Benthic animals live on both hard and soft bottoms

Benthic fauna includes those invertebrate animals that spend most of their life cycle on the seafloor, either buried in the soft sediment or attached to the hard substrate.

The extensive range of benthic animals includes species from several different groups. The species composition of benthic fauna changes according to the local environmental conditions. The community is mainly determined by the seafloor structure, depth, and salinity, as well as the openness of the shoreline.

The isopod Saduria entomon is the largest benthic invertebrate on the Finnish shores

Although most benthic animals are quite small, they can still be detected easily with the naked eye. The largest benthic invertebrate on the Finnish shores is the benthic isopod species, i.e. Saduria entomon, which may grow up to ten centimetres in length. In Finnish it is called a "kilkki". Like many other benthic fauna, these large isopod crustaceans spend a part of their life either digging in or crawling around on the  seafloor and occasionally they also feed in the water column. Fun fact! An isopod crustacean is one whose every pair of legs are identical and not adapted for different functions.

Other benthic fauna that occur alongside Saduria in the Baltic Sea include amphipods, prawns, mysid shrimps, oligo- and polychaete worms, and of course, various species of clams and snails.

In shallow areas, various aquatic insects also thrive in benthic faunal communities, such as caddis-fly and dragonfly larvae, whose aquatic phase is usually longer than their adult flying stage.

The isopod crustacean, i.e. Saduria entomon, crawling on an algal mat.
Despite their fierce appearance and the fact that they are quite large invertebrates, the isopod Saduria entomon is a placid carrion eater.

The benthic faunal community of the soft seafloor is diverse

Many different species, such as clams, snails, crustaceans, oligo- and polychaete worms, as well as priapulid worms inhabit the sediments of the soft seafloor. Some of the largest of these species are the bivalve clams. The most common clam species found along almost the entire Finnish coastline are the so-called Baltic tellin (Limecola bathica) and the sand-gaper (Mya arenaria). The former thrives in muddy sediments while the latter prefers somewhat coarser bottom deposits with a higher sand content.

A close-up shot of the Baltic tellin (Limecola balthica) on sand.
The Baltic clam or tellin, i.e. Limecola balthica, is the most common benthic animal of soft seafloor sediments in the Baltic Sea.

Worms and amphipod crustaceans crawl about in the soft
bottom deposits

Various species of marine annelid worms crawl about in the soft bottom sediments alongside the clams. One group is the oligochaete worms, which are the aquatic cousins of the terrestrial earthworm. Oligochaete worms are pale, transparent and quite fragile. These worms get their name from their relatively few stiff bristles or chaetae, which protrude from their bodies and allow them to crawl through the sediments.

Among the oligochaete worms and clams are also found the polychaete worms, which
somewhat resemble terrestrial millipede insects. Unlike oligochaetes, polychaetes can have multiple bristles per segment along the entire length of their bodies and often also have tentacles arranged around their heads. Polychaetes prey on other benthic fauna smaller than themselves either within the sediment or in the water currents immediately above the bottom. The largest species of polychaete in Finnish waters is the ragworm, i.e. Hediste diversicolor, which can reach up to 20 centimetres in length.

The amphipod crustacean species, i.e. Monoporeia affinis and the red-eyed Pontoporeia femorata are the hardest-working of the benthic fauna as they scurry about their tasks. These tiny crustaceans live in the surface layers of the bottom sediments, only a few centimetres below the sediment-water interface. They occasionally also venture out to the water above to feed. 

A wide variety of benthic fauna hides on hard bottoms

A wide variety of benthic animals can be found living on hard bottom types, such as rock, boulders and stones. Some species attach themselves directly to the hard substrate, while others require the protection of seaweeds. The most visible are the blue mussels, which grow up to several centimetres in length and are the most common in the outer reaches of the archipelagos on the southwest coast of Finland.

In terms of visibility, at the other end of the scale are the polyps, whose transparent colonies are easy to miss among the growths of filamentous algae. Both polyps and blue mussels alike grow attached on the hard substrate and cannot dart away from observers or predators.

Snails, gammarid amphipods and sea slugs graze on mats of diatoms that have accumulated on seaweeds growing on the hard substrate. These benthic animals can be easily seen just by slightly moving the mass of seaweed.

The shell of the blue mussel is open, showing the orange mantle. The siphon is used to filter seawater.
Blue mussels diligently filter nutrients from the water.

Benthic fauna:

  • Benthic isopod crustacean (Saduria entomon)
  • Mud shrimp/amphipod (Corophium volutator)
  • Baltic tellin (Macoma balthica)
  • Sand-gaper clam (Mya arenaria)
  • Oligochaetes (Oligochaeta)
  • Ragworm (Hediste diversicolor)
  • Red-gilled mudworm (Marenzelleria spp.)
  • Amphipod crustaceans (Monoporeia affinis, Pontoporeia femorata)
  • Marine Leaf beetle (Macroplea spp.)
  • Blue mussel (Mytilus trossulus)
  • Polyps (Cordylophora caspia, Laomedea loveni)
  • Gammarid amphipods (Gammarus spp.)
  • Herbivorous isopods (Idotea spp.)
  • Brackish water sponge (Ephydatia fluviatilis)
  • Encrusting bryozoan (Electra crustulenta)
  • River nerite snail (Theodoxus fluviatilis)
  • Hydrobid mud snails (Hydrobiidae)
  • Broad-headed lanceolate slug (Limapontia capitata)