Page 315 - (DK) Ocean - The Definitive Visual Guide

SMALL, BOTTOM-LIVING PHYLA 313

Small, Bottom-living Phyla

MANY DISPARATE INVERTEBRATES play important parts in marine ecosystems but
DOMAIN Eucarya
are seldom seen, because they are small or their habitats are difficult to study. Like
KINGDOM Animalia
all animals, they are grouped into phyla, each phylum representing an apparently
PHYLA 10
distinct body plan. Most of these small, bottom-living phyla live in seabed sediment
SPECIES Many
and are loosely called “worms” due to their shape and burrowing lifestyle.
However, the superabundant nematodes or roundworms of which there are 28,000 described species and
possibly a million in total, live in a wide range of environments, including
the seabed. A selection of 10 bottom-living phyla are represented below.

Sand-grain Animals SAND COMMUNITY
A community of tiny animals, referred to collectively as meiofauna, lives Many invertebrates
in the surface water film between the sand grains on beaches and in exist in the watery
spaces between
1
1
shallow water. They range in size from / 100 in to / 2 in (0.05 mm to 1 mm) grains of coastal
and so can only be seen well with a microscope. Many have intricate sand. The wormlike
and beautiful shapes. Almost every marine invertebrate phylum has gastrotrich (phylum
representative species that live in this habitat, and several phyla, such as Gastrotricha)
shown in this
tardigrades (water bears) and gastrotrichs, occur virtually nowhere else. photo-micrograph is
A diverse meiofauna is a good indication of a healthy environment, since one of many species
these minuscule organisms are the basis for many marine food chains. found in this habitat.
Mud-swallowers

FEEDING APPARATUS The consistency and structure of seashore and seabed sediments
Female spoonworms (phylum
Echiura) sweep up organic depends largely on the many wormlike phyla that live there. Millions
material and sediment with of these burrowing animals continually mix and rework the sediment,
a scooplike proboscis, seen a process called bioturbation. Lugworms (see p.274) are famous for their
here extending from a burrow. ability to eat sand, depositing the inedible material on the sand surface
in the form of coiled heaps, or casts, but many of the less well-known
groups, including peanut worms, acorn worms, and kinorhynchs, are
just as important. Organic material washed onshore with each tide or
carried by currents is quickly incorporated into the sediment as the
animals move around, or is processed as the surface mud is eaten.

New Phyla

Scientists have so far described only a fraction of the species
that live in oceans. New species are being discovered all the
time, mostly in groups such as sponges and soft corals that
traditionally have been neglected. Occasionally a species is
found that is fundamentally different from all other known
organisms and so it is classified as belonging to a new
phylum. Most of these exciting discoveries are from
inaccessible areas such as deep-sea mud, and the animals are
usually small. But when the abundant life around deep-sea
hydrothermal vents (see p.188–89) was sampled in the
1970s, gigantic tube worms like no others were found.

NEW SPECIES
Symbion americanus
(above) lives on the
mouthparts of the
American lobster
(left). It was first
HORSESHOE WORMS described in 2006,
These sedentary worms (phylum and is the second
Phoronida) live in small tubes buried in species in a new OCEAN LIFE
sand or mud or (as here) attached to phylum of animals,
seabed rocks. To feed, the worms extend the Cycliophora
a horseshoe shaped net of tentacles. (see p.316).