Molluscs, crustaceans and marine invertebrates

Five views of the same small, oval, glossy cream-coloured shell, laid side by side: top, side, underside, back and front. — A 2.6 cm money cowrie shell from the Batu Islands in Indonesia, shown from five different angles (photograph by H. Zell, 2012). H. Zell, CC BY-SA 3.0, source

Nine out of ten marine species do not yet have a name.

This is the most frequently cited figure: 91% of the species in the sea remain to be described, compared with 86% for all species on the planet. It comes from an estimate published in 2011, which predicts about 8.7 million species on Earth, of which 2.2 million are in the ocean.¹

This figure is disputed, and that should be said from the outset. A team bringing together more than one hundred and twenty marine taxonomists replied as early as 2012 that such a total is unlikely, and that only between one third and two thirds of marine species remain to be described. The gap does not come from the observations but from the method: the first estimate extrapolates from the shape of the classification, the second directly asks the specialists of each group.²

What no one disputes is the direction: the sea is the main area of our ignorance. It is the exact extent that is debated.

This page lies at its heart.

The place of this page in the encyclopaedia

In a classic animal encyclopaedia, priority generally goes to the large vertebrates: lions, whales or primates. The figures from scientific research confirm this attention bias.

Yet the groups covered here — molluscs, crustaceans, sea urchins, sponges, jellyfish, marine worms — make up the greater part of animal biomass and diversity.

This page is deliberately extensive. Not out of aesthetic preference, but because these organisms dominate the living world in numbers and in ecological impact.

The octopus: established facts and the limits of knowledge

The study of cephalopods benefits from separating rigorous observations from speculative interpretations.

Anatomy and lineage

Evolutionary divergence: The mollusc lineage split from the one leading to vertebrates a very long time ago; the first cephalopods appeared about 530 million years ago.
Nervous structure: A key part of their brain retains the basic organisation of invertebrate nerve centres. Most of their neurons are located in their arms.³

Experimental results on behaviour

In 2016, a study put seven octopuses through a five-level test. They were first taught to open an L-shaped container to take the food. Then they had to pass it through a narrow hole cut in a transparent partition — which required them to pull, release, then push. Next, the object’s orientation changed. Then the partition became opaque. Then the orientation was drawn at random on each trial.⁴

All seven octopuses succeeded at every level.⁵

The most interesting result, however, is not the one you might expect. At each change of rule, their performance dropped — they took longer. Then they adapted so quickly that, taken together, working times did not differ significantly from one level to the next.⁶

In other words, what the experiment measures is not that they knew how to do it: it is that they relearned quickly. The authors call this behavioural flexibility.⁷

What remains to be established

The authors of the study say so from the outset: octopuses have large brains and complex behaviours, but relatively little is known about their cognitive abilities. Seven animals make a small sample. Variability from one individual to another is high, and its causes are poorly identified.⁸

What is established is the capacity for learning, adapting and changing strategy. What is not established is the nature of their subjective experience.⁹

Scientific neutrality and errors of interpretation

Two opposing analytical biases distort the study of living things:

Anthropomorphism: Projecting human emotional categories onto a radically different nervous system.
Dogmatic reductionism: Rejecting on principle any form of inner experience in the absence of experimental evidence to the contrary.

These two approaches share the same flaw: settling on a conclusion before anything has been measured.

Editorial rule

This encyclopaedia applies a strict method:

The state of scientific research is reported without extrapolation. Where science has not settled a question, the doubt and the subject of the disagreement are made explicit.

Asserting an unsupported certainty risks leaving the reader with false information. Correcting a statement that has taken hold among the public is a complex process, as the history of reference publications on wildlife shows.

The cowrie: the role of a mollusc in the global economy

The impact of invertebrates extends beyond the strictly biological field. The shell of the cowrie (Monetaria moneta), a small marine mollusc, served as a medium of exchange on an international scale.¹⁰

Fishing: Fished mainly in the Maldive Islands, in the Indian Ocean.
Export: Shipped in billions of shells.
Use: Used as everyday currency in West Africa and Asia over several centuries.

This trade required structured maritime networks, precise accounting and a global commercial agreement. It was a major economic system based on a marine biological resource.

Exploring marine biodiversity

Whichever estimate is adopted, the biological inventory of the oceans is still largely to be done.

Most current data concern coastal and shallow areas, which are easy to reach. Knowledge of the marine environment depends directly on the state of the technology for reaching the depths and observing them.

Ecological functions: the case of filtration

Numerical data on the role of marine organisms must be analysed rigorously.

Revising the oyster example

The often-quoted filtration figure of “190 litres of water per day per oyster” corresponds to optimal laboratory conditions (targeted temperature and salinity). It does not reflect an annual average. In the natural environment, the rate is far more variable: on reefs measured on site, small oysters filtered on average about 1.2 litres per hour.¹¹

A more representative indicator comes from a historical comparison of the same ecosystem:

Chesapeake Bay (historical): Oyster populations filtered the entire volume of the estuary in 3 to 4 days.
Chesapeake Bay (present): In the northern part of the bay, oysters have fallen to less than 1% of their numbers in the early nineteenth century. Overall filtration now takes more than 300 days.¹²

Impact on the environment

A restored oyster reef can remove up to ten times more nitrogen from the environment than a nearby unrestored bottom. In a single Maryland waterway, Harris Creek, the restored reefs remove each year as much nitrogen as 20,000 bags of garden fertiliser.¹³

Other filtering organisms

The work of filtration is shared by many sessile organisms:

Organism	Indicative filtration capacity	Ecosystem impact
Oyster (Crassostrea virginica)	4 to 34 L/h (depending on conditions)	Clarification of estuaries, nitrogen retention
Sponge (one-kilogram individual)	up to 24,000 L/day	Bacterial purification, nutrient cycling

Sources for the table¹⁴

The clarity and chemical quality of coastal waters depend on the combined action of these immobile benthic organisms.

Ecosystems and human pressures

Marine invertebrates hold a central place in the balance of the ocean, and two pressures weigh on them today:

Ocean acidification: The absorption of atmospheric CO₂ lowers marine pH and reduces the availability of carbonate ions, which are essential to the calcification of molluscs and corals. (See Entry 7.13, Human impact and climate change.)
Bottom trawling: The mechanical destruction of benthic habitats directly affects populations of filter-feeding invertebrates and the associated food webs.¹⁵

Note 1

it predicts ∼8.7 million (±1.3 million SE) eukaryotic species globally, of which ∼2.2 million (±0.18 million SE) are marine […] some 86% of existing species on Earth and 91% of species in the ocean still await description

Mora, Camilo et al., “How Many Species Are There on Earth and in the Ocean?”, PLoS Biology, vol. 9, no. 8, e1001127, 2011

In spite of 250 years of taxonomic classification and over 1.2 million species already catalogued in a central database, our results suggest that some 86% of existing species on Earth and 91% of species in the ocean still await description.

Mora, Camilo et al., “How Many Species Are There on Earth and in the Ocean?”, PLoS Biology, vol. 9, no. 8, e1001127, 2011, open access