Marine Invertebrates

Invertebrates make up over 97% of all animal species on Earth, forming the largest group in the animal kingdom. Boston Harbor is no exception – its most abundant residents are marine invertebrates. Invertebrates are classified based on lifecycle, morphology, molecular similarity, and evolutionary history, but they all share a key trait: the absence of a backbone.

Marine invertebrates play a critical role in coastal ecosystems. They are more than just residents; they serve as engineers, recyclers, and frontline defenders. They filter and clean the water, recycle nutrients, build habitats, stabilize shorelines, and provide food for countless species.

Continue reading to learn about the diverse groups of marine invertebrates that inhabit Boston Harbor.

The Base of the Food Web

At the base of the food web, zooplankton form a diverse group of drifting organisms that fuel the harbor’s food web. They provide a vital food source for fish, seabirds, and even whales. Some are holoplankton, such as copepods, which remain planktonic for their entire lives. Others are meroplankton, the larval stages of animals such as mollusks, worms, barnacles, and other invertebrates. Meroplankton disperse and colonize new habitats before settling into adult life.

 

Close up of Bread-crumb Sponge (Halichondria panicea) pores.

Observation on iNaturalist by Jean-Paul Boerekamps, public domain.

Sponges

Sponges, belonging to the phylum Porifera, are amongst the earliest and simplest animals, dating back 600 million years. They feed by filtering water through tiny pores in their walls, removing bacteria, plankton, and pollutants, which improves water quality. They also serve as hosts for various microorganisms, such as algae. The bread-crumb sponge (Halichondria panicea) is one of the most common sponges in New England.

Bryozoans

Bryozoans are microscopic aquatic invertebrates that live in colonies made up of hundreds or even thousands of individual animals known as zooids. Together, the colony functions as a single cooperative organism. Each zooid is genetically identical, yet not all perform the same role. Some zooids are responsible for feeding, using a crown of tentacles to sweep plankton and other particles from the water column. Others specialize in reproduction, defense, or structure. In Boston Harbor, the kelp lace bryozoan often encrusts seaweed fronds.

 

Zoomed in photo of Kelp Lace Bryozoan (Membranipora membranacea).

Observation on iNaturalist by Rob Wallace, public domain.

 

Plumose Anemone (Metridium senile) attached to rocky substrate.

Observation on iNaturalist by Jean-Paul Boerekamps, public domain.

Cnidarians

The phylum Cnidaria includes soft bodied, stinging animals. Anemone, hydrozoa, and jellyfish are all examples of cnidarian that call the harbor home. All possess specialized stinging cells called cnidocytes, which are used to capture prey. Species like the plumose anemone (Metridium senile) attach to pilings and rocks. Jellyfish—including moon (Aurelia aurita) and lion’s mane (Cyanea capillata) jellies—appear seasonally in Boston Harbor.

Worms

A wide variety of aquatic worms inhabit the harbor, including segmented worms, flatworms, ribbon worms, and peanut worms. Many worms are especially abundant in the mudflats, where they recycle nutrients and provide a crucial food source for fish and shorebirds. Others are parasitic, preying on mollusks and other invertebrates.

 

A bed of Blue Mussels (Mytilus edulis).

Observation on iNaturalist by Alexander Lydon, public domain.

Mollusks

The phylum Mollusca is represented by many familiar species. Bivalves, such as our clams, oysters, mussels, and scallops, filter water and form reefs that buffer our coastlines. Gastropods, such as snails, limpets, and nudibranchs (sea slugs), graze on algae or prey on other invertebrates. Cephalopods, such as our squid, are agile predators in the harbor. Chitons are oval-shaped algae eaters that cling to rocky intertidal zones. Mollusks play a central role in both ecosystems and diets: they improve water quality, stabilize shorelines, and feed birds, fish, and mammals.

Arthropods

Arthropoda is the largest phylum in the animal kingdom. All arthropods have a hard protective exoskeleton made of a protein called chitin. Most of Boston Harbor’s arthropods are crustaceans – such as isopods, amphipods, copepods, crabs, lobster, barnacles, and shrimp. Crustaceans play a vital role in breaking down organic material and recycling nutrients back into the ocean. Barnacles carpet rocks and pilings in the intertidal zone, while copepods provide food for whales. Larger crustaceans, such as lobsters and crabs, support New England’s iconic fishing industry. The Atlantic horseshoe crab (Limulus polyphemus) is another type of arthropod that can be found in the harbor. Despite its name, horseshoe crabs are more closely related to spiders and scorpions than they are to true crabs.

 

Common Sea Star (Asterias rubens) in the intertidal zone.

Observation on iNaturalist by Jean-Paul Boerekamps, public domain.

Echinoderms

The phylum Echinodermata, meaning “spiny skin” in Greek, includes starfish, sea star, sea urchin, brittle star, and sea cucumber, all of which inhabit the harbor. These creatures have a remarkable superpower – the ability to regenerate lost body parts. This ability helps them survive attacks, recover from injury, and maintain their roles in the ecosystem as predators, grazers, and recyclers. The common sea star (Asterias rubens), often seen in the intertidal zone, preys on mussels and clams, helping regulate populations.

Tunicates

Tunicates belong to the phylum Chordata because their larvae possess all four chordate traits, including a notochord, which provides skeletal support. They do not, however, develop a backbone, like most other chordates. Their name comes from their protective tunic; a gelatinous outer layer that deters predators. There are two forms of tunicates: colonial tunicates, made up of individual zooids, similar to bryozoans, and solitary, or noncolonial tunicates, which are often referred to as sea squirts. Many colonial species, like the star tunicate (Botryllus schlosseri), are invasive in the harbor, and can be seen encrusting docks, ropes, and rocks in dense mats.

 

Star Tunicate (Botryllus schlosseri)

Observation on iNaturalist by Jean-Paul Boerekamps, public domain.

The Backbone of the Harbor

Despite lacking spines, marine invertebrates are truly the backbone of Boston Harbor. They support ecosystems, stabilize shorelines, fuel the food web, and sustain biodiversity - quietly shaping life above and below the waterline.

 

 

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