. Two plant species that cannot live outside these gardens are the bromeliad Aechmea tillandsioides and the orchid Coryanthes picturata. Apparently, the gardening ants have a good taste, for these plants have beautiful flowers.

Three million years

The ant Camponotus femoratus of the Amazonian lowland forest plants similar gardens. Mutualism is obligate for the plant Peperomia macrostachya, that only lives in the nests of this ant. Elsa Youngsteadt and colleagues showed that Camponotus femoratus is the only ant species to collect the seeds of this plant. The ant takes them from the plant, from the soil or from the feces of birds and mammals that have eaten the fruits. Probably, the seeds emit volatiles that only only Camponotus femoratus appreciates. The ant inserts many Peperomia seeds in the walls of its nest. Each seed has only a small chance to germinate there, but the seeds that are not brought into the ant’s nests have no chance to sprout at all.

According to Chomicki, Philidris nagasau in Fiji descends from ancestors that, just like their American colleagues, made carton nests in trees and planted seeds in the wall. But at some time, Philidris nagasau stopped making nests and planted the seeds in the bark instead; at roughly the same time Squamellaria species developed the hollow, bulbous stems that can house the ants. So, ant and plants co-evolved; their co-evolution started about three million years ago.

Willy van Strien

Photo’s:
Large: a Squamellaria major plant, grown by ants on macaranga. © Guillaume Chomicki
Small 1: workers of Philidris nagasau inspecting seedlings. © Guillaume Chomicki
Small 2: hanging garden of Azteca gnava. © Jonas Morales-Linares

Sources:
Chomicki, G. & S.S. Renner, 2016. Obligate plant farming by a specialized ant. Nature Plants 2: 16181. Doi: 10.1038/nplants.2016.181
Chomicki, G., Y.M. Staedler, J. Schönenberger & S.S. Renner, 2016. Partner choice through concealed floral sugar rewards evolved with the specialization of ant-plant mutualisms. New Phytologist, online May 9. Doi: 10.1111/nph.13990
Morales-Linares, J., J.G. García-Franco, A. Flores-Palacios, J.E. Valenzuela-González, M. Mata-Rosas & C. Díaz-Castelazo, 2016. Vascular epiphytes and host trees of ant-gardens in an anthropic landscape in southeastern Mexico. The Science of Nature 103: 96. Doi: 10.1007/s00114-016-1421-9
Youngsteadt, E., J. Alvarez Baca, J. Osborne & C. Schal, 2009. Species-specific seed dispersal in an obligate ant-plant mutualism. PLoS ONE 4: e4335. Doi: 10.1371/journal.pone.0004335