Dipl. Biol. (botanics, ecology). Not a trained marine biologist, I just like it a lot.
I declared myself a zoologist when I was a little boy, as soon as I could put on a snorkeling mask.

OBSERVATIONS: Thank you for your IDs and comments!

My heart has always been on the underwater life of the Croatian island Krk (Kvarner), where I identified (and also misidentified) roughly 450 marine species in the last ten years, still counting. So far I have only uploaded observations from this region.
My uploads of in-vitro shells and carapaces are all dead finds, no living specimen were collected. In recent years I have been using a small foldable blackbox for light and background control. It is easy to bring to the beach ... take the pics on site and leave the shells in the sea.
For my in-vitro finds from 2012-2020 I use fixed dates somewhere in between the time those shells were collected, and a fixed location that is best suited to draw a big circle around the whole island. Holidays. I was lazy. Didn't document my shells then.

IDENTIFICATIONS: -> Identification Etiquette on iNaturalist

Since attending university my main speciality has been "identifying". I did not focus on a certain group, but I seem to have a general knack for spotting details and differences in shapes and patterns (provided the relevant literature is available to me).
Right now I am focussing on the marine life of the Adriatic Sea > Mediterraneis > Europe. I am not a specialist in a taxonomic sense ( I am a generalist, so to speak). If anything, I am slowly turning into a regional specialist for the Northern Adriatic after more than a decade of increasing research.
As a botanist I will occasionally ID plants as well, especially from Germany, Austria and Croatia.
As a trained rainforest ecologist I will not be able to abstain from also IDing a few rainforest organisms every now and then.

MEDITERRANEIS and EUROPEAN SEAS - books, monographs & keys that I use and recommend for identification :

Alf, Brenzinger, Haszprunar, Schrödl, Schwabe: A Guide to Marine Molluscs of Europe (2020)
Antolic, Zuljevic, Vukovic: Check list of bethic marine macroalgae on the eastern Adriatic coast (2001)
Baldacconi, Trainito: Spugne del Mediterraneo (2013)
Bay-Nouaihat, Bay-Nouaihat: Guide des Tuniciers de l'Europe de l'Ouest (2020)
Bowen, Goodwin, Kipling, Picton: Sea Squirts and Sponges of Britain and Ireland (2018)
Brunetti, Mastrototaro: Fauna d'Italia - Ascidiacea of the European waters (2017)
Cabioc'h, Floc'h, Toquin, Boudouresque, Meinesz, Verlaque: Guide des Algues des Mers d'Europe (1992)
Falciai, Minervini: Guida dei Crostacei Decapodi d'Europe (1992)
Gil: The Europaean fauna of Annelida Polychaeta (2011)
Hayward, McKinney: Northern Adriatic Bryozoa from the vicinity of Rovinj, Croatia (2002)
Igić: Cirripedia of Adriatic (2007)
Kovačić, Renoult, Pillon, Svendsen, Bogorodsky, Engin, Louisy: Identification of Mediterranean marine gobies (...) of the continental shelf from Photographs of in situ individuals (2022)
Le Granché, André, Rochefort: Spongiaires de France (2018)
Louisy: Europe and Mediterranean Marine Fish (ital.: 2020, eng.: 2015, ger.: 2002)
Müller: Reconnaître les principaux bivalves fouisseurs ou foreurs au moyen de leurs siphons (2016)
Neumann, Paulus: Mittelmeer-Atlas - Fische und ihre Lebensräume (2005)
Nikolić: Flora Croatica - Vaskularna Flora Republike Hrvatske 2-3 (2020)
Noel: Cle preliminaire d'identification des Crustacea Decapods de France er des principales autres espèces d'Europe (1992)
Pansini, Manconi, Pronzato: Fauna d'Italia - Porifera I (2011)
Pillon: Astropecten of the Mediterranean Sea (2020)
Poppe, Goto: European Seashells Vol. II (1993)
Renoult, Pillon, Kovačić, Louisy: Gobies of the North-eastern Atlantic and the Mediterranean: Gobius and Thorogobius (2022)
Riedl: Fauna und Flora der Adria (1970)
Rodríguez-Prieto, Ballesteros, Boisset, Afonso-Carillo: Alghe e Fanerogame del Mediterraneo (2015)
Tiralongo: Blennies of the Mediterranean Sea (2020)
Tortonese: Fauna d'Italia - Echinodermata (1963)
Trainito, Baldacconi: Atlante di Flora e Fauna del Mediterraneo (6th ed. 2021)
Trainito, Baldacconi: Coralli del Mediterraneo (2016)
Wirtz, Debelius: Niedere Tiere Mittelmeer/ Atlantik (2003)
...
and an ever increasing number of other publications ...

BOTANIK DEUTSCHLAND/ ÖSTERREICH - Bücher, die ich zur Bestimmung verwende:

Buttler: Steinbachs Naturführer Orchideen
Eschrich: Gehölze im Winter - Zweige und Knospen
Fischer, Adler, Oswald: Exkursionsflora Österreich Liechtenstein Südtirol
Fritsch: Exkursionsflora von Österreich
Godet: Bäume und Sträucher
Godet: Blüten der heimischen Baum- und Straucharten
Grau, Kremer, Möseler, Rambold, Triebel: Steinbachs Naturführer Gräser
Häupler, Muer: Bildatlas der Farn- und Blütenpflanzen Deutschlands
Kremer: Steinbachs Naturführer Strauchgehölze
Mitchell: Parey - Die Wald- und Parkbäume Europas
Oberdorfer: Pflanzensoziologische Exkursionsflora
Rothmaler: Exkursionsflora von Deutschland 2-5
Schmeil-Fitschen: Flora von Deutschland und angrenzender Länder
Grey-Wilson, Blamey: Parey - Bergblumen
...

WEB:
Sources of information I frequently use.
(Not everything posted on those sites is certified truth. Handle with care. But you know that.)

algaebase.org
colors.artyclick.com (color shade determination tool for your observation pics - very useful!)
doris.ffessm.fr
eol.org
fish-watch.org
gbif.org
idscaro.net
inaturalist.org
naturamediterraneo.com
marinespecies.org
molluscabase.org
pflanzenbestimmung.de
researchgate.net
...

COMMENTS I frequently do not post:

"You should have left it in the ocean."
"Science is not a matter of opinion."
"Geneticists tell us we are apes, let's not forget that."
"That may as well be, but did you check out the latest WEBB pics?"

In the last centuries naturalists killed everything they wanted to ID without second thought.
This is typical human behaviour. But we can do better.
Thank you, iNaturalist, for proving it.