Hydras

Hydras are small, sedentary, tentacled freshwater animals whose relationships with algae offer a compact example of how the route of inheritance can change the balance between parasitism and cooperation. The algae are harmful in some hydra species but beneficial and reliably transmitted in another; the decisive difference, in the account presented, is whether the algae reach future generations through the hydra’s eggs.

The Hydra–Alga Spectrum

The tissues of hydras are often parasitized by algae. In Hydra vulgaris and Hydra attenuata, the algae are described as genuine parasites that make their hydra hosts ill. Chlorohydra viridissitnay presents the contrast: its tissues are never without algae, and the algae contribute usefully to the hydra’s well-being by providing oxygen.

The contrast is not simply between an unhealthy association and a healthy one. It links the algae’s effects on the hydra to how the algae are inherited. In Chlorohydra viridissitnay, algae pass to the next generation by way of the hydra’s egg. In the other two species, they do not. The example makes transmission route central to explaining why an association can remain debilitating in one case and become beneficial in another.

Shared Transmission and Diverging Interests

The book’s explanation turns on what each set of genes stands to gain. In Chlorohydra viridissitnay, both hydra genes and alga genes depend on the production of hydra eggs to enter the future. The algae therefore have an evolutionary interest in the host’s reproduction, not merely its survival. Their interests converge with those of the hydra genes around producing more eggs.

In Hydra vulgaris and Hydra attenuata, the algae do not share that route of inheritance. Both host and parasite genes may benefit from the hydra body surviving, but only the hydra genes depend on hydra reproduction. The algae consequently remain debilitating parasites rather than moving toward benign cooperation. The distinction echoes other parasite examples in which host survival and host reproduction carry different consequences for the parasite: a parasite’s effects cannot be judged as host-serving merely because they prolong the host’s life.

A Rule for Host–Parasite Relations

Hydras help establish a broader proposed rule: ask whether a parasite’s genes reach future generations through the same vehicles as the host’s genes. If they do not, the parasite is expected to harm the host in some way; if they do, the parasite is expected to help the host survive and reproduce. Over evolutionary time, the book argues, such cooperation may become so close that the parasite merges into the host’s tissues and is no longer recognizable as a parasite.

This reasoning treats the hydra–alga cases as part of a spectrum rather than isolated curiosities. It also distinguishes shared bodily survival from shared reproductive destiny: the latter is what aligns the algae in Chlorohydra viridissitnay with hydra reproduction. The example thus supports the larger account of gene effects, in which selection can favor changes to a host’s body when those changes promote the transmission of the genes responsible for them.

Significance in the Gene-Centered Argument

The hydras provide a concrete case for the book’s argument that apparent cooperation need not arise from foresight or conscious agreement. Genes are not purposeful agents; language about their interests and calculations is a metaphor for the consequences of natural selection. When parasite and host genes depend on the same route into future generations, their evolutionary outcomes can converge, making cooperative effects more likely.

The example also sharpens the book’s distinction between an organism’s welfare and the propagation of particular genes. In the two harmful associations, algae and hydras do not share the same reproductive route; in Chlorohydra viridissitnay, they do. That difference supplies the explanatory hinge: a parasite may become a partner when its future is carried forward with its host’s eggs.