By the Late [Roman] Republic, the general idea of res communes was already well known in Roman culture. Plautus’ comic play Rudens, thought to date from c.190 B.C., features a fisherman (his occupation is important!) proclaiming that “the sea is unquestionably common to all persons” (mare quidem commune certost omnibus) in a longer speech that emphasizes fish as belonging to their captor.... [T]he concept becomes much more familiar in Latin sources from the Late Republic and Early Empire. Forms of common property are most often the air and the sea, but Cicero adds in the seashore (litus) as well. Although the familiar idea of common property was further elaborated in Early Imperial poetic or Stoic theories on the origin and development of human culture, there is no sign that these theories had any pronounced influence on the law; certainly they lead to no demonstrable legal conclusions.
By the Late Republic, a second cultural element had emerged: a widespread and often expressed disquiet about the startling proliferation of villas along the coast of central Italy and the Bay of Naples.... The architects of these villas, capitalizing on recently-discovered hydraulic concrete, often put down substructures extending beyond the shore and out into the sea — a phenomenon quite commonly referred to in legal texts, and one of particular concern to poets such as Horace who perceived the structures as morally hubristic. More exciting, however, is the discovery that not a few of the maritime villas incorporate fishponds so large that they were plainly intended to produce fresh fish not just for the villa, but for local markets.... [T]hese fishponds may well have brought the villa-owners into direct confrontation with more humble local fishers.
The ambiguity of late classical law is famously captured in a fragment of Ulpian (D. 220.127.116.11).... Ulpian confronts two questions, which he answers in quite different ways:
• first, is there liability for iniuria (affront) if someone prevents me from fishing in the sea or using a dragnet from the shore; here, Ulpian stresses the right of individuals to act or move about, without obstruction, in public places (i.e., the seashore as public property, like baths or theatres).
• second, is there liability for iniuria if, not just someone, but a villa-owner, prevents me from fishing in front of his villa? Again, the answer is yes, but for what seems, at first, like an entirely different reason: "the sea and its shores are the common property of everyone (commune omnium), like the air; and imperial rescripts have often held that no one can be prohibited from fishing."
For all that the idea of Common Things had long been ‘in the air’,... the legal category of res communes was a creation of the Severan jurists late in the classical period, a fact that helps to explain a good deal of the seeming conflict between these sources and earlier ones. Less clear is whether the Roman jurists ever worked their way through to a concept at least very broadly resembling (or at least presaging) the modern Public Trust Doctrine. But the Anglo-American concept of a trust, with its division of ownership between a trustee who holds property and a beneficiary who has an equitable title in it, is not altogether unlike — at least in the case of res communes that the jurists describe as public, namely larger rivers and the seashore — the division between public ownership and the common “ownership” of beneficiaries; and that parallelism is reinforced by the jurists’ insistence on the Natural Law origin of res communes.
On the other hand, it is also true that Roman law private itself does not usually recognize a divided ownership of this type, and later lawyers in the European Continental tradition have also found it troubling. Nor do the jurists impose on the Roman State itself any positive duty to protect res communes from appropriation by private individuals. However, such a duty was arguably implicit in the very creation of the category, as the second half of Ulpian, D.18.104.22.168, suggests.
Somewhat confirming the general public trust nature of res communes omnium in Roman law is the State’s reversionary right to shore land on which private buildings have been erected, in the event these buildings are later demolished.... The State’s reversionary right indicates that a private person is deemed to have only temporary or interim ownership of the developed property. Put another way, the State cannot permanently transfer res communes to private individuals, unlike with ordinary public property. Such a constraint on State power to alienate is a prominent feature of the modern Public Trust Doctrine.