A new species of Cerapus (Amphipoda, Senticaudata, Ischyroceridae) from Mae Klong Estuary, with a discussion on their nesting and types of mating behaviour

The first representative of the genus Cerapus in the Gulf of Thailand, Cerapus rivulus sp. nov., is described from specimens sampled from Mae Klong Estuary, the inner Gulf of Thailand. The main identifying characteristics of this new amphipod species are pere-onites 1 and 2 without constriction; male gnathopod propodus palm transverse with long posterior defining tooth and well-developed anterodistal recurved tooth adjacent to propodus articulation; pereopod 6 coxa without fine fringe setae ventrally, basis with setae on posterior margin; and telson with deep cleft. An updated identification key for the 25 known species in the genus is also presented. A discussion on their nesting and types of mating behaviour is provided.

Most species of Cerapodini are found on the soft bottom of estuaries and various coastal habitats, i.e. seagrass beds, algal beds, mangrove forests and coral reefs (Shen 1936;Lowry and Berents 2002;Lowry and Berents 2005;Nurshazwan et al. 2020).There are various types of tubes, with Lowry and Berents (2005) classifying Cerapus tubes into three types: the detrital tube, algal tube and sandy tube.Most tubes are constructed in a turf of the interwoven tube with various substrates attached, including hydroids, algae, fleshy and calcareous gorgonians and sponges (Lowry and Thomas 1991).The study of Nurshazwan et al. (2020) reported 23 species, but did not include C. longicervicum Lim, Park & Min, 2008 from South Korea (Lim et al. 2008).Adding the newly-discovered C. rivulus sp.nov., there are now a total of 25 Cerapus species worldwide.From that, only six species of Cerapus have been described in the waters of Southeast Asia, including Singapore: C. tubularis Say, 1817;Thailand: C. chaomai Lowry &Berents, 2002 andC. yuyatalay Lowry &Berents, 2002;Korea: C. longirostris Shen, 1936 andC. longicervicum Lim, Park &Min, 2008; Malaysia: C. bumbumiensis Nurshazwan, Ahmad-Zaki & Azman, 2020.However, there have been no recorded species from the South China Sea, including the Gulf of Thailand (Lowry and Berents 2002;Lim et al. 2008;White 2015;Nurshazwan et al. 2020;Azman 2022).
The present study describes Cerapus rivulus sp.nov.as a new species from the Gulf of Thailand.A distribution map with brief biological notes and a key to the world species of the genus are also provided.

Materials and methods
The Cerapus sampling was carried out using the experimental model set, based on Aikins and Kikuchi (2001).The experimental models were set up around Mae Klong estuarine for two and four months (Fig. 1).
The Mae Klong River is located in west-central Thailand and passes through Kanchanaburi, Ratchaburi and Samut Songkhram before reaching the upper Gulf of Thailand.Along the river, there are urban and aquaculture zones which make this area important for fishing activities and aquaculture.The Mae Klong Estuary is situated in the Amphawa District of Samut Songkhram Province with salinity ranges from 0.05-2.00ppt due to tide and water runoff.
The specimens were selected from experimental material and preserved in 95% ethanol.The male holotype and female paratype specimens were transferred from ethanol on to a glycerol slide for morphological study in the laboratory.Drawings were made using a camera lucida attached to an Olympus CH30 light microscope.The pencil drawings were scanned and digitally inked using a WACOM bamboo CTH-970 graphics board in Adobe Illustrator CC 2017, following the method described in Coleman (2003).Terminology for setae and mouthparts described by Zimmer et al. (2009).
Remarks.Cerapus rivulus sp.C. tubularis was described from Long Island Sound, New York, U.S.A. and after that reported in Japan by Morino (1976).However, Lowry and Berents (1989) re-described C. tubularis and excluded the record of Morino (1976).Lastly, White (2015) reported Cerapus sp. in Singapore and noted that the amphipod resembles C. tubularis and possibly was introduced to Singapore.The presence of C. tubularis in the eastern Pacific and South China Sea is still unclear and needs clarification.
Geographic distribution.The Cerapus are distributed worldwide and mainly inhabit marine and brackish water (Fig. 11) (Lowry and Thomas 1991;Lowry and Berents 2002;Drumm 2018;Nurshazwan et al. 2020).Interestingly, C. rivulus sp.nov.was found in freshwater and brackish water (salinity less than 3 ppt), so this is the first report for freshwater habitats.They were dominant in the coarse filter substrate used for benthos sampling because the tubes are retained on the coarse filter.
Biological notes.Generally, tube-building amphipods build tubes using amphipod silk which is observed as silk strands adhering to the tip of the dactylar surface close to the pore on P3 and P4 (Kronenberger et al. 2012).Cerapus rivulus sp.nov.also presents this characteristic (Fig. 12).The outer surface of the tube consists of uniformly coarse sediment and detritus with a mix of fragmented fibres and algae (Fig. 2) (small tubes of juvenile attached to the larger tube) (Fig. 13A-D); the inner surface, on the other hand, exhibits a fine network of amphipod silk.Laboratory observations on C. rivulus sp.nov.: feeding behaviour, both antennae 1 and 2 were used to gather food and move chunks of organic material back inside their tube.From the gut con-tent analysis, the major food items consist of fungus, algae, diatoms, organic substances and protozoa.Moreover, mating behaviour showed that males wander about the habitat investigating the tube of female individuals.During these encounters, males try to contact females with their antennae.After that, females withdraw the posterior part of the body into their tubes, while males use their urosome to stimulate the ventral surface of the female pleosome.These situations are assumed to be a possible prelude to mating (Fig. 14).

Figure 1 .
Figure 1.Map showing the three river mouths along the Gulf of Thailand.Red circle represents the type locality of Cerapus rivulus sp.nov.

Figure 12 .
Figure 12.Distal end of P3 and P4 of Cerapus rivulus sp.nov.Red dash line represents distal end of P3 dactylus with amphipod silk opening and yellow dash line represents distal end of P4 dactylus with amphipod silk opening.

Figure 13 .
Figure 13.Cerapus rivulus sp.nov. A. Showing a mix of fragmented fibres and algae on its tube; B. Showing small tubes of juvenile attached on the larger tube; C. Uniformly coarse sediment on the outer surface of the tube; D. Ultrastructure of uniformly coarse sediment on the outer surface of the tube (scanning electron microscope); E. Ultrastructure of fine network of amphipod silks on the inner surface of the (scanning electron microscope).