How Much Do Starfish Weigh?

Starfish are fascinating marine creatures and vary a lot in size, weight, and even the environment they live in, depending on the species. When underwater, you can’t really check their weight, so have you ever wondered how much starfish weigh? In this post, we’ll talk all about it but let’s begin with a quick answer:

Starfish vary in weight and size depending on the species. On average they weigh between 3.3 and 6.6 lbs (1.5 – 3 kg). The heaviest starfish such as Catala’s sea star can weigh up to 13 lb (6 kg).

However, this certainly doesn’t tell the whole story, and in this post, I’ll explain more about how much starfish weigh and how big they get. Furthermore, I’ll explain why starfish weigh that much. Read on!

How much do starfish weigh? (Explained)

The weight of starfish largely depends on the species, their size, and diet. Some starfish are very flattened and, even when large, can be not as heavy as expected. Others can have very thick bodies and be very heavy compared to their length. Firstly, let’s answer this question with the heaviest and the lightest species.

The heaviest starfish species

Catala’s sea star (Thromidia catalai) is the heaviest sea star recorded, and its weight was reported as much as 13 lb (6 kg). This star specie’s size is about 24 to 26 in (60 to 65 cm), which is large but doesn’t make it the largest sea star in the world.

Thromidia catalai has long, massive and cylindrical arms, which makes the starfish heavy. The tips of its arms are also rounded, while most starfish species’ arms are tapering.

The other heavy starfish is the sunflower sea star which is one of the largest sea stars ever recorded. It can weigh up to 13.4 pounds (5 kg) and has many (less massive than Catala’s sea star) arms, sometimes even more than 20! It can grow to about 3.3 ft (1 m) in diameter.

The lightest starfish species

The lightest sea stars are Parvulastra parvivipara and Allostichaster palmula – they’re also the smallest sea stars in the world, with a maximum diameter of only about 0.4 in (1 cm). Their exact weight is not known, but we can assume it’s closer to 0 than to any other number.

The average size of the starfish

There are about 2,000 different species of starfish, and as you can see, they vary in weight and size a lot. It makes it very difficult to define the average weight, but it’d range between 3.3 and 6.6 lbs (1.5 – 3 kg).

What makes the starfish heavy?

Interestingly, starfish weight is not only defined by their skeleton and other body parts but mostly – by the amount of water inside. Starfish pump the water inside their bodies in order to breathe and walk. If we dried starfish out (which we couldn’t do because it’d be deadly), it would lose a lot of weight.

Starfish use the water vascular system as their circulatory system. They don’t have a heart or blood, so they pump sea water instead. During the circulation, the water enters through a small, smooth plate located on the aboral side of the sea star (madreporite) and flows through the canals, exchanging the nutrients and gases.

How much do baby starfish weigh?

Starfish very often reproduce sexually through the process called spawning. During this event, female starfish release eggs, and male starfish release sperm into a water column, all at the same time. Once they meet and fertilize, starfish begin to transform into larvae.

Starfish larvae can measure about 0.04 in (1 mm) in length, which is the size of a grain of rice. We can assume they weigh something very, very close to 0. During this time, they’re floating as part of the plankton before becoming a juvenile. Read more about it in my other blog post: “How Are Starfish Born?”.

How big do starfish get?

Like in the previous question, let’s start with the biggest and the smallest starfish in the world. Starfish size is usually measured in diameter which is a straight line passing from the tip of one arm to the opposite one, through the center.

The biggest starfish

As I already mentioned, the sunflower sea star is one of the biggest starfish. It can grow to about 3.3 ft (1 m) in diameter and weigh about 13.4 pounds (5 kg). Unfortunately, this starfish is critically endangered due to the sea star wasting syndrome and rising water temperatures.

Another starfish we classify as one of the biggest in the world is Midgardia xandaros. This starfish can grow to 53 in (135 cm) in diameter but there were a lot of research about these species yet. These starfish are unusual as they have a very small central disc but extremely long arms. They were found in deep water in the Gulf of Mexico.

The smallest starfish

The smallest starfish in the world are Parvulastra parvivipara and Allostichaster palmula which are also classified as the lightest starfish we talked about above. They can grow only up to 0.4 in (1 cm) in diameter. They usually live in intertidal rock pools or granite rocks.

If you wanted to see their tiny tube feet, you’d need a microscope. Interestingly, paddle-spined sea stars mostly reproduce by splitting themselves in half (asexual reproduction).