The examples below are taken from two different texts. The first is from an MA thesis and the second is from an online magazine article.
You can see that the two texts both have signal phrases but the sources are cited in different ways. The MA thesis is an academic text and follows APA style rules. The second is a journalistic piece (albeit of an academic nature) and merely offers links to original sources, some with signal phrases and some not, but no citations.
Academic essays and reports normally follow the first style with signal phrases and citations. Articles in specialist journals may follow other styles as in the examples below.
If you are writing an academic paper, you should follow the academic style for citing your sources in whatever style (ASA, MLA, etc) your institution requires.
The following extract was taken from The Journal of Basic and Applied Zoology . You can see that there are normal citations but no signal phrases but that the citations contain a hyperlink which takes you directly to the cited work (or an abstract).
Some research studies have confirmed that incorporating long chain polyunsaturated omega-3 fatty acids in the diet regime of overweight individuals/animals resulted to a reduced adiposity and an increase in lean tissue growth on a calorie-for-calorie basis (Krebs et al. 2006; Thorsdottir et al. 2007; Hassanali et al. 2010), while a negative effect was derived with the use of omega-6 fatty acids (Birch et al. 2002; Nuernberg et al. 2011).
The next extract was taken from the BMJ . You can see that there are no normal citations and no signal phrases. There are simply clickable numbered links which take the reader to the reference list at the bottom of the article. You can see this if you go to the original article on the BMJ site. The numbered "links" in the example below are not clickable.