How Cordgrass Helps Ribbed Mussels

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A beach is a lovely place for people to go on a holiday, but it can be a challenging place for plants and animals to survive. The waves are constantly buffeting and shifting the sands. It may be fun for a while, to watch the waves sweep the ground away from under your feet when you are walking down the beach. Yet, even people avoid the sea during storms and powerful waves.

Plants and animals need to adapt if they are to live in these harsh and dynamic environments. Crabs burrow into the sand to avoid the constant turmoil of waves. The non-burrowers have to find other ways of dealing with the vagaries of the sea, like growing with the cordgrass. If the beach is made of cobblestones instead of sand, life can be harder for plants and animals.

The Cobblestone Beaches

Smooth cordgrass (Spartina alterniflora) is a grass that grows half a metre to three metres high and is a summer bloomer. The taller members of this species grow on the water edge while the shorter grass is found in pannes and upper marshes.

Smooth cordgrass occurs on sand and cobblestone beaches. It is the only plant that can thrive on its own in the rough conditions prevalent on the coast. All other plants grow only where the cordgrass grows.

So how does the smooth cordgrass help other plants and animals?

In the Narragansett Bay in Rhode Island, United States of America, the estuary is protected, and the waves are below one metre in height. Here two algae and 12 herbs grow associated with the smooth cordgrass. Four are perennials, and eight are annuals. At any place though there are only two to three species growing together (1).


Cordgrass is an Ecosystem Engineer

The tall cordgrass grows to form narrow, compact beds that reduces wave action and stabilizes the substrate.

You may have heard of trees being used as windbreaks to protect a farm. Cordgrass also acts as a barrier and reduces the speed of waves by half. During storms, the cordgrass reduces wave speed to a tenth of its force. So the water behind the cordgrass is buffered from the sea and is calmer. This keeps the substrate stable.

A substrate or ground is the surface in which a plant or other immobile animals grow. On land, the soil is the common substrate. In Narragansett Bay, the substrate is made mainly of cobblestones.

The cobblestone beach has large cobbles five to 25 centimetres in size. So this means the soil is not waterlogged, nor saline. Individual cobbles can be swept and tossed around by waves. So the substrate is unsettled.

The cordgrass grows above the Mean Low Water (MLW); about one to one and a half metres above the Mean Low Water (MLW). The Mean Low Water is the average height of water at low tides and is calculated based on a specific 19 year period for a place (2).

Scientists wanted to find out why other plants and animals are found only with cordgrass.

There are many annual and perennial plants that manage to grow thanks to cordgrass. These include sea blite (Suaeda linearis) and common glasswort (Saliscornia eropaea), algae (Chondruscrispus and Ulva spp.), and two perennials sea lavender (Limonium nashii) and woody glasswort (Salicornia virginica).

Image credits: Donna Bilkovic/CCRM-VIMS

Then there is a bivalve the ribbed mussel (Geukensia demissa) that the cordgrass also helps. The ribbed mussels are ten centimetres long and have shells that are yellowish-brown to brownish black coloured on the outside and glossy on the inside.

The ribbed mussels live partially buried in the soil and are attached to the smooth cordgrass roots by byssal threads. Byssal threads are strong, elastic thread-like strands secreted from the mussel’s byssus gland.

Ribbed mussels live for 15 years and can tolerate the salinity of sea water. They are, however, sensitive to heat. Temperatures more than 45 degrees Celsius can be lethal (3).

The bare cobblestones can reach temperatures of 40 degrees Celsius during summer. So the shade from the cordgrass can sometimes be a lifesaver. However, the main reason they grow with the smooth cordgrass is that they offer a stable substrate.

There were 80 ribbed mussels in a square metre found growing with smooth cordgrass. That is twice as many as the 30 ribbed mussels found in a square metre of empty cobblestones in inter-beds. The ribbed mussels avoid being dislodged and washed away by strong waves or storms by binding themselves to cordgrass roots. Also, there are three times more young ribbed mussels where there are adult mussels. So the ribbed mussel community quickly grows large (4).

Cordgrass is an ecosystem engineer, because it influences the survival of many plants and animals. Plants and animals which create new habitats, or change existing ones are called ecosystem engineers. The effect of theirĀ  activities or presence can be good or bad of others. In the case of cordgrass, the effects are beneficial to others.

Ribbed Mussels

When ribbed mussels live tangled in the roots of the cordgrass, the ground becomes stronger. This reinforces the positive effects of the smooth cordgrass. Other animals are quick to exploit the firm surface and move in.

Thus the ribbed mussel acts as a secondary ecosystem engineer. So a hierarchy of ecosystem engineers is formed that boosts a habitat.

Where there is no cordgrass, there are no amphipods (Gammarus spp.) or blue mussels (Mytilus edulis) found. These two beneficiaries grow with cordgrass even when there are no ribbed mussels in the community.

Cordgrass and ribbed mussels, together, increase the number of animals that live in cobblestone beaches. With cordgrass alone, there are 2000 barnacles, 100 blue mussels and 80 ampipods in a square metre. With ribbed mussels and cordgrass, there are five times more barnacles (10,000) and blue mussels (500), and 20 percent more ampipods in a square metre.

Acorn barnacles (Semibalanus balanoides) and periwinkle snails (Littorina littorea) snails, are attracted to ribbed mussels and grow on them whether they are with cordgrass or not.

Small crevices exist between the shells of ribbed mussels. Small animals such as the gammarid amphipods live in these crevices and escape predators. There are 200 species of these tiny creatures. Gammarid amphipods are one to 140 millimetres in length. They are an important food source for many fish, invertebrates and birds (5).

Young blue mussels a delicacy much sought out by people, also thrive in the crevices between ribbed mussels (6).

In this hierarchy of ecosystem engineers, the cordgrass can grow without help from any other organism. All the other plants or animals including the ribbed mussel are dependent on cordgrass. Then there are others dependent on ribbed mussel and therefore indirectly also on cordgrass. The joint beneficial effect produced by both ecosystem engineers on the survival of other animals is the third reason.

Some negative interactions also happen in the cobblestone beach community. There is competition between barnacles and blue mussels for space. Snails graze on the cordgrass and crabs predate on blue mussels. But that is part of nature too.

  1. Bruno JF. 2000. Facilitation of cobble beach plant communities through habitat modification by Spartina alterniflora. Ecology 81:1179-1192
  2. (Retrieved on 18.05.16)
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  4. Altieri AH, Brian R, Silliman BR and MD Bertness. 2007. Hierarchical Organization via a Facilitation Cascade in Intertidal Cordgrass Bed Communities. The American Naturalist 169: 195-206
  5. (Retrieved on 18.05.16)
  6. (Retrieved on 18.05.16)

Carob as Nurse-plant

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Trees can accumulate seeds from other trees and plants. This happens as trees act as perches for birds or attract them with the fruits they bear. Birds use stones, post or trees to rest. When this habit of the birds is combined with the ideal conditions produced by carob trees the results are interesting.

In the Mediterranean regions, summers are warm and dry, and winters are wet and cold winters. Rainfall is around 27 to 90 centimetres annually. The vegetation is adapted to survive the stresses of the warm and dry summer. Plants are evergreen and leaves are sclerophyllus, with thick surfaces to prevent loss of water vapour (1).

In the Mediterranean Valcenia, in east Spain, the carob (Ceratonia siliqua) a legume acts as an important perch for birds. The tree grows four to six metres high and is used as fodder and or in food as a chocolate-flavour by people. This tree has been around for a long time. People in the middle East have cultivated it for 4000 years. From there it was taken to Europe, and later to the Americas and Asia (2).

Once the orchards of carob are abandoned, they begin their role as nurse-plants collecting bird dispersed seeds under them. While they are being tended, the seeds they attract as perches are cleared away by farmers. After the trees are abandoned, there is nobody to clear away the seeds, and they begin to accumulate, germinate and grow (3).

Many species of trees get collected this way under it, like the Mastic (Pistacia lentiscus), kermes oak (Quercus coccifera), prickly juniper (Juniperus oxycedrus), and Aleppo pine (Pinus halepensis). Most of these are themselves also sought after for their products by people (4, 5, 6, 7). Most individuals of these species are found under isolated carobs, than in any other site. The association of mastic with carob is particularly strong.

The frugivorous birds that bring in the seeds are the common songsters seen in Europe. They are small to medium sized. Among them are the brown and white European robin (Erithacus rubecula), true thrushes (Turdus spp.) and the Mediterranean warbler (Sylvia spp). The true thrushes are grey to brown with speckled underparts. In summer these birds usually eat more insects. By the end of autumn there are no more insects available, so the birds’ diet consists mainly of fruits. This diet continues through the winter (8).

The Mastic seems to be their favourite fruit. A Mastic shrub can produce 2,500 to 5,000 red to black drupes and most of them are eaten by birds. Drupes are small single seeded fruit with some fleshy content, for example cherries or almonds. The Mastic fruits fit even in the tiny gape (12 millimetre) of the songster’ beaks. The birds eat their fruits, and then carry their seeds in their bellies. The seeds are regurgitated, or defecated in roughly half an hour. When this happens as they are perched on the carob, the seeds accumulate under the tree. So a group of shrubs and trees start growing around the carob (3, 4).

However, the carob also acts as a nurse-plant by improving the soil conditions under it (3).

During heavy rains when rainfall is more than 25 centimetres, the bare soil cannot absorb all the water in a short time. Much of this water is lost as surface run-off. The thousands of leaves in the carob trees break the force and flow of rainwater falling on trees. So rainwater falling on trees, reaches the ground gradually, and more of it is absorbed (3).

Hence, it happens that water levels optimum for germination are found for at least five days under the trees, while soil in the open dries up faster. Seeds need water to germinate. Under the trees where seeds get more water, their chance of germinating improves. More seeds can germinate, and the process proceeds faster in the presence of water, boosting their growth. When water decreases below a certain level, germination is no longer possible (3).

Water seeping into the ground also changes the soil structure. Soil remains loose and friable while it is wet, and becomes hard and compact as it becomes dry. Therefore increased levels of soil water under the canopy keeps the soil loose. In the open, soil compaction shows half a kilo of pressure more for every square centimetre, than the soil under the trees (3).

The soil under the trees also become more compact in time, but remain looser than the soil in the open. Three days after the rains, the difference in hardness gets pronounced. Soil with more than 4.5 kilo per square centimetre is considered highly compacted. This degree of compaction is widespread in open areas after five days. Seven times more of the area in the open is shows this hard than under the tree. So the distribution of water and loose soil is patchy also under the canopy. When soil compaction crosses 1.5 kilo per square centimetre, germination is critically affected (3).

There is more water available the year around under the trees than in the open, so it is not just germination but later growth also that is helped. So the combination of attracting seeds and providing the right conditions for plant growth makes carob a nurse plant that helps other plants (3).


1. (Retrieved on 10.2.17)

2. (Retrieved on 10.2.17)

3. Verdu M. and P. Garcia-Fayos. 1996. Nucleation processes in a Mediterranean bird-dispersed plant. Functional Ecology, 10:275-80

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8. Jordano P. 1986. Frugivory, external morphology and digestive-system in Mediterranean Sylviid warblers Sylvia spp. Ibis 129:175-189. DOI: 10.1111/j.1474-919X.1987.tb03199.x

Red Devil Crayfish and the Heni’s Emerald Dragonflies

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Predators usually decrease the number of their prey. The opposite happens with the Heni’s emerald dragonflies (Somatochlora hineana), whose populations are larger when their predators the Red devil crayfish (Cambrus diogenes) is around.

Heni’s emerald dragonfly is the only endangered dragonfly in the United States of America. Endangered species are organisms that are likely to go extinct because there are very few of them alive. This nocturnal insect has bright green eyes, and yellow stripes on its thorax. The wing-span is nine to ten centimetres and greater than the body which is only six to seven centimetres long (1).

Heni’s emerald dragonfly lives only near streams or in marshes in the temperate areas. It is found only in a few places in Illinois, Michigan, Missouri and Wisconsin. They are endangered because their habitat is being cleared away to make place for urban development. This urban development is also affecting the ground-water level crucial for the survival of dragonflies. In addition, pollution of streams by pesticides is lethal to dragonflies living in them. Hence, these beautiful dragonflies are very rare these days (2).

Dragonflies have been around for 300 million years, so it is a pity the Heni’s emerald dragonfly has to die out now (3). So recognition of help from an unexpected source like the nocturnal Red devil crayfish is timely for their conservation.

The streams and marshes where these two species live dry up completely for a few months in the summer from June to August. Heni’s emerald dragonflies have three stages in their life-cycle, egg, the immature stage called nymph (also called larva) and adult. They spend up to four years of their lives as nymphs, which is most of their lives as they survive as adults for only two to six weeks. The brown-coloured nymphs are only two and half centimetres long. Since they are aquatic as larvae and inhabit streams, they are at risk during summer droughts many times during their life before they reach adulthood and can fly. Being young is not fun for many animals. As it is, the larval dragonflies are a common and important food for several other aquatic animals.

The Red devil crayfish are common in the United States of America. Though they are called crayfish, they are a kind of insect. The head and thorax is fused into a thick cephalothorax which has eyes on stalks to improve the range of vision. Then there is a relatively smaller abdomen. They are green, blue, brown, to brownish red, and are four to five centimetres long. They are omnivorous and feed on worms and insects living in water-bodies and dead animals. Red devil crayfish live near ponds or streams and make burrows to a depth of a metre to reach underground water. They also have a second channel that goes horizontal into the stream-bed. Their burrows are easily recognisable by a characteristic chimney made of loose mud above the ground (4).

The crayfish is entirely aquatic and can survive only in water, and when it is out of the water it needs to remain wet. The underground burrows provide access to water that is not available in summer droughts on the land surface. In winter the burrows also protect it from freezing temperatures.

Dragonflies live in the open streams, when it is full of water, feeding on other small invertebrates and insects (1). When streams dry in summer they move to the burrows of crayfish, as these are the only places that have water the whole year, due to access to underground water. However, the red devil crayfish is their predator, which end up eating 65 percent of dragonfly larvae. Only the three to four year old dragonflies that are too big for the smaller and younger crayfish, are completely safe from predation (5).

If they were to live in the open streams the larvae would all die after just 13 days due to desiccation when there is no water. Since drought usually lasts for one to two months in these regions, no dragon fly would be able to survive. Inside burrows at least some have a chance to survive. Since the Red devil crayfish prey on dragonfly nymphs even in open streams, the threat from predation is not any greater in burrows compared to streams.

Again during winter when the water in the stream freezes, they could die if they were outside. Within burrows the temperature is always above zero degrees Celsius, so both crayfishes and dragonflies are safe throughout winter.

In Wisconsin streams that Red devil crayfishes inhabit, the Heni’s emerald dragonflies were the most common type of dragonflies. There are 95 percent of them, i.e., of 100 dragonflies of all kinds 95 are Heni’s emerald dragonflies. The other dragonflies which have different life-cycles or behaviour were unable to survive the dry summer months. So that left more resources like food and space at the disposal of Heni’s emerald dragonflies, and they manage to thrive. In a nearby stream that had water flowing in the streams even in summer, other species were abundant, and there were few Heni’s emerald dragonflies; only two to three in every 100 dragonflies was a Heni’s emerald. So Heni’ emerald dragonflies have an advantage over other dragonflies in streams that go dry in summer.

Since the Heni’s Emerald dragonfly is endangered, it is important to know as much as possible about them to conserve them. If habitats without the crayfish had been chosen because people thought Emerald dragonflies would be safe from their predators they wouldn’t be able to survive dry summer streams. Or if only ‘best’ and bountiful habitats, in this case perennial streams were set aside to preserve Heni’s emerald dragonflies, they could not compete with other dragonflies in permanent streams.

The obvious interaction between Heni’s emerald dragonflies and Red devil crayfishes is predation and negative. But the previously unknown effect of getting refuge in summer and winter turns out to be more important for the dragonflies and worth the risk of being eaten up.


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5. Pintor LM and DA Soluk. 2006. Evaluating the non-consumptive, positive effects of a predator in the persistence of an endangered species. Biological Conservation 130: 584-591. doi:10.1016/j.biocon.2006.01.021