Ceratium has long been a popular choice as a representative dinoflagellate genus for textbooks, because as micro-organisms go, they're fairly specky. The theca of Ceratium is characterised by protruding horns, with an elongate anterior horn and one to three posterior horns. The posterior horns may be directed back from the theca, or they may curve around towards the front to produce an anchor-like shape. These horns increase the cell's buoyancy, though they do make them fairly slow swimmers. The concept of Ceratium has been fairly stable since the early 1800s, but Gómez et al. (2010) found when conducting a molecular analysis of a number of 'Ceratium' species that there was a deep divide between freshwater and marine Ceratium species. As well as the molecular divide, there is also a morphological difference: freshwater species have six plates around the cingulum (the groove around the theca body in which sits one of the flagella), while marine species have five cingular plates. As a result, Gómez et al. proposed dividing the two clades between two genera, with the name Ceratium being restricted to the freshwater species. The marine species were all transferred into a new genus Neoceratium. However, Gómez (2013) later recognised that there were a number of older generic names floating about that had been given to marine taxa, and the marine species were moved again into a resurrected genus Tripos. Among the taxa affected by this double transfer was the species shown in the photo above, now known as Tripos humilis.
There are a large number of anchor-shaped Tripos species, and distinguishing them is apparently a difficult process. Tripos humilis has the anterior part of the theca in front of the cingulum (excluding the anterior horn) fairly low, the upper surface of the theca (i.e. the side away from the origin of the flagella) strongly convex, and the right-hand posterior horn much longer than the left, with the right horn tending to converge towards the anterior horn while the left horn diverges (Subrahmanyan 1968). The cingulum is also distinctly angled relative to the posterior margin of the theca. While other Tripos species are found in a range of habitats, T. humilis appears to be a more specifically tropical species. It is found pantropically, though seemingly nowhere abundantly.
Dinoflagellates can sometimes form long chains when dividing individuals don't fully separate but continue to multiply. In Ceratium and Tripos species, the members of a chain remain connected through the apical horns. Chaining individuals may be somewhat morphologically distinct from isolated individuals; in T. humilis, the horns of chained individuals are relatively much shorter. Chains are apparently commoner when dinoflagellates form 'red tides' or algal blooms, and one suggested function is that a chain is able to swim faster overall than an individual, improving the dinoflagellates' ability to compete when moving to occupy suitable places in the water column for light or food.
Gómez, F. 2013. Reinstatement of the dinoflagellate genus Tripos to replace Neoceratium, marine species of Ceratium (Dinophyceae, Alveolata). CICIMAR Oceánides 28(1): 1-22.
Gómez, F., D. Moreira & P. López-García. 2010. Neoceratium gen. nov., a new genus for all marine species currently assigned to Ceratium (Dinophyceae). Protist 161: 35-54.
Subrahmanyan, R. 1968. The Dinophyceae of the Indian Seas. Part I. Genus Ceratium Schrank. Marine Biological Association of India, Memoir 2: 1-129.