Seaweed and seagrass play a significant role in the marine ecosystem. They are vital for oxygen generation and provide food and shelter to many marine organisms. Seagrass and seaweed are both eukaryotic marine organisms. And like all plants, they have a green pigment called chlorophyll for photosynthesis.

Although they are frequently mistaken, they are two different organisms. Below we will look more into the main differences between seagrass and seaweed. Read on to learn more.

Seagrass (Left) vs. Seaweed (Right) Visual Comparison

Credit: HYDRA Marine Sciences GmbH (Left), Gruibio–1 (CC 4.0) (Right)


Main Differences Between Seagrass and Seaweed

  • Structure: Like terrestrial grasses, seagrasses are vascular plants with well-specialized roots, leaves, and underground stems, also known as rhizomes. In contrast, seaweeds are multicellular algae with no vascular tissue and lack true roots, leaves, and stems.
  • Habitat: Both organisms need a lot of sunlight for photosynthesis; therefore, they mainly live in shallow coastal areas. Seagrasses are, however, highly versatile and grow on all substrates. Seaweeds, on the hand, need something hard to attach to. They then attach themselves to rocks or shells.
  • Reproduction: Another main difference between seagrasses and seaweeds is their mode of reproduction. Seagrasses produce flowers, fruit, and seeds, while seaweeds produce spores. Nonetheless, they both exhibit sexual and asexual reproduction.
  • Transport system: Seagrasses have a well-specialized vascular system that transports nutrients throughout the plant. On the other hand, seaweeds lack a vascular system and extract nutrients from the water through diffusion.

Let’s explore more on these main differences between seagrass and seaweed below.

What is Seagrass?

Seagrass is a flowering marine plant that is adapted to the marine environment. But, unlike other flowering plants, seagrasses lack stomata.

However, since they are photosynthetic, they mainly occur in shallow depths with abundant sunlight. Moreover, seagrasses form dense underwater meadows. These meadows provide food and shelter to thousands of marine organisms.

Dense seagrass meadow in clear water
Dense seagrass meadow in clear waters

There are about 72 species of seagrasses, and they belong to four families. These families are; Posidoniaceae, Zosteraceae, Hydrocharitaceae, and Cymodoceaceae, with the most common seagrass species being eelgrass and manatee grass.

Structure

Seagrasses are believed to have evolved from terrestrial grasses and have well-specialized tissues. They possess well-defined roots, leaves, and stems, also referred to as rhizomes. Rhizomes penetrate through the sediments and hold the plant in place. Besides, they also absorb nutrients from the sediments.


Habitat

Generally, seagrasses occur in shallow coastal waters such as bays, estuaries, and lagoons. They grow all over the world except in Antarctica. Seemingly, they prefer to grow in intertidal zones with unlimited sunlight and soft sediments such as mud and sand.

Seagrass occurring on a sandy substrate

They grow extensively to form seagrass beds or meadows, which can survive up to depths of sixty meters. However, the depth to which they occur primarily depends on the availability of sunlight. This is because, like plants, they need sunlight for photosynthesis.

Reproduction

Seagrasses are flowering plants that produce seeds and can reproduce sexually or asexually. In sexual reproduction, they have separate male and female flowers. The male flowers produce pollen into the water, which is then carried through the water currents to fertilize the female flowers.

In asexual reproduction. Like land grasses, rhizomes connect seagrass under the sediments, spreading extensively and sprouting new shoots. A single seagrass plant can produce an entire underwater meadow.

Transport System

Additionally, seagrasses are vascular marine plants with a well-structured network of xylem and phloem. These vascular tissues transport nutrients and dissolved gasses throughout the plant. The roots and rhizomes extract nutrients from the sediments, while the leaves extract nutrients from the water column.

Threats

In recent years, there has been a decline in seagrass meadows. This is highly due to climate change. However, human activities account for the highest percentage of seagrass meadows loss. Similarly, diseases have also contributed to their decrease too. For example, in the early 1930s, a severe eelgrass die-off was caused by a ‘wasting disease.’

The good thing is that there are attempts to conserve seagrass meadows. And they include limiting harmful human practices that contribute to their vulnerability and restoring seagrass beds by planting seagrasses in the affected areas.

What is Seaweed?

Seaweed is a name commonly used to refer to multicellular marine algae. They are non-vascular organisms, meaning they are simple with no complex structure.

Although seaweeds are not plants, they play a similar role in their ecosystem since they are primary producers. In addition, they also provide food and habitat to thousands of marine organisms.

Seaweeds are generally divided into three groups, red algae, green algae, and brown algae, with the most common ones being kelp and sea lettuce.

Seaweed forest
Seaweed forest

Credit: NOAA’s National Ocean Service (CC 2.0)


Structure

Although they are plant-like in appearance, seaweeds are not actual plants. In other words, they lack roots, stems, and leaves. Instead, they have a thallus body. The thallus is an undifferentiated tissue of three significant parts, stipe, holdfast, and blade.

The holdfast is a root-like structure that only anchors the seaweed to a surface, thus serving as the seaweed’s root. The blade, also referred to as the lamina, is a flattened leaf-like structure of the seaweed. At the same time, the stipe is the stem-like structure that connects the blade to the holdfast. However, the stipe might be absent in some species.

Habitat

Like seagrasses, seaweeds prefer to live in cold, shallow brackish waters with sufficient sunlight. They are usually found on rocky shores rather than sandy shores.

Further, seaweed can grow as far as 250 meters below the ocean’s surface, especially in clear waters. They also form dense underwater forests, thus supporting marine life by providing food and shelter.

Different seaweed species gowing on a rock substrate
Different seaweed species anchored on a rock substrate


Reproduction

Seaweeds display both sexual and asexual reproduction. In sexual reproduction, they produce spores that settle and develop into male and female plants known as gametophytes.

The gametophytes then produce the male and female gametes (sperm and egg) and release them into the water. Fertilization occurs when the sperm and the egg fuse together to form a zygote. In the end, the zygote grows and becomes a sporophyte, and the life cycle continues.

In asexual reproduction, reproduction occurs through fragmentation or division. This happens when parts of the seaweed break off and grow into new individuals. Asexual reproduction produces clones, meaning the offspring is genetically identical to the parent seaweed.

In some cases, some species exhibit both sexual and asexual reproduction.

Transport System

Unlike seagrasses, seaweeds lack roots, stems, and vascular tissues. Instead, they use their blades to absorb nutrients directly from the water through diffusion.

Threats

Despite the ecological significance of seaweed, just like other marine organisms, they are also under threat. Currently, seaweeds have been dying off at an alarming rate.

One major threat to seaweed is climate change. For instance, elevated temperatures have critically reduced seaweed forests in our oceans. In addition to climate change, other threats to seaweeds are from sea herbivores and human activities, such as poor fishing practices and pollution.

However, some simple step everyone can take to protect seaweeds is to minimize harmful human interaction. Alongside this is restoring and creating new seaweed forests by companioning seaweed cultivation.

FAQs

Which one is edible; seaweed or seagrass?

Most species of seaweed are edible, while seagrass is not. However, some Australian natives consume the fruit of the tape grass. On the other hand, seaweeds contain a wide range of nutrients and are consumed as food worldwide. Most species, like kelp, gut weed, and dulse, are extensively used as food in coastal cuisines.

Why are seagrass and seaweed important to marine life?

Besides providing food and habitat to thousands of marine organisms, seaweeds and seagrass act as carbon sinks. They play an essential role in producing oxygen. This is because, during photosynthesis, they utilize carbon dioxide from the atmosphere and, in return, generate oxygen used by marine life for respiration.

Can seaweed and seagrass be farmed?

Yes, both seaweed and seagrass can be farmed. In fact, seaweed farming is one of the fastest-growing aquaculture sectors. Seaweed is mainly cultivated to provide food products and for restoration measures. On the other hand, seagrass is highly endangered and mostly farmed to restore lost seagrass meadows.

Between seaweed and seagrass, which one is toxic?

None of the two is toxic. Even though we classify seaweeds as macroalga, no known toxic or poisonous seaweed species exist. Likewise to seagrass, it is not harmful but is unsuitable for the human digestive system. Human stomachs cannot digest seagrass, so consuming it is pointless.