CEPHALOPODS AND FISH: THE LIMITS OF CONVERGENCE
Authors: PACKARD A. 
Published: January 2008

Resemblances between cephalopods and fish

1. Modern cephalopods (coleoids < 1000 species) resemble modern fish (30,000 species) more closely than any of their ancestors did. They have not been replaced by the more diversified group in geological time.

2. The main body of the article (pp. 245–283) reviews these resemblances. They are to be found at all levels of analysis.

3. Basic physiological mechanisms of molluscs (pp. 262–5) have been incorporated into systems with performances comparable to those of vertebrate systems. For instance the cephalopod locomotory system (pp. 249–56) and hydrostatic control system (pp. 256–60), structurally very different from their fish counterparts, have similar adaptive radiation.

4. Behaviour (p. 278) and growth of the brain (p. 265) are characteristically vertebrate‐like.

5. Cephalopods and fish are considered as occupying the same broad adaptive zone though modal differences (pp. 283–5)‐in reproductive habits, growth rate and light‐dependent behaviour where extraocular photoreceptors appear to be important ‐ mean that they occupy different areas within that zone.

(ii) Evolution of convergence

6. Evidence is presented (pp. 287–293) for considering the convergence as due not merely to similar physical demands of the marine environment, but to dynamic interactions between cephalopods and vertebrates from the late Palaeozoic onwards.

7. The convergence was set on its way when the two groups, independently of each other, acquired locomotory methods that allowed them to increase in size.

8. It is argued that reduction and eventually complete loss of the chambered shell (in all but sepiids) was an evolutionary response to the needs of increased mobility and to the need to go deeper as vertebrate predators pushed out into oceanic waters.

9. The ammonites (pp. 291–2) present a partial model of the course that coleoid ancestors may have taken.

10. Coleoids subsequently reinvaded surface and coastal waters, competing successfully in a teleost habitat. Their most striking adaptations are ones that they share with teleosts.

11. Behavioural interactions in a vertebrate‐dominated environment have probably been responsible for the vertebrate‐type eye of cephalopods (p. 293).

12. The conclusion that the common adaptive zone shared by cephalopods and fish has been achieved by mutual interactions between the two groups evokes no special evolutionary principles. It assumes that all cephalopod species have (at some period of their evolutionary history) been in competition for food with some vertebrate species and that vertebrates are a source of selection pressures ‐ largely operating through visual behaviour ‐ that maintain and promote convergence upon the ‘fish’ modal type.

13. Selection pressures also operate within behaviour space to maintain and promote the special differences that separate any competitively successful species from all others. Coleoids as a group appear to have retained adaptations associated with such molluscan features as high growth rate and rapid turnover of the population. They are still characteristically crepuscular in habits and have extensive vertical mobility.