Gutweed (Ulva intestinalis) – Comprehensive Guide


Gutweed, also known as Ulva intestinalis, is a type of green alga found in the family Ulvaceae. It has other common names such as Sea Lettuce, Green Bait Weed, and Grass Kelp.

Before the early 2000s, Gutweed and other tubular members of the sea lettuce genus Ulva were classified under the genus Enteromorpha.

This changed after genetic research showed that they should be part of the Ulva genus.

Where Is Gutweed Found?

Gutweed has a worldwide distribution. It’s found in various parts of the globe, including the Bering Sea near Alaska, the Aleutian Islands, Puget Sound, Japan, Korea, Mexico, the Philippines, and Russia.

In addition, Gutweed is found in Israel and European countries such as the Azores, Belgium, Denmark, Ireland, Norway, and Poland.

It’s also found in the Baltic and Mediterranean Seas, as well as the shores of the Pacific Ocean, including New Zealand.

What Does Gutweed Look Like?

Gutweed has fronds that are completely tubular and branch out, expanding in width towards the middle of the thallus. These fronds can reach 15 cm or more in length.

The cells of gutweed are irregularly arranged, and the chloroplast is hood-shaped, usually with only one pyrenoid.

Gutweed can be 10 to 30 cm long and 6 to 18 mm wide, with rounded tips.

It may reproduce at any time of the year and has a life cycle with alternating generations, where the gametophyte and sporophyte have the same shape.

Subspecies of Gutweed

Some sources classify Ulva intestinalis into two subspecies: ssp. intestinalis (L.) Link and ssp. compressa (L.) Link.

What Is the Origin of The Name Gutweed?

The name “Gutweed” comes from the Latin word “intestinalis,” which means relating to or found in the intestines.

This name was chosen because of the appearance of the long, thin, inflated tubes that make up the seaweed.

How Can Gutweed Be Identified?

Identifying Gutweed and other species within the genus Ulva can be challenging.

Identification relies heavily on details such as cell arrangement and cell detail, as well as the overall shape of the alga.

However, the shape of a single species can change depending on environmental conditions, making identification even more difficult.

The Distinction Between Gutweed and Ulva compressa

Gutweed and Ulva compressa are two distinct species that are genetically different and reproductively isolated. However, they are hard to tell apart.

The presence or absence of branching fronds is the most useful characteristic for distinguishing these species, with Gutweed having unbranched fronds.

However, low salinity or salinity shock can cause Gutweed to have branching fronds, making identification more challenging.

By considering environmental factors like salinity, most thalli can be identified correctly.

In What Habitats Is Gutweed Found?

Gutweed is found in a wide range of habitats, from all levels of the shore to rocks, mud, sand, and rock pools.

It’s abundant in brackish water areas with significant freshwater runoff and in wet areas of the splash zone.

Gutweed is also found growing as an epiphyte on other algae and shells.

Sometimes, it becomes detached from its habitat and floats to the surface, where it continues to grow in floating masses.

Depth Range and Environmental Factors

Based on 49 specimens, Gutweed has a depth range of 0 to 9.37 meters.

The water temperature and chemistry ranges are based on one sample, with a temperature of 21.061°C, nitrate levels of 1.956 umol/L, salinity of 35.349 PPS, oxygen levels of 5.197 ml/l, phosphate levels of 0.384 umol/l, and silicate levels of 5.808 umol/l.

Gutweed Characteristics

Care Level:Easy
Growth Rate:Fast
Nutrient Uptake:Excellent
Required Lighting:Moderate High
Required Water Flow:Low Moderate
Is Palatable?Yes
Maximum Size:12 in (30.5 cm)

Scientific Classification

Scientific Name:Ulva intestinalis
Also Known As:Gutweed, Green Bait Weed, Grass Kelp, Sea Lettuce
Conservation Status:Unknown

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