---
title: "Classifying Ocean Animals"
description: "Classify ocean animals into major groups: fish (breathe through gills, have scales), marine mammals (breathe air, warm-blooded, feed milk), and invertebrates (no backbone — jellyfish, octopuses, crabs"
canonical: https://lightmysky.com/learn/science/classifying-ocean-animals-mt__aHSZTm5k5
source: https://lightmysky.com/learn/science/classifying-ocean-animals-mt__aHSZTm5k5.md
retrieved: 2026-10-19
---

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# Classifying Ocean Animals

Classify ocean animals into major groups: fish (breathe through gills, have scales), marine mammals (breathe air, warm-blooded, feed milk), and invertebrates (no backbone — jellyfish, octopuses, crabs, starfish)

Subject: Science · Area: Ocean Life · Ages 7 to 9
Page: https://lightmysky.com/learn/science/classifying-ocean-animals-mt__aHSZTm5k5

## Ready when they can

- Sort ocean animals into fish, marine mammals, and invertebrates
- Give defining features of each group (gills vs lungs, backbone vs none)
- Correctly classify at least two animals in each group

## Lesson: Sorting ocean animals into groups

Every ocean animal fits into a group of animals with the same kind of body. Three big groups are fish, marine mammals, and invertebrates. Some ocean animals belong to smaller groups, like the reptiles. A sea turtle is an ocean reptile. To find the right group, look at how the animal breathes and whether it has a backbone.

**Example.** A tuna is a fish. Fish breathe through gills, slits on the side of their head that pull oxygen out of the water. Fish also have scales, small hard plates that cover their skin. Sharks and clownfish are fish too.

**Example.** A seal is a marine mammal, just like the whales and dolphins you already learned about. Marine mammals breathe air through lungs, so they swim up to the surface again and again. Their bodies stay warm even in cold water, and mother marine mammals feed their babies milk.

**Example.** A jellyfish is an invertebrate. Invertebrates have no backbone at all. Octopuses, crabs, and starfish are invertebrates too. Some have a soft body and some have a hard shell, but none of them have bones inside their bodies.

*(drawing: Line up 6 ocean animals and sort them into the 3 big groups: 2 fish, 2 marine mammals, 2 invertebrates. Animals in the same group have the same kind of body.)*

**Tip.** Ask two questions to sort the animals in these three big groups. Does it breathe air or water? Does it have a backbone? Air and a backbone means marine mammal. Water and a backbone means fish. No backbone at all means invertebrate.

**Recap.** Fish have gills and scales, marine mammals breathe air and feed their babies milk, and invertebrates have no backbone at all.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Whales & Dolphins Are Mammals](https://lightmysky.com/learn/science/whales-and-dolphins-are-mammals-mt_mRCPP_Ab2W)
- [Ocean Animal Variety](https://lightmysky.com/learn/science/ocean-animal-variety-mt_ytUG3yjCYt)

## Opens up

- [Grouping Living Things](https://lightmysky.com/learn/science/grouping-living-things-mt_AylKwhbDWM)
- [Ocean Food Webs](https://lightmysky.com/learn/science/ocean-food-webs-mt_OnV_DTp5i8)