It seems that there are a number of animal and plant names that can either be used with or without 類 as a suffix. For example, in case of 'fish', I have seen both 魚類 and 魚 being used to denote the respective animals. Another example would be 'mammal', which I have seen both as 哺乳動物 and 哺乳類. Is there any difference in meaning involved in these cases, and if yes, in what situations would each be used? I have the impression that the 類 versions may be used more in the context of biology, but I am not sure about that.
Generally speaking, yes, words like 魚類, 人類, and 哺乳類 sound more technical and scientific than 魚, 人, or 哺乳動物.
魚【さかな】(和語) is the word we usually use when we want to say fish in daily life, for example at supermarkets, while 魚類 (漢語) is only used in the biological context. I think the basic difference between 人 and 人類 is the same as the difference between person and mankind/human beings.
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On the other hand, from the expert point of view, 類 is a rather non-scientific word. The word 類 is not used in the current Japanese biological classification system. There are only ドメイン (domain), 界【かい】 (kingdom), 門【もん】 (phylum), 綱【こう】 (class), 目【もく】 (order), 科【か】 (family), 族【ぞく】 (genus), 種【しゅ】 (species), and the minor supra-/sub-categories of these. The most technically authoritative name for 哺乳【ほにゅう】類 (mammalians) is 哺乳綱【こう】.
じつは、「類」とは、正式な分類単位の名前ではありません。いわば、俗語です。何らかの共通点がある生物を、便宜上、まとめて呼ぶ名です。 ... 「類」は、綱だけを指すのではありません。門や目や科や属を指すこともあります。どの分類単位を指すのかは、決まっていません。
( http://blog.zukan.net/blog/2009/08/12-812_1.php )
Japanese "類" is a customary and ambiguous term for grouping any kind of organisms, at any level. It seems that certain words like 哺乳類 or 魚類 already have strict (customary) definitions, and are safely used by experts.
But there are cases where using 類 is obviously troublesome. For example, the biological classification of "dog" is as follows:
- Class: Mammalia 哺乳綱
- Order: Carnivora ネコ目
- Family: Canidae イヌ科
- Genus: Canis イヌ族
- Species: Canis lupus タイリクオオカミ種
- Subspecies: Canis lupus familialis イエイヌ
The word "イヌ類" is not strictly defined in this system, and it might refer to イヌ科 (includes foxes), or イヌ族 (includes wolves), or イエイヌ (scientific name of what we usually call dogs, and there are of course many types of dogs).