Lontra felina is the only species of the genus Lontra that is found primarily in marine habitats. The habitat of the Marine Otter is mainly the Pacific coast of South America. Marine Otters are, for the most part, restricted to salt water, but may occasionally live in freshwater rivers (Brownell 1978, Cabello 1978, Redford and Eisenberg 1992, Valqui et al. 2018a), some of them far from the coast (Ugarte-Nuñez 2021). The species inhabits marine areas exposed to heavy seas and strong wind (Cabello 1978, Ostfeld et al. 1989) and prefers rocky shores with caves that are above the high tide mark, suggesting high ecological requirements for breeding space. The species primarily uses coastlines from about 30 m inland to 100 to 150 m offshore (Castilla and Bahamondes 1979). In coastal environments of southern Peru and in Chile, it is present near large algae communities, which offer a high abundance and diversity of prey (Castilla and Bahamondes 1979). Sandy beaches offer marginal habitat (Sielfeld 1989) and typically are used only for resting during long-range travelling, for travelling between dens, and to access dens some distance from the water’s edge (Ebensperger and Castilla 1992). Because not all coastlines are suitable, Marine Otters are found in disjunct populations throughout their distribution range (Redford and Eisenberg 1992). The species has a resident population up to 170 km far from the coast (Valqui et al. 2018a), but there is no information regarding its ecology and habitat use in those habitats.

The Marine Otter is one of the smallest otters and the smallest marine mammals. It has coarse dark brown fur, with a dense insulating underfur, and webbed paws with strong claws. It is typically solitary, with a group size of seldom more than two to three individuals. The fact that Marine Otters are solitary or gather only in small groups suggests high ecological requirements regarding space.

The Marine Otter diet is variable, regionally and seasonally, and the species shows an opportunistic feeding behaviour (Medina-Vogel et al. 2004). Diet is composed mostly of invertebrates, including crustaceans (decapods, shrimps, and crabs) and molluscs (bivalves and gastropods), and vertebrate prey, including fish belonging to the families Blennidae, Cheilodactylidae, Gobiesocidae, and Pomacentridae, and occasionally birds and small mammals (Cabello 1978, Castilla and Bahamondes 1979, Ostfeld et al. 1989, Sielfeld 1990, Medina 1995, Mangel et al. 2010). Along the Valdivian coast in the south of Chile, the diet of Marine Otter consisted of 25 species; 52% (13/25) of the species identified were crustaceans, 40% (10/25) were fish, and 8% (2/25) were molluscs. Crustaceans were found in 78% of 475 spraints, 100% of 929 prey remains, and 90.8% of prey, determined by direct observation, fish in 20% of spraints and 9.0% of prey determined by direct observation, and molluscs in 2% of spraints and 0.2% of prey determined by direct observation. Observed seasonal variation in prey availability was reflected in the otter diet. Fourteen prey species were trapped; 43% (6/14) were crustaceans and 57% (8/14) fish, crustaceans were 93% of 566 trapped individuals, fish 7%. Lontra felina showed opportunistic feeding behaviour, selecting prey seasonally according to their availability rather than to their energy input (Medina et al. 2004).

Some studies have found that fruits (Greigia sphacelata, Fascicularia bicolor) may also be consumed on occasion (Brownell 1978, Cabello 1978, Medina 1995). Marine Otters may compete with gulls (Larus) and the South American Sea Lion (Otaria flavescens) for similar species of prey fish (Cabello 1978). The most important natural predator of the Marine Otter is the Killer Whale (Orcinus orca; Cabello 1978), but adults also may be killed by sharks (Parera 1996) and birds of prey may capture juveniles when on land (Cabello 1983).

The species´ preference for coastal waters offering a wide abundance and diversity of prey species (Castilla and Bahamondes 1979) conflicts with the constant artisanal and industrial fishing effort in such habitats. Marine Otters are top predators with a high metabolic rate, thus pollution of their environments may affect them more than other species, as their position in the food chain leads to high bioaccumulation of heavy metals, pesticides and other toxic elements.

Reproductive behaviour is observed all year long with litter sizes from two to four cubs (Valqui 2012). Young remain with their parents for about ten months. The Marine Otter is most likely a monogamous species. Mating typically occurs during December or January (Caballo 1978) with gestation of 60–65 days (Housse 1953, Sielfield 1983). Parturition usually occurs from January to March. It takes place in a den or on shore between rocky outcroppings and vegetation. The litter size varies from two to four young, with two being observed most frequently. Young Marine Otters remain with their parents for approximately ten months. Adults transport their young by carrying them in their mouths or resting the young on their bellies as they swim on their backs. Both adults in the monogamous pair bring prey back to the den to feed their young (Parera 1996). When not breeding, Marine Otters are mostly solitary. The group size is seldom more than two to three individuals. Its activity pattern is generally diurnal, with peaks of activity noted in early morning, mid-afternoon and evenings, with nocturnal activities reported on bright nights (Medina et al. 2006). Marine Otters are much more agile in the water than on land.

The Marine Otter is distributed along the Southern Pacific Coast of South America from Chimbote (9°S) in northern Peru (Valqui 2012), to Isla Grevy (56°S) at the southern tip of Chile (Sielfeld 1997) and eastwards to the Isla de los Estados (54°S), in Argentina (Parera 1996). There is discussion and considerable dispute concerning the northern as well as around the southern limits of its distribution range.

In 1964, Schweigger recorded Lontra felina up to Isla Lobos de Tierra (6°26´S) in northern Peru. More recent studies registered the northern range limit at Chimbote (9°S) (Brack Egg 1978, Brownell 1978, Larivière 1998, Apaza et al. 2004, Sánchez and Ayala 2006, Valqui et al. 2010). Sightings in Huanchaco (8°S) suggest at least occasional events of recolonisation north to the former northern limit of distribution range, yet reasons for appearance or disappearance in these areas remain unclear (Alfaro-Shigueto et al. 2011).

In the south, the species is uncommon in the Bernardo O'Higgins, Maule, Ñuble and Bío bío regions of Chile (Vianna et al. 2010) and in the Tierra del Fuego region in Argentina (Cassini 2008).

Brownell (1978) stated that "the species has been nearly exterminated from the regions of Cape Horn and southern Tierra del Fuego" but several works (Sielfeld 1989, 1990, 1992; Sielfeld and Castilla 1999) reported the species´ presence between 49°S (Puerto Orella) and 55°S (Isla Grevy).

The Marine Otter's habitat is naturally fragmented in a very heterogeneous alternation of suitable habitat (rocky shore patches with caves or, sometimes, docks, shipwrecks or abandoned fishing boats) and unsuitable habitat (sandy beaches or rocky shoreline without caves). Thus, Marine Otters may be absent from stretches of several hundreds of kilometres of coast throughout the species' total distribution range (Redford and Eisenberg 1992, Valqui 2012, Vianna et al. 2010). The first known occurrence of this species far (up to 170 km) from the coast, using freshwater (rivers and lagoons) in areas at more than 1,700 m asl as its main habitat was recorded recently (Valqui et al. 2018a, Ugarte-Nuñez 2021). Studies are being held to understand population numbers and genetic characteristics of this freshwater population.

The Marine Otter is protected in Argentina, Chile, and Peru. It is listed in Appendix I of CITES and in Appendix I of the Bonn Convention (Convention on the Conservation of Migratory Species of Wild Animals (CMS)).

The Global Otter Conservation Strategy for the species (Valqui et al. 2018b) recommends priority actions that include:

An initiative to establish a regional conservation area in southern Peru -- Area de Conservación Regional Marino Costero Morro Sama, Tacna -- was started around 2003, but effective implementation stopped in 2006 because of a change in government, and the situation has not progressed since then. The area is the only otter hotspot on the Peruvian coast (with about 88 individuals in 55 km of coast) (Alfaro and Mangel 2008). Thus, the initiative of establishing a protected area containing an otter hot spot should be supported and completed. A strategy to protect otter habitats along the entire coast, coupled with initiatives to create specific regional conservation areas, is a challenging, but ideal goal for Marine Otter protection. Thirty-four existing natural protected areas in Peru and another ten in Chile could host artificial habitats, such as wave breakers, and specially designed artificial otter habitats. These should be visibly advertised to raise awareness of the Marine Otter’s presence to local communities. A reassessment is needed concerning the construction of two hydroelectric power stations, on the Colca-Majes River and the Cotahuasi-Ocoña River, both in Peru, and inhabited by the Marine Otter. Specific mitigation measures for impacts on wildlife should be considered. Dam construction should be approved only if careful and explicit mitigation measures are put in place. An effective programme to control invasive feral dogs, cats, and rats should be developed, and should be paired with studies on prevalence of transmittable diseases, especially in populations near human settlements. In Chile, studies on disease transmission to otters are already being conducted. Environmental education programmes should be carried out, such as the one by the NGO Chinchimén and Pro Delphinus, aimed at the general public and the young people who are the stakeholders of the future and will help to develop sustainable activities in their region. In order to stem further declines, studies should be undertaken to explain why the distribution of the Marine Otter is discontinuous along its coastal range. Moreover, both the northern limit and southern limits of its distribution need to be clarified through surveys (Alfaro et al. 2011). Although not all fishermen consider otters to be pests, some do, and may kill them “just in case.” Fishermen can be informed through awareness programmes that the amount of damage otters do has a minor economic impact. Reintroduction programmes are not recommended until more is known about the reasons for the decline of Marine Otter populations. High logistical and economic efforts are needed for such programmes, and efforts should concentrate on habitat protection and research for the time being.